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Stars look ok?


Allinthehead

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Hi all. I picked up an Esprit 100 recently and i was hoping to get a second opinion on my stars. At first glance i felt they were good but zooming in shows them to be not quite round but slightly pinched looking. Have i something to worry about or am i being a little over critical?

This is a stack of roughly 30 minutes in poor hazy conditions.

Thanks for looking.

Richard.

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24 minutes ago, Han Solo said:

I can’t see much on my mobile, but Field flatterer to sensor spacing?

It might be a tiny bit out but that wouldn't explain the stars at the centre of the image i would have thought.

 

12 minutes ago, Davey-T said:

Stars aren't perfect, guess it depends how fussy you are :grin:

Dave

Very fussy unfortunately. When you say they're not perfect are you referring to the pinched look?

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I'm not an imager but I'm always curious to see what they do. Looking at the full-size image in the corners and at the center stars seem to be tight. Try shooting some very bright single stars of different magnitudes and colors, center and edge, that will reveal more. That is one of the tests Teleskop Austria's boss does to judge telescopes at prime focus, but the magnified patterns in a star test are a must for complete evaluation. 

http://interferometrie.blogspot.com/2015/11/120840-skywatcher-esprit.html

Edited by Ben the Ignorant
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6 minutes ago, Allinthehead said:

Very fussy unfortunately. When you say they're not perfect are you referring to the pinched look?

They look a bit squashed with centres offset to the right.

Seeking  perfection Richard ? that way madness lies, I gave up years ago :D

Dave

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14 minutes ago, Ben the Ignorant said:

I'm not an imager but I'm always curious to see what they do. Looking at the full-size image in the corners and at the center stars seem to be tight. Try shooting some very bright single stars of different magnitudes and colors, center and edge, that will reveal more. Tht is one of the tests Teleskop Austria's boss does to judge telescopes at prime focus, but the magnified patterns in a star test are a must for complete evaluation. 

Thanks. I'll try that and i agree the stars look nice and tight across the frame. I'm very happy about the sharpness of the scope.

 

12 minutes ago, Davey-T said:

They look a bit squashed with centres offset to the right.

Seeking  perfection Richard ? that way madness lies, I gave up years ago :D

Dave

Dave that ship sailed when i first decided to point a camera at the stars about 4 years ago, and in Ireland of all places. ?

Your observations are exactly what i can see. 

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Stars indeed don't look perfect to my eye. There is some level of distortion, but I can't really tell what it is - nor even describe it.

I would however do couple of things first - one would be to remove impacts of stacking - let's examine what individual frames look like? For example, I get the sense that there is some sort of "+" shape to the stars. This can happen in the stacking if some frames have stars elongated in X axis, while other frames have stars elongated in Y axis - which can happen for number of reasons - guiding issues, something not being tight in imaging train, etc ...

Also worth examining would be In/Out focus patterns (star testing with images - just shoot short subs on bright star with focus racked in and out).

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6 minutes ago, vlaiv said:

Stars indeed don't look perfect to my eye. There is some level of distortion, but I can't really tell what it is - nor even describe it.

I would however do couple of things first - one would be to remove impacts of stacking - let's examine what individual frames look like? For example, I get the sense that there is some sort of "+" shape to the stars. This can happen in the stacking if some frames have stars elongated in X axis, while other frames have stars elongated in Y axis - which can happen for number of reasons - guiding issues, something not being tight in imaging train, etc ...

Also worth examining would be In/Out focus patterns (star testing with images - just shoot short subs on bright star with focus racked in and out).

Hi Vlaiv thanks.

Single sub here.

Pinwheel_single.thumb.jpg.5ebc8daf526d30ccca6844eacda40168.jpg

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7 minutes ago, Allinthehead said:

Single sub here.

Well, it looks like it's not down to stacking, I think I see same issues on single sub.

Problem is that different stars seem to have different distortion, and it's not related to position in the field. Sure, top right corner shows "regular" type of distortion - that would be some tilt or something like that, but oddly shaped stars are just scattered around. Maybe something to do with color correction? This is OSC camera, have you tried mono/filters with this scope?

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2 minutes ago, vlaiv said:

This is OSC camera, have you tried mono/filters with this scope?

No not yet. Only managed about 1 hour of clear sky since i got it. I have a new Idas lps filter in the imaging train, i'll try again without that too. 

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Now this may be all down to the light pollution filter. I was just looking at subs taken with the Epsilon on the first light with the filter and it looks like a similar issue. The filter is loose in the holder and i can actually see it move, which might explain the random distortion.

 

new_neb_1.jpg.777a20dd6a06d2f67ec157f248a25548.jpg

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On 27/04/2019 at 15:54, Allinthehead said:

Now this may be all down to the light pollution filter. I was just looking at subs taken with the Epsilon on the first light with the filter and it looks like a similar issue. The filter is loose in the holder and i can actually see it move, which might explain the random distortion.

