Orion StarBlast 4.5 EQ VS. StarBlast 6
wish1267
and I think I'm ready for the next step.
I been looking at these two telescopes:
http://www.telescope.com/control/product/~category_id=fall2008new/~pcategory=telescopes/~product_id=09964/~sSearchSession=777afa07-cad4-4407-bb39-2039e0172da2
and
I know the 4.5 has a 450mm and the 6 has a750mm focal length,
therefore I would get higher possibility (atmosphere allowing)
magnitude, but I want to know about the experience. Which is better
for hauling around ? Is the extra 15lbs a deal breaker?
Between these two, would planets look the same?
Thanks for any input.
Tom Royer
news:1164a13e-ae24-4fc6...@k29g2000prf.googlegroups.com...
The focal length isn't too important other that as it drives the physical
length of the telescope. All other things being equal, the larger objective
of the 6in will stand you in good stead. Also, if you're a normal sized
adult, you'll find the very low eyepiece level of the 4.5 awkward. You're
the only one who can decide if the extra 15lbs is a deal breaker -- for most
adults, it isn't. If it were me, I'd buy the 6in.
Curtis Croulet
> level of the 4.5 awkward.
The assumption is that you'd put it on a table or on the car hood.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
astros...@gmail.com
> Between these two, would planets look the same?
Assuming similar optical quality, the planets ought to look better
(brighter, sharper, and of higher contrast) with the 6-inch scope.
Yet, if the price of the StarBlast 6 represents your maximum budget -
and if you're primarily concerned with how good planets will look, you
might want to consider the XT6. While not as friendly to haul around,
the longer focal length of the XT6 stands a reasonable chance of
providing better planetary views than either of your original choices.
Bill Greer
To sketch is to see.
Margo Schulter
Hi, there, Bill and all,
Looking at a Spring 2009 Orion catalogue, I see another
choice that might help with price, weight, and versatility
with some compromise of aperture: the Orion SkyQuest XT4.5
Dobsonian telescope for $199.95, with an assembled weight
of 17.6 pounds. This is f/8, which might be better for
planetary observing and for "reasonable but imperfect"
wide-angle eyepieces.
This isn't any kind of recommendation -- only throwing
another possibility into the discussion.
Most appreciatively,
Margo Schulter
msch...@calweb.com
joni...@aol.com
Margo:
These are all reasonable telescopes and good values, each has it's
pluses and minuses... This is my experience/thinking:
- Starblast 4.5 inch: A popular and compact telescope but at F/4 it
requires careful collimation and will only be sharp on axis. Handy
and portable but I suspect better in the hands of an experienced
observer.
- Starblast 6 inch: The 6 inch aperture definitely offers a step up
in brightness and resolution. At F/5 it will be easier to collimate
and easier on the eyepieces. You will definitely see more with this
than the Starblast 4.5, whether you are looking at the planets or the
deep sky. Both the Starblast's require a table or some sort of
additional mount, this may make viewing awkward and one needs a super-
steady table to avoid the shakes.
- XT-4.5 : A cute little DOB, probably best as a child's scope as it
is too short for an adult to comfortably sit beside. The high power
will be better than the Starblast 4.5 inch but this scope has a
spherical mirror which is something of a hinderance..
- XT-6: Bill was right, you ought to be considering this one... The
downside is that it is physically larger than the others, the tube is
longer, the tube is heavier, the base is bigger and heavier as well.
Still, it is a reasonable size, the base is the heaviest component at
about 20lbs and will fit in any normal sized car.
On the plus side, this is a serious telescope, there are some good
reasons that the 6 inch F/8 Newtonian has long been a favorite amateur
scope. This is particularly true in the Dobsonian configuration...
The OTA is about 48 inches long. This means that an adult can sit
along side it comfortably and view for long periods. One does not
need a table or car to rest the scope on.
Like the Starblast 6, the 6 inch aperture will show the planets and
deep sky objects well but with it's slower F/8 focal ratio it will be
very easy to collimate and will show wonderful views of pinpoint stars
nearly right up to the edge with simple eyepieces like Plossls and
even Kellners. The Planets will show nicely in this scope, better
than the Starblast 6...
One down side of the XT-6 is that it's longer focal length restricts
the field of view, the maximum possible is about 1.3 degrees whereas
the Starablast 6 has a possible 2.1 degree TFoV. I think the trade
off is worth while, you are essentially giving up one view for a more
comfortable scope and one that performs better across the board.
So, while all the scopes you mention are decent quality scopes capable
of providing good views, my experience is a 6 inch DOB like the XT-6
is the best choice from both an optical and ergonomic standpoints. In
the old days, a 6 inch F/8 was the dream of many an astronomer, these
days people consider them beginners scopes but I attest to the fact
that they can provide wonderful views of the night sky...
jon isaacs
Margo Schulter
> Margo:
>
> These are all reasonable telescopes and good values, each has it's
> pluses and minuses... This is my experience/thinking:
Thank you for putting my mention of the XT-4.5 in better perspective
for the original poster, and bringing up the important point about
the limitations of a spherical mirror, a detail I didn't even notice
when quickly noticing this catalogue item!
