Which Way Is Out on a SCT Focus Knob?
W. Watson
attached to a long screw thet is on the back of the mirror. It would seem to
me that if one turns the screw CC that it would pull the mirror toward the
observer; thereby moving the focal point of the mirror inward. I tried this
with a 4" Meade SCT, and the focal point moves outward? Perhaps this depends
on where one things the head of a typical screw is in the device. I would
guess it essentially be where the focus knob is located. Can anyone clear
this up?
Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin
Web Page: <home.earthlink.net/~mtnviews>
Brian Tung
> My understanding of the focus knob found on SCTs (all?) is that it is
> attached to a long screw thet is on the back of the mirror. It would seem to
> me that if one turns the screw CC that it would pull the mirror toward the
> observer; thereby moving the focal point of the mirror inward. I tried this
> with a 4" Meade SCT, and the focal point moves outward? Perhaps this depends
> on where one things the head of a typical screw is in the device. I would
> guess it essentially be where the focus knob is located. Can anyone clear
> this up?
Don't know about the mechanics of the situation, but for the purposes
of star-testing, toward infinity is inside focus. This direction is
indicated on many SCT knobs.
--
Brian Tung <br...@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
RMOLLISE
W. Watson wrote:
> My understanding of the focus knob found on SCTs (all?) is that it is
> attached to a long screw thet is on the back of the mirror. It would seem to
> me that if one turns the screw CC
Clockwise...
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland>
The Annual SCT User Imaging Contest is Underway!
<http://www.rothritter.com/contest/2006/>
EdZ
The focal point of an SCT mirror is not constant. If you move the
mirror forward, it would not move the focal point out. Due to the fact
the SCT has a magnifying secondary, as you move the mirror further from
the secondary, the focal length increases. That explains why the focal
point moved out when you pulled the mirror further out.
If you turn the focus screw to pull the mirror out, the focal length
increases. If you turn the screw to push the mirror forward the focal
length decreases. Also, the focal length of an SCT is generally given
for the length when it focuses at the back end of the tube. If you
have a diagonal in your f/10 SCT, it is not operating at f/10, it's
longer.
edz
William Hamblen
<wolf_...@invalid.com> wrote:
>My understanding of the focus knob found on SCTs (all?) is that it is
>attached to a long screw thet is on the back of the mirror. It would seem to
>me that if one turns the screw CC that it would pull the mirror toward the
>observer; thereby moving the focal point of the mirror inward. I tried this
>with a 4" Meade SCT, and the focal point moves outward? Perhaps this depends
>on where one things the head of a typical screw is in the device. I would
>guess it essentially be where the focus knob is located. Can anyone clear
>this up?
The mirror is attached to a fitting that runs up and down the threaded
rod. When you turn the knob clockwise the mirror moves to the back of
the telescope and when you turn the knob counterclockwise the mirror
moves to the front of the telescope.
--
The night is just the shadow of the Earth.
W. Watson
between mfgers? Further what does out mean? the Mirror moves out or the
focus movies out?
W. Watson
> The focal point of an SCT mirror is not constant. If you move the
> mirror forward, it would not move the focal point out.
>
> increases. If you turn the screw to push the mirror forward the focal
> for the length when it focuses at the back end of the tube. If you
> have a diagonal in your f/10 SCT, it is not operating at f/10, it's
> longer.
>
> edz
>
mirror is moved upward without moving the eyepiece, then the eyepiece needs
to be moved upward to regain focus. If you complain that your head would
block the light hitting the mirror, do the same with a diangonal (as in a
Newtonian). You might need to increase the size of the diagonal, but you'd
have to move the eyepiece out towards you. A 6" SCT f/10 has a constant
focal length, right?. Same for a Newtonian. How would it change?
W. Watson
observed is that when the mirror is pulled back the focal point shifts
outward. I say pulled back, since CC, in my mind, equates to lowering the
mirror. When I move CC, then I need to pull the eyepiece further out.
Ah, I just found Ken Novak's Cass Notes. Let's see if he says anyting about
focus. It doesn't seem to be of much help. I think I have another book on it.
W. Watson
down, and the focal point would move into the eyepiece further. I observe
the opposite. However, if the screw thread is the reverse action of a
conventional screw, that would explain it. I'm not sure the screw analogy
completely explains this all. It would seem like a descending screw would
force the focus knob down too, which obviously dosn't happen. A drawing of
the innards would help. I have a 4" SCT that might get sacrifices. :-)
Chris L Peterson
wrote:
>EdZ wrote:
>> The focal point of an SCT mirror is not constant. If you move the
>> mirror forward, it would not move the focal point out.
