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Are there any telescopes or lenses that don't shift focus

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19 replies to this topic

#1 joelin

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Posted 14 February 2018 - 03:36 AM

One of the annoyances is when you get a good focus early in the night and the cooling during the night throws your telescope out of focus for astrophotography.

 

Is there any scope or even camera lens that is immune to a focus shift due to temperature?



#2 bobzeq25

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Posted 14 February 2018 - 03:49 AM

The problem is that astrophotography requires extremely precise focus, nothing would be "good enough".  The problem is solved by motorized focusers with software that adjusts focus automatically.



#3 happylimpet

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Posted 14 February 2018 - 04:49 AM

Yup. Its an eternal problem.



#4 einarin

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Posted 14 February 2018 - 06:08 AM

I guess slower (F ratio) scopes are less immune.



#5 ChrisWhite

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Posted 14 February 2018 - 06:49 AM

Actually slower (higher f number) scopes are more immune. The issue is compounded with fast scopes.

If your scope can cool to ambient quickly, AND your temperature drop throughout the night is less severe than the problems are not quite as bad. My newt stabilizes much quicker than my refractor for example. Both suffer from focus shift though.

As mentioned above, motorized focuser with Automatic focus routine is the solution.
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#6 Jim Davis

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Posted 14 February 2018 - 07:06 AM

Listened to a talk from the man who built the camera for the New Horizons mission to Pluto. The camera had to stay in focus from launch, through the cold dark transit and then work perfectly upon arrival. The entire telescope was made from silicon carbide, since it doesn't thermally expand. May be a new area to research for ATMs. Not sure how you could grind the mirror, since the mirror would grind most everything else.


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#7 Benach

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Posted 14 February 2018 - 07:38 AM

One of the annoyances is when you get a good focus early in the night and the cooling during the night throws your telescope out of focus for astrophotography.

 

Is there any scope or even camera lens that is immune to a focus shift due to temperature?

Yup, speaking as a professional optomechanical engineer here. Yes, they exist but making a design athermal requires some extra engineering. Some things in random order that are within the realm of amateur telescopes:

  1. Making the mechanics athermal by putting the isothermal point on the optical axis
  2. Do not overconstrain the optics
  3. Use Zerodur for the mirrors
  4. Making the tube from a material that has a very low expansion coefficient such as invar or carbon fibre for reflective telescopes or a matched material for refractive/catadioptric systems.
  5. Making athermal optical designs such as Houghtons, Maksutovs
  6. Do a thermal analysis in your optical calculation software. Requires a few steps extra but certainly doable.
  7. Using a temperature calibrated focuser

etc. etc.

 

At my club we have a 300mm f/3.8 Takahashi Epsilon (one out of three in the world afaik) and it is not having any focus shift due to temperature and only some of the things mentioned above are used to achieve this.


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#8 einarin

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Posted 14 February 2018 - 07:40 AM

Actually slower (higher f number) scopes are more immune. The issue is compounded with fast scopes.

 

Can you elaborate this ?

My reasoning is that critical focus area in slow scope is much wider.

On the other hand slower scope has more material which expands/shortens.

But I think is not so significant.



#9 baron555

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Posted 14 February 2018 - 08:20 AM

One of the annoyances is when you get a good focus early in the night and the cooling during the night throws your telescope out of focus for astrophotography.

 

Is there any scope or even camera lens that is immune to a focus shift due to temperature?

Do you know WHY this is happening?    The actual physical phenomena involved?



#10 Alex McConahay

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Posted 14 February 2018 - 09:13 AM

>>>>>>> Do you know WHY this is happening?    The actual physical phenomena involved?

 

The physical parameter is known as coefficient of expansion. I am sure you know that things expand when they get warm and contract when cold. This is what makes asphalt streets crack, what makes a can of soda bulge when stuck in a freezer, etc. the coefficient of expansion is how much a material changes per degree of temperature change.  

 

The material in the tube of your telescope is not immune to expansion and contraction in the changing temperature. If you have 2.000 feet of warm metal between your primary optic and your sensor at the beginning of the night, that same tube will shrink to 1.997 feet of metal (or something) when it gets cold. Depending on how far the temperature drops and what material is in the tube that number will be different for different types of "metal" (or non metallic) tube.