> - XT-4.5 : A cute little DOB, probably best as a child's scope as it
> is too short for an adult to comfortably sit beside. The high power
> will be better than the Starblast 4.5 inch but this scope has a
> spherical mirror which is something of a hinderance..
>
> - XT-6: Bill was right, you ought to be considering this one... The
> downside is that it is physically larger than the others, the tube is
> longer, the tube is heavier, the base is bigger and heavier as well.
> Still, it is a reasonable size, the base is the heaviest component at
> about 20lbs and will fit in any normal sized car.
>
> On the plus side, this is a serious telescope, there are some good
> reasons that the 6 inch F/8 Newtonian has long been a favorite amateur
> scope. This is particularly true in the Dobsonian configuration...
> The OTA is about 48 inches long. This means that an adult can sit
> along side it comfortably and view for long periods. One does not
> need a table or car to rest the scope on.
Here I might add from personal experience on the last point that an
OTA of about 48 inches (or the very slightly shorter 1200mm) is indeed
comfortable for a typically-sized adult to use from an observing
chair. As it happens, I have a 200mm (7.9") f/6 SkyWatcher Dob -- but
the XT6 at 150mm f/8 (according to the Orion catalogue) is exactly the
same length of 1200mm.
> One down side of the XT-6 is that it's longer focal length restricts
> the field of view, the maximum possible is about 1.3 degrees whereas
> the Starablast 6 has a possible 2.1 degree TFoV. I think the trade
> off is worth while, you are essentially giving up one view for a more
> comfortable scope and one that performs better across the board.
This matter of calculating maximum TFoV is an interesting one. With my
200mm Dob at f/6, in practice I get a tad more than 2 degrees with an
2" ultrawide 30mm eyepiece (82 degrees AFoV) at 40x. This is an
interesting problem. With this same eyepiece, the XT6 should give the
same 40x (1200mm/30mm), and therefore the same field of about 2.05
degrees -- with a smaller exit pupil of 3.75mm rather than 5mm, if
I'm right.
If this is correct, we might chalk it up as another advantage of the XT6,
since 2 degrees is a nice wide field if one gets interested in admiring
extended DSO's as well as planetary observing. Of course, as you point
out, the f/8 will be more forgiving of eyepieces in terms of edge
quality, etc. I've found f/6 reasonable -- but f/8 should be yet better.
> So, while all the scopes you mention are decent quality scopes capable
> of providing good views, my experience is a 6 inch DOB like the XT-6
> is the best choice from both an optical and ergonomic standpoints. In
> the old days, a 6 inch F/8 was the dream of many an astronomer, these
> days people consider them beginners scopes but I attest to the fact
> that they can provide wonderful views of the night sky...
> jon isaacs
Let's hope that this helps the original poster -- and I'd appreciate
any corrections or comments on my impressions about field of view with
the XT6.
With many thanks,
Margo
joni...@aol.com
>
> This matter of calculating maximum TFoV is an interesting one. With my
> 200mm Dob at f/6, in practice I get a tad more than 2 degrees with an
> 2" ultrawide 30mm eyepiece (82 degrees AFoV) at 40x. This is an
> interesting problem. With this same eyepiece, the XT6 should give the
> same 40x (1200mm/30mm), and therefore the same field of about 2.05
> degrees -- with a smaller exit pupil of 3.75mm rather than 5mm, if
> I'm right.
>
> If this is correct, we might chalk it up as another advantage of the XT6,
> since 2 degrees is a nice wide field if one gets interested in admiring
> extended DSO's as well as planetary observing. Of course, as you point
> out, the f/8 will be more forgiving of eyepieces in terms of edge
> quality, etc. I've found f/6 reasonable -- but f/8 should be yet better.
>
> Let's hope that this helps the original poster -- and I'd appreciate
> any corrections or comments on my impressions about field of view with
> the XT6.
>
> With many thanks,
>
> Margo
Margo: The difference in field of view is simply because your 8 inch
F/6 Dob has a two inch focuser and the XT-6 has a 1.25 inch focuser...
Put a two inch focuser on the XT-6 and it will provide the same TFoV.
jon
Margo Schulter
>
>
> Margo: The difference in field of view is simply because your 8 inch
> F/6 Dob has a two inch focuser and the XT-6 has a 1.25 inch focuser...
> Put a two inch focuser on the XT-6 and it will provide the same TFoV.
>
> jon
>
Ah, that's a very important difference! It's curious that I would just
assume a 2" focuser on a Dob -- thanks for the correction.
Best,
Margo
Daniel Birchall
try out (as well as other more "serious" scopes). As an adult of more than
reasonable mass, I can easily single-hand the 4.5 or the 6; the 8 I have to
carry with 2 hands, and the 10 is an unwieldy beast. :)
--
Dan Birchall, Operator, UH 2.2-meter (88") Telescope
"Science is a wonderful thing if one does not have to
earn one's living at it." - Albert Einstein
joni...@aol.com
wrote:
Dan: Are you carrying the scope and the mount together or
separately? The only Dobsonian I ever carried assembled was a 6 inch
F/5 I had there for a while. All the others I move in two pieces, the
mount and the OTA. Doing it that way, I find my 10 inch Asian DOB to
be very manageable...
