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Equipment Discussions >> Refractors

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buckeyestargazer
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Help me diagnose this - odd stars new
      #5635885 - 01/21/13 10:49 PM Attachment (73 downloads)

OK guys/gals, I need some help figuring out why my stars are suddenly looking strange. Equipment: APM115 CNC Classic apo refractor, TS flattener, Atik 383L camera.
Below is a crop showing the anomaly in a few of the stars. This effect shows up on all the stars in the full image. To me it looks like the stars are not focused, but I have focused the image using two different methods (bahtinov mask and autofocus) so the focus should be very good. It is not a filter issue because the stars look the same with different filters.

Anybody have a guess what is causing this? This only started recently. A few weeks ago I completed a CCD image and the stars were nice and round and didn't show this anomaly. The only thing that changed was the temperature; it's been much colder the last couple of weeks. Is it possible this is pinched optics due to cold? My equipment is in an observatory and nothing was changed between images.


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Scott in NCAdministrator
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5635913 - 01/21/13 11:07 PM

Joel, since the stars appear to have notches spaced 120 degrees apart, your scope may have collimation screws which are pressing slightly too tightly upon the edges of the lenses in the cold, leading to a "pinching" effect. I had this happen to a scope once, several years ago. Fortunately it was easily remedied.

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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: Scott in NC]
      #5635939 - 01/21/13 11:18 PM

Scott, thanks for the reply. I believe there are collimation screws, but I've never touched them and don't know what to do. What was your solution?

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Jon Isaacs
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636039 - 01/22/13 12:37 AM

Quote:

Scott, thanks for the reply. I believe there are collimation screws, but I've never touched them and don't know what to do. What was your solution?




Joel:

It's a triplet so I would not mess with the collimation. It's an APM so I would Email Markus and see what he has to say. It could be some sort of thermal pinching caused by cold weather.

Jon


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Itz marcus
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Re: Help me diagnose this - odd stars new [Re: Jon Isaacs]
      #5636046 - 01/22/13 12:49 AM

You could try using a dew strap. To keep it warmer ifthe anomoly goes away then at leat you know that the cold caused the pinching.
clear Skies
itz


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GlennLeDrew
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Re: Help me diagnose this - odd stars new [Re: Itz marcus]
      #5636089 - 01/22/13 01:19 AM

The tight localization of the 120 degree spaced artifacts on the brighter stars looks very much like the diffraction induced by foil spacers separating air-spaced objective elements. If pinching was in effect, stars should take on something more like a triangular aspect, but the fainter (not saturated) stars are fairly round, with perhaps a bit of astigmatism.

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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636409 - 01/22/13 08:50 AM

Quote:

Scott, thanks for the reply. I believe there are collimation screws, but I've never touched them and don't know what to do. What was your solution?




OK so I obviously don't know what I'm talking about ! I was in touch with Markus at APM, and he informed me that there are no such collimation screws on this APO. So...I guess I have to do the hard work of narrowing down the piece of equipment that is being affected by the cold (flattener, filters, ccd?).

Any other ideas would be appreciated. I've just never run into this before.


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tomcody
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636455 - 01/22/13 09:15 AM

Still could be from the cell itself contracting and pinching the optics. I like the suggestion of the dew strap to see if that prevents it. If the dew strap does not work, you may have found the lower lint of temp that you can use that scope in. A couple of years ago, when Europe was in that real bad cold spell, a astronomer (in Poland, if I remember correctly) posted that he had the same problems with his FSQ 106 and came to the conclusion that it just would not work below a certain temperature due to contraction of the cell.
Rex


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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: tomcody]
      #5636473 - 01/22/13 09:26 AM

Here's what I intend to do to eliminate components...if any of this looks dumb let me know!

1. Set the filter wheel to an open spot to eliminate filters
2. Turn up the dew strap to warm the objective
3. Remove the flattener/CCD camera and see how things look through an eyepiece
4. Replace the CCD camera (w/o flattener or filter wheel) and take a defocused star image (instructions per Markus Ludes at APM).

Any other ideas to try and eliminate problem components?


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GlennLeDrew
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636574 - 01/22/13 10:19 AM

The form of the defect, and that it is limited to the bright stars only, is most suggestive of diffraction. If pinching were occurring, *all* stars would be afflicted.

The reason diffractive effects do not show up fir the fainter stars is because there is insufficient light to bring out this subtle effect; only above some brigtness threshold will it appear. An optical defect which arises from a large scale deformation (area wise) must affect even the faint star images as much as the bright ones.


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tomcody
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Re: Help me diagnose this - odd stars new [Re: tomcody]
      #5636606 - 01/22/13 10:32 AM

FSQ cold link
Here is the posting I mentioned from the guy in Poland with a similar cold weather issue. (hope the link works, it is to the Yahoo unsensored Takahashi group)
Rex


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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: GlennLeDrew]
      #5636627 - 01/22/13 10:49 AM

Quote:

The form of the defect, and that it is limited to the bright stars only, is most suggestive of diffraction. If pinching were occurring, *all* stars would be afflicted.

