Oil spaced vs. Air spaced refractors?
Nick Smith
I would like to buy a (true) APO refractor around 100-115mm =<f7.5
range for airplane portability with a view to do DS colour photography
later on. I want to narrow my choices down, so I need others'
expertise to do it. I have seen ads for oil-spaced & air spaced
refractors and I have read reviews that indicate there are v. good
versions of both.
=============================================================================
Here are the Q's:
Q1: What is the difference in image quality btw. the two in laymans
terms?
Q2: Is there a chance of oils spaced lenses leaking or going out of
collimation becaus of extreme temp ranges? (I want to take my
telescope to extremes of temp. To give you some idea, in Namibia it
regularly gets to 44 degs in summer and then temp drops to 5 or even
less at night in certian places. I have this nightmare image of oil
leaking out of the thing - probably not possible but I want to know)
Q3: Which collimation is easier to fix on if it goes out?
Q4: Which lasts longer? Oils change in structure over time - that much
I know and I rekon I'll keep it for around 10 years maybe more.
============================================================================
Right, that's it - any (objective (heehee!)) feedback & advice would
be very much appreciated appreciated.
All the best,
Nick
PS: Bit of a newbie so don't get too technical on me...
Chris1011
terms?>>
Shouldn't be any difference if the two are made to the same accuracy.
>>Q2: Is there a chance of oils spaced lenses leaking or going out of
collimation becaus of extreme temp ranges?..... I have this nightmare image of
oil leaking out of the thing - probably not possible but I want to know)>>
The oil space is only a few microns thick, so the oil is really just a thin
film. It cannot flow. Capillary action keeps it in place
>>Q3: Which collimation is easier to fix on if it goes out?>>
Depends on cell design. Generally, oil spaced lenses themselves do not lose
collimation. In the case of a push-pull cell design, it would be the cell that
gets collimated, but this is usually not required because high end refractors
are made in a much more rugged manner than other types of instruments. They
don't go out of collimation from the factory setting unless the users wishes to
re-collimate the original settings.
>>Q4: Which lasts longer? Oils change in structure over time - that much
I know and I rekon I'll keep it for around 10 years maybe more.
>>
Some oils change structure, but there are now special oils that do not. These
were developed for the space program for long term use in a vaccuum under
extremes of temperature, etc. Normal petroleum based oils would slowly
evaporate under those conditions, but these oils do not.
Roland Christen
Astro-Physics
Howie Glatter
> I would like to buy a (true) APO refractor . . I have
>seen ads for oil-spaced & air spaced refractors . .
Forgive me for not answering your primary question; I am not very
knowlegeable as to all the refractor choices available.
I'm posting because the term "oil spaced" in this context is one
of my pet peeves. I copy below a post I made 2 1/2 years ago on the
subject. By the way, this information may allay people's fears
of "leaking objectives"
Maybe I'm wrong. If I am, I hope Roland, Tom or Valery
will correct me :
From: howieglatterSubject: Re: Astro-Physics Lemon?
View: Complete Thread (39 articles)
Newsgroups: sci.astro.amateur
Date: 1999/10/21
Some posts to this thread have been using the term "oil spaced"
incorrectly, and creating the misconception of oil filling a
perceptible space between elements. When we say "air spaced objective"
we mean the elements are separated by a distance called for in the
optical design.
In Roland's objectives, the facing internal surfaces match, and they
are in contact, save for the capillary film of very pure oil between.
This oil is not intended to space the elements apart, but to act as an
optical coupling or index matcher to facilitate the transmission, and
prevent absorbtion , scattering or reflection of light rays. The lens
is more properly call an oiled objective.
Howie Glatter
William Hamblen
> I would like to buy a (true) APO refractor around 100-115mm =<f7.5
> range for airplane portability with a view to do DS colour photography
> later on. I want to narrow my choices down, so I need others'
> expertise to do it. I have seen ads for oil-spaced & air spaced
> refractors and I have read reviews that indicate there are v. good
> versions of both.
There's not any difference as far as image quality is
concerned. The oil spaced lens is one where the faces surfaces
have the same curvature and the space between is filled with
a thin layer of microscope oil. The reason for the oil is
to prevent reflections that lose light or cause ghosting.
In small lenses you can permanently cement the two surfaces
together but in big lenses differential thermal expansion
between lens elements won't let you use cement. The oil is
held in by capillary attraction and is slow to evaporate so
the lens stays good for a long time if not tampered with.