Jon
tony_f...@yahoo.com
> - Starblast 4.5 inch: A popular and compact telescope but at F/4 it
> requires careful collimation and will only be sharp on axis. Handy
> and portable but I suspect better in the hands of an experienced
> observer.
Actually, my experience is that beginners and kids love this scope;
it's unbelievably easy to use at low and medium power. However, it's a
bit of a specialist instrument. The short focal length makes it very
hard to focus at high power, and hardly anybody -- even kids -- can
use it when it's sitting on the ground.
For an experienced deep-sky observer, this scope is magnificent
because of its immense field of view. The finest view I ever had of
the Veil Nebula as a whole was through a StarBlast 4.5.
> - XT-4.5 : A cute little DOB, probably best as a child's scope as it
> is too short for an adult to comfortably sit beside. The high power
> will be better than the Starblast 4.5 inch but this scope has a
> spherical mirror which is something of a hinderance..
As it happens, I find the XT-4.5 exceedingly comfortable when I'm
sitting on a low stool. This scope definitely performs better than
most people expect; it yields startlingly good planetary images. No
doubt it would be better still with a parabolic mirror, but at 4.5-
inch f/8, being spherical does not disqualify a mirror from being
diffraction-limited. I find the scope more pleasant to use at high
power than the Starblast 4.5 simply because it's a lot easier to
focus.
> - XT-6: Bill was right, you ought to be considering this one...
I agree. Unless ultra-portability or cuteness is a major
consideration, a 6-inch f/8 Dob is the obvious choice for a beginner
on a tight budget. Six inches puts you in the big time, and the
eyepiece height is easier for many people to deal with than on
physically smaller scopes. On the other hand, it's really a tad too
high for a conventional chair when viewing objects high in the sky,
while still being a tad low for standing.
wsne...@hotmail.com
> So I am thinking of purchasing my first telescope. I have used binos
> and I think I'm ready for the next step.
>
>
> and
>
> http://www.telescope.com/control/product/~category_id=reflectors/~pca...
>
> I know the 4.5 has a 450mm and the 6 has a750mm focal length,
> therefore I would get higher possibility (atmosphere allowing)
> magnitude, but I want to know about the experience. Which is better
> for hauling around ? Is the extra 15lbs a deal breaker?
>
> Between these two, would planets look the same?
>
> Thanks for any input.
I would suggest the 6-inch, simply because of the added light-
gathering power. The longer focal length XT-6 currently ships with
only one eyepiece, so you would have to include at least one more
eyepiece in your budget. A 2X or 3X Barlow lens is also a good idea
for either scope. The XT-6 would also take up space in the back seat
of a small car, while either Starblast would probably fit easily in
the trunk.
The very portable 4.5 -inch would be a better choice if you think that
you might be able to buy an 8-inch or larger scope in the near
future.
rmollise
> and I think I'm ready for the next step.
>
>
> and
>
> http://www.telescope.com/control/product/~category_id=reflectors/~pca...
>
> I know the 4.5 has a 450mm and the 6 has a750mm focal length,
> therefore I would get higher possibility (atmosphere allowing)
> magnitude, but I want to know about the experience. Which is better
> for hauling around ? Is the extra 15lbs a deal breaker?
>
> Between these two, would planets look the same?
>
> Thanks for any input.
While the 6-inch will go considerably deeper and will show more detail
on planets as well, yes, the larger size and larger weight is a deal
breaker for me for this kind of scope. The 4.5-inch is perfect for
what it is. If you want a six inch, I'd look at one of Orion's
"normal" 6-inch f/8 models.
wsne...@hotmail.com
...which at an additional 21 pounds and 2.5x the weight of the alt-az
StarBlast 4.5-inch, would be even more of a deal-breaker. A 24 pound
6-inch f/5 resting on a lightweight plastic crate (the kind you BUY at
the discount store) would have a comfortable eyepiece height for
someone seated in a lawn chair.
A 6-inch f/8 has its advantages, but we don't knock 10-inch and larger
scopes for being f/5.
rmollise
>
> - Show quoted text -
I ain't knockin' nothing...just obsrving that if you don't mind a
little more weight and bulk, an f/8 will produce some excellent
images...and ain't exactly a beast to haul around. LOL
The six inch StarBlast, you will find, needs more than a plastic milk
crate. ;-)
joni...@aol.com
Until I passed it on to someone who could make better use of it than
I, I had a rather nice 6 inch F/5 on a Dob mount that someone had
built from a kit. All the bearings were Ebony Star-Teflon, the wood
was furniture grade plywood and the optics were quite nice. It sat on
a wooden tripod and was comfortable but the tripod was considerably
taller than the lightweight plastic crates I have seen, one of those
would have been uncomfortably low...
Based my experiences with my 6 inch F/5 minidob, I believe the 6 inch
Starblast is a viable option if space is critical. The reason we
accept 10 inch F/5's is because they are so much more manageable than
10 inch F/6's and 10 inch F/8's. A 10 inch F/5 fits easily across the
seat of most cars, a 10 inch F/6 does not.
