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ED doublets and cold...

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#1 Darren Drake

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Posted 24 February 2021 - 09:31 PM

I've noticed that several of Chinese ED doublets that perform  very nicely in warmer temps seems to have issues in colder  temps.  I believe that the optics are sufficiently cooled and acclimated but there seems to be a tendency to show significant undercorrection and some flaring possibly due to stress on the lens.  I suspect that the lens cell is contracting more than than the lenses and causing a pinching effect but I'm not sure of this.  These scopes are all from China but I have noticed similar effects on other apos some made in the US.  Is this a common effect for higher end apos as well?  Do most high end apos perform at the same high level regardless of temps?  Thanks



#2 KBHornblower

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Posted 24 February 2021 - 09:53 PM

If the cell material has a larger coefficient of expansion than the glass and not enough clearance is provided, the cell will pinch the glass if it gets cold enough.  For a typical doublet leaving it a bit loose at normal temperature is satisfactory.  If it is a triplet with much tighter centering tolerance, a bimetallic temperature compensating cell is needed.  That means $$$.



#3 junomike

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Posted 24 February 2021 - 10:56 PM

It's possible the OTA's are not fully acclimated.  It takes longer than most people realize.


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#4 gwlee

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Posted 24 February 2021 - 11:55 PM

I haven’t noticed a difference, but the temperature seldom goes lower than the mid twenties here, and Winter is short. 



#5 bobhen

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Posted 25 February 2021 - 08:06 AM

If cells are not made properly or the lenses are not assembled properly, as can be the case with less expensive refractors, the cold can cause pinching of the optics. You can see this in the defocused star test as astigmatism or an irregular shaped (not circular) defocused star.

 

Test in warm and cold conditions and compare.

 

Sometimes the fix is as easy as loosening a retaining ring that was over tightened during assembly.

 

High-end refractors usually do not have issues with cold temperatures, as the cells are properly designed and executed. Also, oil-spaced refractors have less complicated lens cells and do not suffer from pinching. 

 

Bob


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#6 peleuba

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Posted 25 February 2021 - 09:40 AM

If the cell material has a larger coefficient of expansion than the glass and not enough clearance is provided, the cell will pinch the glass if it gets cold enough.  For a typical doublet leaving it a bit loose at normal temperature is satisfactory.  If it is a triplet with much tighter centering tolerance, a bimetallic temperature compensating cell is needed.  That means $$$.

 

You bring up a good point...   Temperature compensating cells are a requirement in all but the most benign climates.

 

I want to expound on one point you mentioned - many are surprised to know that in ED (and Fluorite) doublets, centering can be more of an issue then in triplets.   The reason being is optical prescription - the doublets require steeper curves then a triplet to maintain correction.  Steeply curved lens surfaces have very small tolerance for lens centering and wedge.  This is what did in the larger Meade doublets of the 1990's.  These lenses could not maintain centering without tilt and wedge creeping in because the curves were steep, the cell was poorly designed, and the glass elements were too thin at the edge.


Edited by peleuba, 25 February 2021 - 09:54 AM.

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#7 Darren Drake

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Posted 02 March 2021 - 07:50 AM

I'd like to know if others have noticed optical issues with ED doubletsin the cold made in China.   I've noticed them in several scopes but so far no one here is confirming similar experiences..



#8 licho52

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Posted 02 March 2021 - 08:01 AM

No question about it, I noticed that most refractors from China arrive with retaining rings screwed on so tight they produce pinched optics and CA.



#9 alphatripleplus

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Posted 02 March 2021 - 09:53 AM

I have not noticed an issue with cold temps, but I haven't used my ED doublet in anything below the high 20s.



#10 ngc6352

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Posted 02 March 2021 - 01:43 PM

I haven't seen an issue with my SV80A (FPL53/lanthanum), but I'm not generally out at <20F.

My TV102 doesn't fit the poster's criterion of China-sourced.



#11 Zubenelgenubi17

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Posted 02 March 2021 - 06:07 PM

I have noticed no optical problems in my 1st gen Zenithstar 61 (which, afaik, is the same scope as the AT60ED, the Apertura 60mm, and the TS Optics Photoline 60mm). With the matching flattener, stars are good to the edges of my APS-C sensor.



#12 barbie

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Posted 02 March 2021 - 09:52 PM

I haven't noticed any problems with any of the ED refractors I've had when used down to 32 degrees F. Any colder than freezing, I don't observe and it's usually cloudy or snowing anyway.



#13 laedco58

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Posted 03 March 2021 - 05:58 PM

I haven't seen an issue with my SV80A (FPL53/lanthanum), but I'm not generally out at <20F.

My TV102 doesn't fit the poster's criterion of China-sourced.

I've observed at -20F with my SV80A, NP127, and FC100DL, with no optical issues. When it gets nippy like that, I set the scopes up about an hour before I plan to observe, so they have plenty of time to acclimate. I have had issues focusing though when the feeling and mobility go out of my fingers from the cold.



#14 russell23

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Posted 03 March 2021 - 06:12 PM

I think my SW120ED has this issue.  In the winter it often seems that stars seem to look like seagulls in really cold temps with the scope.  It is not much of an issue with my SV102 Access and I did not notice the issue with my Starwave 102ED last night.  


Edited by russell23, 03 March 2021 - 06:13 PM.


#15 Pelle

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Posted 04 March 2021 - 10:07 AM

I have done one star test in -15C with my new TS 102 (FPL53/Lant) and it showed good round stars at 285x, at least as far as the air let me see. I have also done a couple of astrophoto sessions in below -10C and the star images look fine with the small pixels of the ASI183.

This scope is advertised with a temperature compensated cell, which was one of the main selling points for me. I guess the Access and Starwave has the same cell so it looks like these are good in the cold.



#16 csrlice12

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Posted 04 March 2021 - 11:45 AM

The cold affects me before it affects the scope....but so far, no problems with the ED81S.


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#17 Magnus Ahrling

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Posted 04 March 2021 - 07:00 PM

No problem with my C80ED doublet at -19C.

I have had my 120 S-W Equinox ED doublet out at -5 with no problems.

 

Clear Skies,

Magnus



#18 duck2k

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Posted 04 March 2021 - 07:53 PM

I think the coldest I used my 72mm was at 32 degrees (below freezing) in CO. I let the scope acclimate first before using it. I noticed no changes in viewing. I have used it in 65 degrees and no changes.:)


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