Air spaced lenses have air between the lens elements and small
spacers at the edges or a ring to hold the lenses apart.
The facing surfaces don't have to have the same curvature,
which allows the optician a little more freedom in his design.
What matters most is the skill in design and manufacture.
Dan Chaffee
>Some oils change structure, but there are now special oils that do not. T
Would microscope immersion oil change its structure as a function of
time?
DC
Chris1011
time?>>
I don't really know, but that is beside the point anyhow. Microscope immersion
oils are not used in my objectives.
Roland Christen
Jon Isaacs
>immersion
>oils are not used in my objectives.
>
>Roland Christen
What other companies make "oil spaced" refractors? I seems that maybe
Takahashi is one.
jon isaacs
Bill Tschumy
(in message <20030508131342...@mb-m07.aol.com>):
TEC is another. Their 140 refractor is oil-spaced.
--
Bill Tschumy
Otherwise -- Austin, TX
http://www.otherwise.com
Chris1011
Takahashi is one.>>
Not Tak, but TEC is the only other one I'm aware of.
Roland Christen
TMBack
>>Takahashi is one.
>>
>> jon isaacs
>
>TEC is another. Their 140 refractor is oil-spaced.
>
>--
>Bill Tschumy
>Otherwise -- Austin, TX
>http://www.otherwise.com
Takahashi's are all air spaced.
Zeiss APQ apo series (discontinued)
100mm/650mm
100mm/1000mm
130mm/1000mm
150mm/1200mm
Astro-Physics - All years and models, however
some models were cemented, and some air spaced.
TMB
80mm f/6
80mm f/7.5
4" to 20" are air spaced.
TEC
140mm f/7
203mm f/?
APOMAX
5.2" f/12
8" f/9
8" f/12
Aries
I think 4" to 7". Very few were made.
Thomas Back
Nick Smith
So far I was looking at these 5 scopes, first is oiled:
1) Fluoro-Star FLT 110mm F/6.5 SD William Optics refractor. 24"
length. 6.5kgs -
(this is the oiled one which prompted my queries - I was curious about
because it costs almost 2x other scopes, and I wondered what the price
meant in terms of optical quality) Price in London an 'astronomical'
£3,399.00!
2) Astro Physics Traveller 105mm F/5.8 4kgs. 19" length.
(this looked perfect with rave reviews, but not a realistic option -
unknown waiting list period so I can't tell when I would get it)
3) TMB Europe 115/805 f7 Vixen Tube. 24.6" carry length. 6.5kgs (upper
limit of portability) Price: £2695
4) TMB Europe 105/620 f6.2 Vixen Tube. Unknown length - possible
17.5"? This is carry length of heavy CNC version but I need to check.
4,9kgs (looks OK, price OK too, but lighter one may be available & not
sure of length yet) Price: £2095
5) 105/620 carbon-fibre tube APO triplet - sounds promising, but no
more info yet saw it in some search I just did and now I've lost it.
Price: ??
I still need to research a small goto (or with at least setting
circles) mount I can carry on a plane.
This is as far as I've got...
N
TMBack
>
>1) Fluoro-Star FLT 110mm F/6.5 SD William Optics
>refractor. 24" length. 6.5kgs - (this is the oiled one
>which prompted my queries - I was curious about
>because it costs almost 2x other scopes, and I
>wondered what the price meant in terms of optical
>quality) Price in London an 'astronomical' £3,399.00!
This is a TEC oiled lens.
Thomas Back
Dan Chaffee
>>>Would microscope immersion oil change its structure as a function of
>time?>>
>
>I don't really know, but that is beside the point anyhow.
Since microscope immersion oil is a close match for Bk7's index of
refraction, it seems quite relevant, given its common use in
achromats.
DC
Chris1011
refraction, it seems quite relevant, given its common use in
achromats.>>
Why would it be relevent if it's not being used? Yes, Immersion oils come in
all different index values, so can be used with all different glass material.
Nevertheless, these oils have high vapor pressure and would in time evaporate.
The oils we use have essentially zero vapor pressure (so low it cannot be
measured), and also have very good index matching properties with ED and BK7.
Roland Christen
Eric Fuller
Maybe something like DOT-5 brake fluid would work. It's a silicon based
long chain 'oil' that has a very low vapour pressure. Maybe diffusion
pump fluid would be good too. Or DOW-Corning vacuum grease or Apiezon
grease. The grease may be difficult to get a good film, though.