But I think it is important to remember the down side of a 6 inch F/5
as compared to a 6 inch F/8, particularly as a first serious
telescope. For me or your or Rod, it is likely we could achieve very
good high magnification views out of the F/5 but for a first time
observer learning to collimate the scope and using simple eyepieces, I
suggest the slower F/8 focal ratio with its much larger coma-free
version and friendliness towards simple eyepieces is a better choice.
jon
wsne...@hotmail.com
We choose ratios such as f/5 for large telescopes to reduce or
eliminate the use of a step ladder. We might also choose them to save
weight and bulk in mid-size instruments. Assuming that the OP's car
even has a back seat, a 6-inch f/8 takes up the whole back seat, while
the f/5 leaves room for a passenger and might even fit in the trunk.
In a larger vehicle the f/5 might go with you on a vacation, the f/8
seems much less likely to do so. So what we might say about the
advantages of f/5 over f/6 in a 10-inch, applies to some extent to the
advantages of f/5 over f/8 in a 6-inch.
>
> But I think it is important to remember the down side of a 6 inch F/5
> as compared to a 6 inch F/8, particularly as a first serious
> telescope. For me or your or Rod, it is likely we could achieve very
> good high magnification views out of the F/5 but for a first time
> observer learning to collimate the scope
As long as the diagonal mirror remains aligned properly, the primary
will need only a tweak or two at each session, even at f/5. With a 6-
inch f/5 you might even be able to make the adjustments while looking
through the eyepiece.
> and using simple eyepieces, I
> suggest the slower F/8 focal ratio with its much larger coma-free
> version and friendliness towards simple eyepieces is a better choice.
This particular 6-inch f/5 comes with two Plossl eyepieces which
should work fine at f/5. Older types such as Ramsdens, Kellners and
Huygens which don't work well in fast scopes are not seen much
nowadays, especially since Plossls are now reasonably priced.
wsne...@hotmail.com
An 8-inch f/6 can produce some excellent images too, weighs only a
little more and costs not much more than the 6-inch f/8 or f/5. A _6-
inch_ scope, of whatever focal ratio that costs only $250, fits in
almost any car and weighs only 24 lbs seems like a good deal, even if
you do have to set it up on something.
> The six inch StarBlast, you will find, needs more than a plastic milk
> crate. ;-)
From a standing position you would need something higher than a milk
crate, even a 6-inch f/8 might need something higher than a milk
crate, but from a seated position raising the scope up a foot or more
should be sufficient.
joni...@aol.com
> As long as the diagonal mirror remains aligned properly, the primary
> will need only a tweak or two at each session, even at f/5.
It is not a question of whether or not the diagonal and/or the primary
remain properly aligned, rather it is difficulty aligning them in the
first place. The concepts of collimation are not so easy to grasp, an
F/8 is very forgiving, an F/5 is not...
- Plossls are reasonably sharp in the center of a well collimated F/5,
not so hot at the edges but likely acceptable for a beginner.
Inexpensive widefields though are generally quite rough in an F/5.
But Plossls and simple widefields are quite wonderful in a 6 inch F/
8...
- Regarding the the larger 8 inch... Indeed, one of my favorite
sayings is: "The best 6 inch F/8 is an 8 inch F/6."
About the only downside of the 8 inch F/6 over the 6 inch F/8 is the
cost and a somewhat heavier OTA. Physically they are the same length
and the bases both weigh about 20lbs so if one can handle a 6 inch F/
8, one can manage an 8 inch F/6. While not as forgiving when it
comes to collimation, F/6 is still significantly easier than F/5.
The extra ~$100 to step up from the 6 inch to the 8 inch not only buys
one light gathering and resolution but it also typically buys an 8x50
finder, often a right angle correct image finder as well as a 2 inch
Crayford type focuser. Both worthwhile improvements.
Of course this is how these discussions often go... Start of looking
at the Starblast 4.5, then see the Starblast 6, and that leads to the
classic 6 inch F/8 DOB which leads to the 8 inch F/6 DOB. The final
step now is to consider the 10 inch F/5 DOB. The OTA is the same
length as the 6 inch F/8, only about 10-15 lbs heavier than the 8 inch
F/6 and of course provides even more light gathering and resolution
while still fitting across the back seat of even a small car.
I pickup used 6 inch F/8 and 8 inch F/6 DOBs every but as a "grab and
go" DOB my 10 inch F/5 seems to get the nod.... Those others find new
homes, the 10 inch F/5 stays here along with it's siblings, both
larger and smaller...
Jon
wsne...@hotmail.com
> Just a couple of comments:
>
> > As long as the diagonal mirror remains aligned properly, the primary
> > will need only a tweak or two at each session, even at f/5.
>
> It is not a question of whether or not the diagonal and/or the primary
> remain properly aligned, rather it is difficulty aligning them in the
> first place. The concepts of collimation are not so easy to grasp, an
> F/8 is very forgiving, an F/5 is not...
The tricky part of collimation is getting the diagonal positioned and
aimed, not easy even on an f/8. This can be taken care of at the
factory and once aligned diagonals tend to stay aligned, because they
are light and close to the focuser. That leaves the much heavier
primary as the part that usually needs adjustment. Most people would
have no trouble reaching around to adjust the primary set screws on a
StarBlast while looking through the eyepiece to see the results in
real time. Try that with a 6-inch f/8!
>
> - Plossls are reasonably sharp in the center of a well collimated F/5,
> not so hot at the edges but likely acceptable for a beginner.
> Inexpensive widefields though are generally quite rough in an F/5.