>>
>> If you turn the focus screw to pull the mirror out, the focal length
>> increases. If you turn the screw to push the mirror forward the focal
>> length decreases. Also, the focal length of an SCT is given generally
>> for the length when it focuses at the back end of the tube. If you
>> have a diagonal in your f/10 SCT, it is not operating at f/10, it's
>> longer.
>>
>> edz
>>
>Why isn't ia constant? Think of it as a cone and no diagnoal. If the the
>mirror is moved upward without moving the eyepiece, then the eyepiece needs
>to be moved upward to regain focus. If you complain that your head would
>block the light hitting the mirror, do the same with a diangonal (as in a
>Newtonian). You might need to increase the size of the diagonal, but you'd
>have to move the eyepiece out towards you. A 6" SCT f/10 has a constant
>focal length, right?. Same for a Newtonian. How would it change?
Of course, the focal length of an SCT mirror (either of them) is
constant. But the focal length of the entire system is not constant with
changing focus; the focal point of the SCT varies non-linearly with
respect to the position of the primary mirror (because the focal length
is changing, too).
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
W. Watson
OK, I'll buy that. This leaves me to ask 1. Do all SCTs work the same way?
That is, If I drive the focus knob CC, will the focal point always move
towards the viewer's eye? 2. What are the construction details of the
focusing mechanism under the mirror (web site, book), and is the shape of
the bottom of the mirror other than flat?
William Hamblen
<wolf_...@invalid.com> wrote:
>It seems like if you go CC, then the screw would descend--taking the mirror
>down, and the focal point would move into the eyepiece further. I observe
>the opposite. However, if the screw thread is the reverse action of a
>conventional screw, that would explain it. I'm not sure the screw analogy
>completely explains this all. It would seem like a descending screw would
>force the focus knob down too, which obviously dosn't happen. A drawing of
>the innards would help. I have a 4" SCT that might get sacrifices. :-)
It's a right handed thread. The threaded rod is attached to the knob
and doesn't shift fore-and-aft. When you go clockwise (righty-tighty)
you are pulling the primary mirror toward you. When you go
counterclockwise (lefty-loosey) you are pushing the mirror away from
you.
The effective focal length of the telescope depends on the spacing
between the mirrors:
1/efl = 1/fs + 1/fp - d/fs*fp
efl = effective focal length of telescope
fs = focal length of secondary
fp = focal length of primary
d = spacing between the two.
If the mirrors are the algebraic sum of their focal lengths apart you
have a Galilean telescope with mirrors instead of lenses and the
effective focal length is infinity. As the mirrors get closer the
effective focal length becomes shorter. At some point the effective
focal length will be what they say in the manual. Supposedly on
Celestron C8s, the advertised focal length is available at the focal
plane of a 35 mm camera attached to a Celestron camera adapter and a
t-ring.
The offset push and pull of the threaded rod rocks the primary mirror
back and forth to produce the image shift common to
schmidt-cassegrainians.
Bud
Matthew Ota
My understanding is that when you turn the knob CW you are bringing the
mirror back towards you and the rear of the mirror cell. When you turn
it CCW to the stops you have moved the mirror to its furthest extent
away from you and the rear mirror cell.
Hope that this helps and gets off of the tangent discussion about focal
lengths.
Matthew Ota
Chris L Peterson
wrote:
>OK, I'll buy that. This leaves me to ask 1. Do all SCTs work the same way?
>That is, If I drive the focus knob CC, will the focal point always move
>towards the viewer's eye? 2. What are the construction details of the
>focusing mechanism under the mirror (web site, book), and is the shape of
>the bottom of the mirror other than flat?
AFAIK all the Meade and Celestron SCTs use similar mechanisms, with the
mirror riding on the center baffle and a normally threaded screw on the
mirror cell mated with a nut in the focuser knob. There may be
exceptions from other manufacturers; certainly there is no requirement
that an SCT have a moveable mirror for focusing at all.
W. Watson
> On Tue, 01 Aug 2006 01:38:58 GMT, "W. Watson" <wolf_...@invalid.com>
> wrote:
>
>
>>OK, I'll buy that. This leaves me to ask 1. Do all SCTs work the same way?