Eric.
Chris1011
long chain 'oil' that has a very low vapour pressure. Maybe diffusion
pump fluid would be good too>>
Silicon oil does have very low vapor pressure, but it's not anywhere low
enough. Another problem is that silicon oil creeps and eventually will get on
the coated surface. A short while after that happens, it will migrate under the
coating and into the glass. Eventually the coatings will just fall off, and the
glass can never be coated again. You would have to remove a depth of surface
glass by grinding and re-polishing and figuring to restore the lens to original
condition. Silicon oil and optical glass do not produce a good marriage.
Roland Christen
Dan McShane
"Chris1011" <chri...@aol.com> wrote in message
news:20030509154312...@mb-m23.aol.com...
Hi Roland,
I thought the oil negated the need for an AR coatings with those surfaces
(internal). Apparently not the case from what you state above?
Dan McShane
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.478 / Virus Database: 275 - Release Date: 5/6/2003
Alan French
"Dan McShane" <dgmo...@erols.com> wrote in message
news:b9h2p7$agp$1...@bob.news.rcn.net...
>
> I thought the oil negated the need for an AR coatings with those surfaces
> (internal). Apparently not the case from what you state above?
Internal surfaces do not need coatings. I believe the silicon oils can
migrate to the front and back coated surfaces, which are normally coated.
Clear skies, Alan
Chris1011
(internal). Apparently not the case from what you state above?
>>
The inside surfaces are oiled, not airspaced which would require coatings. The
outside surfaces are coated and the silicon oil will creep and attack them.
Roland Christen
Ratboy99
>glass can never be coated again.
So does that mean that a normal refractor lens **can** be recoated without much
trouble?
rat
~( );>
email: remove 'et' from .com(et) in above email address
Eric Fuller
Hadn't heard of that. Learn something new every day. So what do you use
?(without giving away any trade secrets) petroleum based, stuff i guess ?
Eric.
Jan Owen
--
To reply, remove the "z" if one appears in my address
"Eric Fuller" <erichal...@yahoo.ca> wrote in message
news:3ebc...@ccpop.capcollege.bc.ca...
> Chris1011 wrote:
SNIP!
> > Silicon oil and optical glass do not produce a good marriage.
> >
> > Roland Christen
>
> Hadn't heard of that. Learn something new every day. So what do you use
> ?(without giving away any trade secrets) petroleum based, stuff i guess
?
>
> Eric.
Some oils change structure, but there are now special oils that do not.
Dan Chaffee
>>>Since microscope immersion oil is a close match for Bk7's index of
>refraction, it seems quite relevant, given its common use in
>achromats.>>
>
>Why would it be relevent if it's not being used?
It IS being used---by myself and other ATMs. I am not completely
convinced it is better in perfomance than other common oils in
achromats, however.
Dan Chaffee
Nick Smith
> There's not any difference as far as image quality is
> concerned. The oil spaced lens is one where the faces surfaces
> have the same curvature and the space between is filled with
> a thin layer of microscope oil. The reason for the oil is
> to prevent reflections that lose light or cause ghosting.
<........>
> Air spaced lenses have air between the lens elements and small
> spacers at the edges or a ring to hold the lenses apart.
> The facing surfaces don't have to have the same curvature,
> which allows the optician a little more freedom in his design.
>
> What matters most is the skill in design and manufacture.
Hi William,
something just occurred to me as I look over the replies: If the oil
eliminates internal reflections, does this not mean higher total
transmissivity of the lens system (and therefore better contrast than
air spaces could provide) because of the greater difference between
the Refractive Index of each element?
Or do coatings offer the same total transmissivity air or not?
I have only recently begun reading about optics and profess no
expertise in this area, but the reports of the William Optics scope
are really good and I'm wondering if this gives it the edge.
Regards,
Nick
Quite an educational discussion I seem to have kicked off.. This group
is really good :o)
Chris1011
much trouble?>>
Yes, a normal refractor lens can be recoated, but with much trouble. The old
coating must be removed by agressive polishing (poly pad high speed process),
and then normal polishing on pitch, then carefully figured using very fine
polishing compound with pitch lap, and finally have the coatings applied in the
vacuum coater. This process is approximately 85% of the original cost of
manufacture of the lens element, less material cost.
Roland Christen
Llanzlan Klazmon the 15th
Maybe some sort of diffusion pump oil?
Llanzlan.