The reason diffractive effects do not show up fir the fainter stars is because there is insufficient light to bring out this subtle effect; only above some brigtness threshold will it appear. An optical defect which arises from a large scale deformation (area wise) must affect even the faint star images as much as the bright ones.




To my eyes, in the full frame image the defect appears in all the stars just much more obviously in the brighter stars. In any event, what do you mean by "diffraction" and what could be the causes of such "diffraction"?


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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: tomcody]
      #5636630 - 01/22/13 10:50 AM

Quote:

FSQ cold link
Here is the posting I mentioned from the guy in Poland with a similar cold weather issue. (hope the link works, it is to the Yahoo unsensored Takahashi group)
Rex




Thanks for this. I've done business with the person from Poland before, and have sent him a private message.


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csrlice12
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636738 - 01/22/13 11:55 AM

Just fill in the missing spots with white-out, problem solved. Now, let's discuss eyepiece cleaning........

Agree with Glenn, have the mfg check the spacers. I'd check the collimation and everything first though. Get a Cheshire or laser and check to make sure the objective and the focuser are collimated first. Has the scope been dropped or the front Objective cell messed with or have you done any mods (i.e. new foucser)?


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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: csrlice12]
      #5636807 - 01/22/13 12:35 PM

Quote:

Just fill in the missing spots with white-out, problem solved. Now, let's discuss eyepiece cleaning........

Agree with Glenn, have the mfg check the spacers. I'd check the collimation and everything first though. Get a Cheshire or laser and check to make sure the objective and the focuser are collimated first. Has the scope been dropped or the front Objective cell messed with or have you done any mods (i.e. new foucser)?




The scope has not been dropped or mishandled or modded...or moved at all since the last successful image that did not show this star problem. I'm mounted in an OBS and the only change was a large drop in temp from a few weeks ago to recently.

I will eliminate the components I can and see if it is the objective/tube or not. Markus has been helpful...though I think he is skeptical it could be the telescope .


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Itz marcus
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5636822 - 01/22/13 12:42 PM

I had pinching in my EON 120 at low temps and the dew strap eliminated it.

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Darenwh
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Re: Help me diagnose this - odd stars new [Re: Itz marcus]
      #5636851 - 01/22/13 12:54 PM

I would agree that this looks like pinching at low temps by the cell or retaining ring. If you had an achro I would suggest relaxing the pressure slightly on the retaining ring to see what that does. As you have a triplet APO I will simply say 'Don't do that'. The colimation of those scopes are too delicate to take a chance that loosening the retaining ring a tiny bit might resolve the issue. It would be better to just try the dew strap turned up to warm the cell.

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buckeyestargazer
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5639461 - 01/23/13 07:30 PM

Well, I **think** that the issue has gone away by cranking up the dew heater strap. I captured a series of defocused stars to see the diffraction pattern and the results were telling (at least as far as I know!). When the scope was at the ambient temp, the defocused star showed three notches spaced 120 degrees apart. Then I turned on the dew strap to as high as it would go and let it warm for 30min, and those three notches disappeared in the resulting star images.

Here's a link to the star image comparisons: STAR TESTS

Markus Ludes said that indeed those notches are from the metal spacers in between the lens elements.

Unfortunately the clouds came in before I could do a high power eyepiece visual examination of the optics, or a long-exposure image. So next clear night I want to take some 10min test images and see what it shows. Of course, by the time I get some clear sky it might be the warmer weather of spring in which case this will be a non issue!


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GlennLeDrew
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Re: Help me diagnose this - odd stars new [Re: buckeyestargazer]
      #5639565 - 01/23/13 09:08 PM

Interesting! So instead of diffraction primarily, it's the cell contracting in the cold, squeezing the lens elements rather *forcefully*, to the point of warping the glass only at yhe soacers and thus inducing a concomitantly localized defect on the wavefront.

Relying primarily on your heater to alleviate this is not necessarily the best strategy. Why? Suppose this warping commences at, say, 0C. Your heater will raise the temperature of the cell/lens to perhaps +3C. Now, what happens when it gets down to -10C? The heater might bring stuff up to -7C, but you need to raise the temp up to zero. That much differential will cause thermals and thus degraded imagery.

Best to simply relax the retaining ring sufficiently so as to eliminate the squeezing. And if necessary, this could be done for the specific temperature regime. In other words, a certain degree of differing 'looseness' may need to be implemented for summer, spring/fall and winter seasons. That is, slight slackening for winter might just be too floppy/rattling loose for summer.


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buckeyestargazer
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Re: Help me diagnose this - odd stars [Re: GlennLeDrew]
      #5639764 - 01/23/13 11:26 PM

I'm not comfortable messing around with making any adjustments to the lens, so I'll just have to live with this. But really this only happened when the temps were unusually cold. I had been routinely imaging at 0c to -5c without a problem. The last few nights were -10 to -15c and the problem went away with the dew strap. There's only a few nights a year where the temps are that cold.

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