>>That is, If I drive the focus knob CC, will the focal point always move
>>towards the viewer's eye? 2. What are the construction details of the
>>focusing mechanism under the mirror (web site, book), and is the shape of
>>the bottom of the mirror other than flat?
>
>
> AFAIK all the Meade and Celestron SCTs use similar mechanisms, with the
> mirror riding on the center baffle and a normally threaded screw on the
> mirror cell mated with a nut in the focuser knob. There may be
> exceptions from other manufacturers; certainly there is no requirement
> that an SCT have a moveable mirror for focusing at all.
>
important is that the focus knob is where the real action is. The screw
comes down into it. The screw isn't being turned, it's a nut inside the knob
that is being turned, and not the head of a screw. I would guess then the
maximu travel of the mirror then is about the lnegth of the focus knob.
W. Watson
that's being turned. The screw is normal and is fixed. When one turns the
knob CCW, the nut is really going CC.
W. Watson
Thanks. Yes the focal length is a red herring. I just posted the following
to the fellow about you.
They key to my confusion seems to be that it's a nut that's being turned.
The screw is normal and is fixed. When one turns the knob CCW, the nut is
really going CC.
W. Watson
For those who like mnemonics, try this for the focus knob:
tIghten
N
focus moves in
lOosen
U
T
focus moves out
William Hamblen
<wolf_...@invalid.com> wrote:
>Thanks for the info. They key to my confusion seems to be that it's a nut
>that's being turned. The screw is normal and is fixed. When one turns the
>knob CCW, the nut is really going CC.
The "nut" doesn't turn. It is being pushed or pulled by the threaded
rod. Get a nut and bolt from the hardware drawer and play with them a
while. You will see what is going on.
William Hamblen
The primary mirror on schmidt-cassegrainian telescopes is a sphere.
Otherwise you wouldn't get an image at all. The primary miror is
fastened to a short, hollow cylinder that has an arm attached to it.
The short cylinder slides back and forth on a long, hollow cylinder,
the baffle tube you see when you look inside the front of the
telescope. A threaded rod turns inside the arm and pushes or pulls
the primary mirror back and forth.
The thing about focal lengths is that schmidt-cassegrainian telescopes
focus by changing the effective focal length. If the focal plane only
moved the same distance as the primary mirror, you wouldn't have
anywhere near the same range of focus that you can get from an SCT.
When the image is formed near the telescope the focal length is less
than when the image is formed farther away from the telescope. If you
look into an eyepiece placed in the visual back of the telescope the
magnification is different from when you look into the same eyepiece
placed into a star diagonal. The image is farther back when you use
the star diagonal than the image is when you are not using the star
diagonal.
William Hamblen
<c...@alumni.caltech.edu> wrote:
>AFAIK all the Meade and Celestron SCTs use similar mechanisms, with the
>mirror riding on the center baffle and a normally threaded screw on the
>mirror cell mated with a nut in the focuser knob. There may be
>exceptions from other manufacturers; certainly there is no requirement
>that an SCT have a moveable mirror for focusing at all.
Old, pre C8, Celestrons used an arrangement like a scissors jack for a
car to avoid the image shift problem.
You can buy a focuser to put on the back end of your SCT and lock the
primary mirror in place.
This web page has a cross section view of a C8:
http://www.celestron.com/c2/technology_view.php?TechnologyID=2&Style=printable
W. Watson
knob, you are turning the nut. The threaded end remains stationary.
Actually, the knob is the nut.
William Hamblen
<wolf_...@invalid.com> wrote:
>Got one in front of me. The nut is inside the focus knob. When you turn the
>knob, you are turning the nut. The threaded end remains stationary.
>Actually, the knob is the nut.
The knob is not the nut. The knob is the head of the bolt. The nut
is attached to the primary mirror support.
RMOLLISE
William Hamblen wrote:
>
> Old, pre C8, Celestrons used an arrangement like a scissors jack for a
> car to avoid the image shift problem.
>
> You can buy a focuser to put on the back end of your SCT and lock the
> primary mirror in place.
>
Hi William:
Actually, if I remember correctly, the old Celestron Pacific White
tubes used an arrangement that consisted of three spindles connected by
a belt.
Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
and
_The Urban Astronomer's Guide_
<http://skywatch.brainiac.com/astroland/index.htm>
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers:
<http://groups.yahoo.com/group/sct-user>
Chris L Peterson
<wrha...@comcast.net> wrote:
>The knob is not the nut. The knob is the head of the bolt. The nut
>is attached to the primary mirror support.
Not in Meade SCTs. The threaded rod hangs off the back of the mirror
cell. It is on a pin that lets it tilt, but it can't rotate. The nut is
in the hollow focus knob. You rotate the nut with the knob, pulling or
pushing the threaded rod into or out of the knob. That's the reason the
knob is so long. I'm pretty sure Celestrons do it the same way,
otherwise the OTA would have to be a couple of inches longer (unless you
have a hole in the mirror for the threaded rod when the mirror is all
the way back!)
Roger Hamlett
"William Hamblen" <wrha...@comcast.net> wrote in message
news:0ffuc2plrniuu6epb...@4ax.com...
> On Tue, 01 Aug 2006 11:01:40 GMT, "W. Watson"
> <wolf_...@invalid.com> wrote:
>
>>Got one in front of me. The nut is inside the focus knob. When you turn
>>the
>>knob, you are turning the nut. The threaded end remains stationary.
>>Actually, the knob is the nut.
>
> The knob is not the nut. The knob is the head of the bolt. The nut
> is attached to the primary mirror support.
The focus knob, is a tubular nut, supported normally by a couple of
bearings. There is a threaded shaft (the 'bolt'), which attaches to a pin
on the focusser arm. As you turn the knob (nut), the shaft screws into,
and out of this, pulling/pushing the focus arm, and moving the mirror.
This is why the knob is quite long, since the shaft moves in and out
through it as you turn the knob. On some models, you can actually pull off
the rubber sleeve, and watch the rod moving. Both Meade and Celestron,
have the _bolt_ attached to the focus arm.
Best Wishes
W. Watson
This link might help. <http://www.ngc1514.com/Celestron/disassem.htm>. I'm
satisified with my explanation and the info I've gleaned about the
mechanisms that make this all work, and will bow out of this discussion. I
have a Meade. It wouldn't surprise me if Meade and Celestron have a
different mechanism. Some day I may open it up and see for myself. My
curosity is satisfied, however. :-)
W. Watson
Roger Hamlett
"W. Watson" <wolf_...@invalid.com> wrote in message
news:kYLzg.8369$157....@newsread3.news.pas.earthlink.net...
_not_ at the focusser arm. As shown, if the arm was the nut, you would
have no more than half an inch of mirror travel, before the threaded rod,
would hit the mirror...
Notice in the disassembly diagram, the threaded rod being screwed out
through the knob, to expose the cirlip, so it can be unscrewed.
I'm sorry by your 'concept' of how the mechanism works, _is_ the wrong way
round, and the right operation, is nicely shown by these diagrams.
Best Wishes
wrha...@comcast.net
>I'm pretty sure Celestrons do it the same way,
>otherwise the OTA would have to be a couple of inches longer (unless you
>have a hole in the mirror for the threaded rod when the mirror is all
>the way back!)
There is a cross section of a C8 at
http://www.celestron.com/c2/technology_view.php?TechnologyID=2&Style=printable
It had been a while since I had the knob off my C8 to put on a
Motofocus and I'd forgotten exactly how it works. I just looked at my
C8 and the thread rod on mine moves fore and aft while the nut turns
around the rod. It is 39 turns of the knob from all the way in to all
the way out. The pitch of the thread rod looked like 30 to the inch,
so the total movement of the primary mirror would be only 1.3 inches.
Bud
W. Watson
> For the Celestron version of the innards, try
> <http://www.ngc1514.com/Celestron/disassem.htm>.
One other item. About the only difference between the Celestron and Meade
arrangement is the Meade has a hook at the top of the screw.
Willie R. Meghar
the SCT focusing mechanism:
Step 1: With eyepiece firmly seated in diagonal focus SCT on distant
object.
Step 2: Unlock eyepiece set screw. Pull eyepiece partially out of
diagonal. Lock eyepiece in new position.
Step 3: Refocus SCT, paying attention to which direction the focus
knob is turned.
If necessary, repeat experiment changing the direction the focus knob
is turned in step 3.
Upon the successful completion of this experiment simple logic ought
to suffice in arriving at the question's answer.
Willie R. Meghar