> But Plossls and simple widefields are quite wonderful in a 6 inch F/
> 8...
With a 6-inch f/8 you are still limited to about a 1.25 degree true
field of view, unless you upgrade the focuser and buy some pricey
eyepieces. The f/5 comes with an eyepiece that gives almost a 1.75
degree true FOV. This might make aiming the scope a bit easier for
the beginner.
>
> - Regarding the the larger 8 inch... Indeed, one of my favorite
> sayings is: "The best 6 inch F/8 is an 8 inch F/6."
One could also say that the best 4.5 inch f/8 is a 6-inch f/6 (or
maybe f/5.)
> About the only downside of the 8 inch F/6 over the 6 inch F/8 is the
> cost and a somewhat heavier OTA. Physically they are the same length
> and the bases both weigh about 20lbs so if one can handle a 6 inch F/
> 8, one can manage an 8 inch F/6. While not as forgiving when it
> comes to collimation, F/6 is still significantly easier than F/5.
>
> The extra ~$100 to step up from the 6 inch to the 8 inch not only buys
> one light gathering and resolution but it also typically buys an 8x50
> finder, often a right angle correct image finder as well as a 2 inch
> Crayford type focuser. Both worthwhile improvements.
>
> Of course this is how these discussions often go... Start of looking
> at the Starblast 4.5, then see the Starblast 6, and that leads to the
> classic 6 inch F/8 DOB which leads to the 8 inch F/6 DOB. The final
> step now is to consider the 10 inch F/5 DOB. The OTA is the same
> length as the 6 inch F/8, only about 10-15 lbs heavier than the 8 inch
> F/6 and of course provides even more light gathering and resolution
> while still fitting across the back seat of even a small car.
The OP asked to compare the two Starblast scopes... maybe his car
doesn't have a back seat, or he doesn't want to carry a scope in the
cargo bed of his truck, or an XT6 is too bulky to carry on the bus,
or...
joni...@aol.com
> The tricky part of collimation is getting the diagonal positioned and
> aimed, not easy even on an f/8. This can be taken care of at the
> factory and once aligned diagonals tend to stay aligned, because they
> are light and close to the focuser. That leaves the much heavier
> primary as the part that usually needs adjustment. Most people would
> have no trouble reaching around to adjust the primary set screws on a
> StarBlast while looking through the eyepiece to see the results in
> real time. Try that with a 6-inch f/8!
My experience is with Asian scopes is that the diagonal is probably
more likely to need adjustment than the primary. And whether it could
happen at the factory, it doesn't seem to happen that way... It's
true it's easier to reach the collimation screws of a 6 inch F/5 but
it is also true the 6 inch F/8 is much more tolerant of
miscollimation.
As I said, I very much enjoyed my 6 inch F/5 mini-Dob and if room is
an issue it makes a decent choice. However a first serious scope, I
think the 6 inch F/8 makes a scope that allows the observer to spend
less time learning about the scope and more time learning the sky.
Jon
Steve P.
<joni...@aol.com> wrote:
> wsnel...@hotmail.com wrote:
>> The tricky part of collimation is getting the diagonal positioned and
>> aimed, not easy even on an f/8. This can be taken care of at the
>> factory and once aligned diagonals tend to stay aligned, because they
>> are light and close to the focuser. That leaves the much heavier
>> primary as the part that usually needs adjustment. Most people would
>> have no trouble reaching around to adjust the primary set screws on a
>> StarBlast while looking through the eyepiece to see the results in
>> real time. Try that with a 6-inch f/8!
> My experience is with Asian scopes is that the diagonal is probably
> more likely to need adjustment than the primary.
It was once reported that the plastic housing for the secondaries pinched
the optics. While I was never convinced this was true of my Asian
reflectors, I loosened them up just the same. At the other end it was also
reported that the retaining clips were too tight on the primary. I'd always
loosen those up just a bit as well (just enough that I could forceably
rotate the primary in the cell).
In the end though, I figured it was just that my seeing wasn't good, and the
scope hadn't sufficiently cooled, and that explained the somewhat triangular
diffraction pattern that would appear regularly in my star testing of the
scopes not long after I'd set out for a obs session.
I don't know if it's a modified version, or if I finally had the lights come
on in my brain, but the current PDF for the Orion 4.5" imaging reflector (on
their website) has a collimation section that describes the process really
well (I thought). In previous study of these types of docs, I had trouble
with the whole "secondary under the focuser" thing. I don't know if the
problem was the description, the picture, or my dead brain application of
the procedure. I think it's the "shifted toward the primary" that use to
throw me, but the aforementioned version of the procedure uses good focuser
hole imagery to get the idea across in pictures.
-Steve P.
rmollise
>
> - Show quoted text -
Whatever floats your boat. I have actually used the StarBlast 6,
though. ;-)
joni...@aol.com
I don't know about the other Asian DOBs but the GSO 10 inch was quite
well known for a pinched secondary. I bought one second hand several
years ago because the owner thought the optics were poor and I was
pretty sure they couldn't be all that bad. Well it had serious
astigmatism which I traced to a pinched diagonal and later discovered
this was quite common.