 

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That's interesting Richard. I also have an IDAS filter (D1). I noticed recently that mine too is loose in the cell. If I shake it I can feel it move. I'm not sure how long mine's been that way tbh. I just assumed that it wouldn't be an issue, but now you've got me wondering if I should inspect my own subs more closely. 

I can't get to a computer until this evening. But just looking on my phone, I think I can see what you're talking about. Slightly squashed and elongated to the right. But it's very subtle, to my eyes at least. 

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Hi Richard, this article might be worth looking at as it highlights some of the potential problems with star shapes that can arise in the esprit scopes: http://interferometrie.blogspot.com/2014/08/esprit-tuning-how-we-finetune-esprit80.html

I was going to hold off on my own Esprit experiences for now but since it seems relevant to this thread I'll continue... TLDR is, (especially if imaging in colder weather) it can be a delicate balance with these scopes between preserving element centering/collimation and preventing some pinching of the optics.

I bought a new Esprit 120ED last autumn and although perfectly collimated I noticed some funny star shapes & (asymmetric) diffraction spikes in images appearing only when the weather is colder (below about 4 or 5 C) suggesting mild pinching by one or more of the element centering screws (e.g. below).

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Seeking perfection (foolishly perhaps) I decided to very slightly loosen the centering screws for the front element (WARNING this is a high risk procedure!!!) The good news is that I managed to isolate the screw causing the problem and stop the uneven spikes regardless of temperature. The bad news is that I managed to very slightly de-centre the element which has made the star halo slightly asymmetric and resulted in a slight lateral colour shift. I can get good star shapes now if I combine subs from each side of the meridian flip as the distortions essentially cancel each other out (see below) but this is hardly ideal. Also aligning RGB channels is problematic.

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I figured I had two options from here, admit defeat and fork out to have the scope sent back to Es Reid or attempt more advanced adjustment techniques. Since there was no guarantee that pinching wouldn't result again in cold weather after the scope was recollimated  I figured it would be better (and more fun!) if I could attempt this myself at home as required. At a not inconsiderable expense I've managed to put together a double pass autocollimation setup with a beam-splitter, focuser-insertable artificial star and optical flat as described in the Teleskop Austria article above and the test setup seems to check out on my (well collimated non adjustable) Tak FC76. Next step is to try it out on the Esprit...

Paul

 

 

 

 

 

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1 hour ago, Allinthehead said:

Well i tried again without the filter and all seems good. FLO collecting and will inspect. Thanks for all the replies, much appreciated.

Great news Richard. Glad you got to the bottom of it, and good on FLO for sorting things out for you.

Did @FLO mention anything about any particular side effects of the cell being loose Richard?

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2 hours ago, Xiga said:

Great news Richard. Glad you got to the bottom of it, and good on FLO for sorting things out for you.

Did @FLO mention anything about any particular side effects of the cell being loose Richard?

Thanks I'll post a sub without the filter when i get a chance.

No but it certainly shouldn't rattle and it's so bad you can see it move, so focus would be affected all across the frame. 

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13 hours ago, Allinthehead said:

Thanks I'll post a sub without the filter when i get a chance.

No but it certainly shouldn't rattle and it's so bad you can see it move, so focus would be affected all across the frame. 

I would not have thought that the filter would have had that kind of effect. 

I have wanted an Esprit 100 for a long time! Would be interested to hear if the difference in thickness between your AD filters (3mm) and the ZWO filters (2mm) is causing you any issues with optimal spacing of the corrector. 

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6 hours ago, Adam J said:

I would not have thought that the filter would have had that kind of effect. 

Surely a tilted piece of glass in the optical train would cause issues? 

6 hours ago, Adam J said:

I have wanted an Esprit 100 for a long time! Would be interested to hear if the difference in thickness between your AD filters (3mm) and the ZWO filters (2mm) is causing you any issues with optimal spacing of the corrector. 

I'll let you know if i ever get a clear night again. It's been so long since i got out.

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10 minutes ago, Allinthehead said:

Looks better without the lps filter 

 

Got to be honest I am struggling to tell the difference beyond the better colour balance without the LP filter. 

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4 minutes ago, Adam J said:

Got to be honest I am struggling to tell the difference beyond the better colour balance without the LP filter. 

Really? Maybe wishful thinking on my part. 

Here's a 750 second stack. I think it looks better, maybe.

pinwheel_test.thumb.jpg.dec0d9f98dde26e5e514d65c183e69f2.jpg

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2 minutes ago, Allinthehead said:

Really? Maybe wishful thinking on my part. 

Here's a 750 second stack. I think it looks better, maybe.

 

Again all I am seeing in all the images is a flat spot on the star / halo at the 2o'clock position. Perhaps you were looking at something else in the first image. 

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1 hour ago, Adam J said:

Again all I am seeing in all the images is a flat spot on the star / halo at the 2o'clock position. Perhaps you were looking at something else in the first image. 

Ya i had another look and now it's clearly visible. i had convinced myself it was better. Question is, is it really a problem? I'll gather a decent amount of data and see how it looks, and maybe try with the 1600.

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