 

As the distance from the optic to the sensor changes, the sensor moves out of the "critical focus zone."

 

Of course, the CFZ itself can change position, as the optics change. The optics, after all, may also contract and expand with the cold.

 

But it is the simple expansion and contraction of materials during the change of temperature which is at fault. 

 

Alex


Edited by Alex McConahay, 14 February 2018 - 09:15 AM.


#11 fbzolt

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Posted 14 February 2018 - 12:31 PM

For telescope with powered refractive elements, there is another factor that has to be looked at for an athermal solution.  The index of refraction of the material changes as a function of temperature.  In many cases, this is a greater factor than the expansion/contraction of the glasses and as such paying for a carbon fiber tube for a refractor is not necessarily going to get you a scope that behaves better athermally if the optics were not designed for such.

 

For all reflective systems, athermal performance is rather easy to get...just make sure your optics and metering structure have the same thermal expansion coefficient.  Once the optics stabilize at the new temperature, focus will hold.

 

Frank Z...



#12 joelin

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Posted 14 February 2018 - 09:04 PM

So are you suggesting to use a newt astrograph for best focus performance?

#13 RedLionNJ

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Posted 14 February 2018 - 09:21 PM

 

Actually slower (higher f number) scopes are more immune. The issue is compounded with fast scopes.

 

Can you elaborate this ?

My reasoning is that critical focus area in slow scope is much wider.

On the other hand slower scope has more material which expands/shortens.

But I think is not so significant.

 

Critical focus in slower scopes is indeed significantly wider. I suspect this may be an English interpretation issue.

 

More immune = less likely to show symptoms

Compounded = stronger symptoms


Edited by RedLionNJ, 14 February 2018 - 09:21 PM.


#14 schmeah

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Posted 14 February 2018 - 10:46 PM

I got lucky with my Tak FSQ 85. The tube/optics together have a low temperature coefficient of -9 steps (-18 microns)/degC with a fairly linear response, such that I can use only temperature compensation focusing using my Optec TCF-SI. It would be nice if this data were reported in the specs for OTAs.

 

Derek 



#15 joelin

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Posted 15 February 2018 - 01:17 AM

what other compensation is there besides temperature?



#16 Benach

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Posted 15 February 2018 - 02:23 AM

joelin: one can design optical systems that have no temperature dependancy. A simple example would be a Ritchey-Chretien with meniscus shaped mirrors. Since meniscus shaped mirrors do not change thickness over the aperture with a changing temperature, the mirror's axial expansion or contraction at least does not change the radius of curvature (a normal mirror does this).
Thus only radial expansion and contraction remains but this does not affect the focal length of the optical system. Therefore the system is athermal.

Before you start asking why aren't all mirrors meniscus shaped then, I'll give the answer: a meniscus shaped mirror is significantly more difficult to make and to test. One has to hog out two surfaces instead of one, creating axial supports for a meniscus shaped mirror is more expensive etc.

For other methods: read my earlier post in this topic.

#17 schmeah

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Posted 15 February 2018 - 06:30 AM

what other compensation is there besides temperature?

I meant I don't use FWHM or HFR based automated focusing, just temperature compensation focusing, microns/degC based on careful focuser training.

 

Derek



#18 Madratter

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Posted 15 February 2018 - 12:01 PM

I'm pretty sure that even if you used zero coefficient of expansion materials for EVERYTHING in the scope, you would still end up with some temperature shift just because of the air between the optics. As it changes temperature, its refractive index will change. Whether that effect is high enough to actually move the focus outside of the critical zone, I don't know.

 

In practice there are nights that are stable enough in temperature that I could go for hours at a time without refocusing. Other nights, the temperature varies way too much and I'm VERY glad that I have automated focusing.


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#19 baron555

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Posted 15 February 2018 - 12:22 PM

If you park your telescope in space, you may be able to keep it at a constant temperature.



#20 nmoushon

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Posted 16 February 2018 - 05:50 PM

Another way to look at it is to try and keep your setup from changing temperature as much as possible. There are some people that will insulate their scopes to decrease the change as much as possible. Happens more often with big closed tube SCT than other scopes though. Not saying this is a solution just pointing it out as another way.




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