My concern with collimating fast newtonians is simply that the
tolerances are quite tight so making an F/4 Newtonian perform it's
best is no easy task.. As a general rule of thumb, the coma free
diameter goes about like this:
F/4= 1.1mm
F/5 = 2.1mm
F/6 = 3.7mm
F/8 = 8.8mm
jon
On Mar 16, 1:51 pm, "Steve P." <smarshallp...@gmail.com> wrote:
Steve P.
not about to try and straighten that out. Got better stuff to do with my
time. :-)
I've hardly been out at all over the past couple of years. Divested myself
of all my big guns and fancy do-dads, and bought me a motor-sickle. I did
hang onto a StarHOC 6-5 I picked up along the way, and traded something or
other for an AT66 ED Doublet. They both work quite well. The 6-5 is
undergoing experimental surgery to get it to come to focus with an Orion
binoviewer I bought off an astromarter a year or so ago. Cut the tube down,
and intend to make it a sliding configuration so I can change the position
of the primary with the turn of a few thumbscrews.
Just playing, just saying.
One day I'd like to pick up another 12" F4-F5 Dobber. Of all the scopes I
had during the years I was actively getting outside in all manner of
temperatures, the Meade 12" F4.9 optics in the Discovery PDHQ tube I cobbled
together was the best all around performer I've ever had for poking around
the sky, from planets to planetary nebulae.
Got other priorities right now with the economic uncertainty, and kids in
school.
Keep posting.
-Steve
<joni...@aol.com> wrote in message
news:b761bd78-e6ef-45c1...@v13g2000pro.googlegroups.com...
wsne...@hotmail.com
I usually opt for something with quite a bit more light gathering
power most nights. Any small scope that I would use would have to
have low power and a wide field of view. The scopes I use most are f/
4 to f/6; the f/7's and higher sort of became "obsolete."
wsne...@hotmail.com
> Steve:
>
> I don't know about the other Asian DOBs but the GSO 10 inch was quite
> well known for a pinched secondary. I bought one second hand several
> years ago because the owner thought the optics were poor and I was
> pretty sure they couldn't be all that bad. Well it had serious
> astigmatism which I traced to a pinched diagonal and later discovered
> this was quite common.
>
> My concern with collimating fast newtonians is simply that the
> tolerances are quite tight so making an F/4 Newtonian perform it's
> best is no easy task.. As a general rule of thumb, the coma free
> diameter goes about like this:
>
> F/4= 1.1mm
> F/5 = 2.1mm
> F/6 = 3.7mm
> F/8 = 8.8mm
>
> jon
The f/8 might have 4x the coma-free field, but the f/5 has almost
twice the leverage in the adjustment, so the f/5 won't really be that
much harder to adjust. Being able to see what is happening as you
tweak the primary in the 6-inch f/5 helps even the odds.
joni...@aol.com
> On Mar 16, 5:59 pm, "jonisa...@aol.com" <jonisa...@aol.com> wrote:
>
>
>
> > Steve:
>
> > I don't know about the other Asian DOBs but the GSO 10 inch was quite
> > well known for a pinched secondary. I bought one second hand several
> > years ago because the owner thought the optics were poor and I was
> > pretty sure they couldn't be all that bad. Well it had serious
> > astigmatism which I traced to a pinched diagonal and later discovered
> > this was quite common.
>
> > My concern with collimating fast newtonians is simply that the
> > tolerances are quite tight so making an F/4 Newtonian perform it's
> > best is no easy task.. As a general rule of thumb, the coma free
> > diameter goes about like this:
>
> > F/4= 1.1mm
> > F/5 = 2.1mm
> > F/6 = 3.7mm
> > F/8 = 8.8mm
>
> > jon
>
> The f/8 might have 4x the coma-free field, but the f/5 has almost
> twice the leverage in the adjustment, so the f/5 won't really be that
> much harder to adjust. Being able to see what is happening as you
> tweak the primary in the 6-inch f/5 helps even the odds.
>
In my view, One important factor for a beginners scope is how well it
performs when it is miscollimated because at first, it will be
miscollimated. This is where the F/8 focal ratio and its much larger
coma free field helps. The shorter focal length and seeing the image
are a help if one actually knows what they are doing...
Jon
Dennis Woos
> sitting on a low stool. This scope definitely performs better than
> most people expect; it yields startlingly good planetary images. No
> doubt it would be better still with a parabolic mirror, but at 4.5-
> inch f/8, being spherical does not disqualify a mirror from being
> diffraction-limited. I find the scope more pleasant to use at high
> power than the Starblast 4.5 simply because it's a lot easier to
> focus.
This is a common misunderstanding that my son and I have encountered. When
he was 12 years old he made his own 5" f/11.4 mirror, and because of the
size and focal ratio he didn't parabolize, and could use the Foucault test
as a null test and not test zones. Some folks say that it is a "spherical"
mirror because he didn't parabolize. For his mirror the difference between a
spheroid and a parabaloid is not testable/meaningful, at least with our test
tools. Talking about the shape of a mirror - spheroid/parabaloid - is not
meaningful and whether or not the optic was "parabolized" is beside the
point. What is meaningful is knowing how much of the light is going where it
should, and how the optic performs.
Dennis
joni...@aol.com
Dennis: With a 5 inch F/11.4 the difference between a sphere and a
parabola are sufficiently small the optic can be considered
"diffraction limited" and thus the light is going where it is supposed
to be. However with a 4.5 inch F/8, this is not the case, a perfect
spherical mirror is not sufficient, it does need to be parabolized to
reach it's potential.
Jon Isaacs
Dennis Woos
>performs when it is miscollimated because at first, it will be
>miscollimated. This is where the F/8 focal ratio and its much larger
>coma free field helps. The shorter focal length and seeing the image
>are a help if one actually knows what they are doing...
Our club has in the past few years been offering kits and classes building
6" f/8 dobs with very good purchased optics, and the results are great. The
scopes are light and portable, the collimation is relatively easy and
forgiving, and the views are extremely satisfying using moderately priced
eyepieces (e.g. Plossls). I think that a beginner cannot go wrong with a 6"
f/8 assuming that it fits transportation/storage requirements.
I spent some hours last night with our club's new 14" f/4.7 dob, and there
is no doubt that the faster optic is more sensitive to miscollimation and
that the coma is a problem, so much so that we are seriously considering
purchasing a Paracorr. We used my 10" f/6 as a comparison scope (with a
mirror made by the same guy who figured the new 14" - both excellent) and I
have to say that f/6 as compared to f/4.7 has a lot going for it.
So far as height goes, I don't see this as a big deal. We have made a couple
of Denver observing chairs, and these allow us to observe comfortably at
most any height. Getting comfortable allows one to observe better. I wonder
how many folks would be better off getting a chair as opposed to an
expensive eyepiece?
Dennis
Dennis Woos
>Dennis: With a 5 inch F/11.4 the difference between a sphere and a
>parabola are sufficiently small the optic can be considered
>"diffraction limited" and thus the light is going where it is supposed
>to be. However with a 4.5 inch F/8, this is not the case, a perfect
>spherical mirror is not sufficient, it does need to be parabolized to
>reach it's potential.
I think it depends on how one interprets the numbers and especially
"diffraction limited", which is not all that meaningful. However, I bet that
the difference is not, in this case, relevant. Of course, I could be wrong
and maybe I wouldn't like how the scope performs - I haven't observed with
one myself.
Dennis
joni...@aol.com
Dennis: I have a couple of pretty fast scopes, a 12.5 inch F/4.06 and
a 16 inch F/4.42 plus several in the F/5 range as well as a 12.5 inch
F/6. There is no doubt in my mind that optically F/6 is a much nicer
place to be. A Paracorr really does a great job of cleaning up the
views in an F/4-F/5 Newt, with the right eyepieces the edge is as
sharp as one could want. The other night I was watching Rigel drift
past the field stop at about 90x with the companion still a bright
pinpoint in the F/4.06 (=4.7 with the Paracorr)
As far as the 4.5 inch F/8, I have had a few of these, I am sure they
all had spherical mirrors, still they gave nice views. I am not a fan
of fast scopes for beginners, too touchy. Even collimated the
Starblast has a very small circle that is free from obvious coma and
eyepiece aberrations unless used with fancy eyepieces... Inside that
small circle, a properly collimated parabolic F/4 should be marginally
superior to the spherical F/8 but outside that circle the F/8 will be
much more pleasing and if the F/4 is miscollimated, the F/8 will be
better all over. So in that sense, spherical or not, the F/8 puts
more of the light over the entire field of view, where it belongs.
Jon
wsne...@hotmail.com
Both the f/5 and f/8 scopes are sensitive to miscollimation.
If the collimating screws have 1mm threads, then 1/5 to 1/4 turn of
one of the screws would be enough to bring the edge of the
"diffraction-limited" field of what had been a collimated f/5 scope
to the center of a 50 degree AFOV eyepiece working at 200x. It
would only take a little over 1/2 turn to do the same to an otherwise
similar f/8! A full turn of one of the screws on either scope would
put the "diffraction-limited" field completely out of the field of the
same eyepiece.
Of course, the fine adjustments in either scope are made using small
fractions of a turn, but at least with the 6-inch f/5 you can make the
adjustments while looking into the telescope. The instructions that
come with the StarBlast seem OK, and it is up to the newbie to read
and follow them, as it would be with any other newly purchased device.
The f/8 might be easier to collimate, it isn't that much easier, nor
the f/5 that much more difficult.
joni...@aol.com
This is how I look at it:
- An F/8 telescope is much easier to collimate and even if it is
seriously miscollimated the views will still be quite good.
- An F/5 telescope has much tighter tolerances on collimation and if
it is seriously miscollimated the views will not be good.
- Practically speaking, my experience is that most first time scope
owners do have trouble with collimation, if one can sidestep the need
to learn how to carefully collimate, it's a good thing.
Jon
Howard Lester
- Practically speaking, my experience is that most first time scope
owners do have trouble with collimation, if one can sidestep the need
to learn how to carefully collimate, it's a good thing.
-----------------------------------
When I was 10-1/2, my new Edmund 4-1/4" reflector came with collimation
instructions with generous drawings that were so simple to follow. Granted
the primary was f/10. Regardless, it proved even a kid could do it, or
easily get very close just eyeballing the setup in the focuser. We need more
Sam Browns. In the same vein, operating a German equatorial mount was also a
"piece of cake" (contrary to some postings I've seen here in the past), at
least thanks in part to the supplied Sam Brown instructions.
Howard
Brian Tung
> Dennis: With a 5 inch F/11.4 the difference between a sphere and a
> parabola are sufficiently small the optic can be considered
> "diffraction limited" and thus the light is going where it is supposed
> to be. However with a 4.5 inch F/8, this is not the case, a perfect
> spherical mirror is not sufficient, it does need to be parabolized to
> reach it's potential.
Oh, I don't know. Spherical aberration in a 4.5-inch f/8 really isn't
that bad at all. And it's not a binary thing, where once you get to a
certain f/ratio and aperture, the SA just vanishes. You're never
entirely free of it; it's just a question of which is more obvious: the
diffraction or the SA.
Disclaimer: I own, but no longer regularly use, my 4.5-inch f/8.
--
Brian Tung <br...@lunabase.org>
The Astronomy Corner moved to http://www.astronomycorner.net/
Unofficial C5+ Home Page at http://www.astronomycorner.net/c5plus/
The PleiadAtlas Home Page at http://www.astronomycorner.net/pleiadatlas/
My Own Personal FAQ (SAA) at http://www.astronomycorner.net/reference/faq.html
wsne...@hotmail.com
With the 6-inch f/5 most adults and a good many teenagers should be
able to look through the telescope and make adjustments to the primary
while doing so. They would see the results in real time.
With the 6-inch f/8 you must make an adjustment, then go back and look
in the focuser to see happened and then probably repeat this process
several, or perhaps many, times until the result seems good enough.
How is that easier??
> and even if it is
> seriously miscollimated the views will still be quite good.
A serious miscollimation will put the diffraction-limited field
outside of the field of a moderate to high power eyepiece, as I
mentioned earlier.
>
> - An F/5 telescope has much tighter tolerances on collimation and if
> it is seriously miscollimated the views will not be good.
>
> - Practically speaking, my experience is that most first time scope
> owners do have trouble with collimation, if one can sidestep the need
> to learn how to carefully collimate, it's a good thing.
Maybe a small f/15 refractor or Mak would be more appropriate...never
mind the added expense or narrow field of view.
Dennis Woos
>a 16 inch F/4.42 plus several in the F/5 range as well as a 12.5 inch
>F/6. There is no doubt in my mind that optically F/6 is a much nicer
>place to be. A Paracorr really does a great job of cleaning up the
>views in an F/4-F/5 Newt, with the right eyepieces the edge is as
>sharp as one could want. The other night I was watching Rigel drift
>past the field stop at about 90x with the companion still a bright
>pinpoint in the F/4.06 (=4.7 with the Paracorr)
Thanks for the info. Our club's new scope is a 14" f/4.7. So far, the only
eyepiece we have dedicated to this scope is a donated 14mm Meade UWA. This
eyepiece works very well, and stars are nice right to the edge. This
eyepiece has spent the past 8 months in my eyepiece case, and in my 10" f/6
it is great! A few club members want to try a Paracorr for the new 14", and
I am all for it. However, I have cautioned folks not to expect a Paracorr to
turn inexpensive (< $100) wide-field eyepieces into Naglers, as I think that
these eyepieces suffer from distortions other than coma (e.g. astigmatism).
I/we hope that a Paracorr will, for instance, clean up the view in a 35mm
Panoptic. Can you provide some info on how you use the Paracorr, and what
eyepieces work for you in your 12.5" and 16" reflectors? Thanks.
Dennis
joni...@aol.com
Dennis: This is my experience:
Coma is inherent in a parabolic mirror . To see coma most easily, it
takes widefield eyepieces like Naglers and the Meade UWA's that handle
fast light cones without adding their own aberrations. With such
eyepieces, one can see that stars are not perfect pin-points at the
edge. A good test is a double star and let it drift across the field
stop. Rigel is a good example, how sharp is that companion at the
edge vs. the center... With Naglers and the like, the Paracorr
effectively cleans up the coma so that the views really are "refractor
like", one can watch a relatively tight double drift past the field
stop still nicely separated.
As far as how I use the Paracorr, I put it the focuser and leave it
there all night. Whether I am using a low power widefield eyepiece or
viewing the planets at 200x-400x, the Paracorr provides an improved
view. At the lower powers, one is normally viewing big chunks of the
sky and the Paracorr + Naglers etc really makes for pleasing views.
At the high powers for planets and double stars it makes the sweet
spot much larger. Besides the F/4.06 and the F/4.42 I have 130mm, 8
inch and 10 inch F/5 Newtonians. While the coma is not such an issue
at F/5, the Paracorr does noticeably improve and takes it the last
step.
As far as the Paracorr and simple eyepieces, it does add about 15% to
the focal ratio so that it not only greatly reduces the coma but it
also helps the eyepiece aberrations. With simple eyepieces, the way
to make them perform like Naglers in a fast scope is to use them with
a barlow and a Paracorr. The barlow tricks them in to thinking they
are working in a slow scope... :)
This is really quite off topic, about all I can do to bring it back on
topic is say that Uncle Rod Mollise really loves his 4.5 inch F/4
Starblast, I recently sent him my 1.25 inch Paracorr to use with it...
Hopefully he will chime in here. (It did make a difference.)
Jon