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#1 Paradoxdb3

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Posted 29 September 2022 - 10:11 AM

I don't have a video or photo, but last night, under clear and calm skies (pretty good seeing) my SkyWatcher showed a perfect star, with a perfect airy disk, while my C11 showed a star that looked like one of those sparklers you see on a birthday cake. Stars ALWAYS look like this with my C11. What would cause this?

This is better suited for the Cats and Cassels forum. Feel free to delete here....



#2 mikef0924

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Posted 29 September 2022 - 10:23 AM

thermals and off collimation



#3 RedLionNJ

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Posted 29 September 2022 - 10:35 AM

The larger instrument likely takes a LOT longer to reach equilibrium, The patten you describe is pretty normal for a scope undergoing rapid cooling.  The volume of air & glass involved in the C11 is many times that of the smaller Mak.  You can reduce this time by active cooling or make the symptoms less visible by thermal wrapping.

 

[Saw your Moderator Action, btw - this topic is appropriate for here, also]



#4 dcaponeii

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Posted 29 September 2022 - 10:44 AM

Thermals and not collimated properly.  Oh wait, but your Poisson spot was perfectly centered...so I guess nevermind??  NOT

Note:  Just messing with you now.  Sorry not really trying to be an A$$ about it honestly, but I am connecting the dots between threads to try and make the point for you.  The C11 should be performing much better than it is and if it's not then time needs to be spent correcting the situation (IF it's correctable, and I call your attention to my two years of fighting with my 12" ACF as documentation that sometimes you just can't fix the scope.)



#5 Paradoxdb3

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Posted 29 September 2022 - 11:10 AM

Thermals and not collimated properly.  Oh wait, but your Poisson spot was perfectly centered...so I guess nevermind??  NOT

Note:  Just messing with you now.  Sorry not really trying to be an A$$ about it honestly, but I am connecting the dots between threads to try and make the point for you.  The C11 should be performing much better than it is and if it's not then time needs to be spent correcting the situation (IF it's correctable, and I call your attention to my two years of fighting with my 12" ACF as documentation that sometimes you just can't fix the scope.)

The scope certainly needed a TON of work. Stripped bolts shifting the front cell when fitting a dew shield on, causing out of collimation (what I thought at first was mirror flop). Took it all apart, replaced bolts, greased the primary baffle and focus threads. Recollimated and it held collimation like a cool Summer breeze. There's still a TINY bit of shift when slewing from one target to the next, but it used to go from almost perfect collimation to squashed donut in the matter of ten seconds of slewing (or putting in the dew shield).  My next suspicion is that the secondary is not centered with the primary baffle. Might take apart once again and see if shimming the corrector in a different orientation will help. Honestly, I don't know. All I know is that the BEST I can collimate this thing is with a tight defocus (almost at focus, but not quite). AT focus doesn't show me an airy disc. Do you think I shouldn't have tried scrubbing the primary mirror with a steel wool? (Okay, see I'm having fun too! Lol)

 

Anyway, I was able to get this image of the King planet with the C11 and I don't know how much better I could have hoped for, given my location and low planetary altitude. So if I could get THIS image once before, it suggests that I am either not getting as well collimated as I did here, or the seeing just isn't supporting the aperture. Or, other scope problems. Until I can test with a new unit, I don't know if I can rule anything out yet.

 

IMG_20220929_101018.jpg


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#6 Paradoxdb3

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Posted 29 September 2022 - 11:17 AM

The larger instrument likely takes a LOT longer to reach equilibrium, The patten you describe is pretty normal for a scope undergoing rapid cooling.  The volume of air & glass involved in the C11 is many times that of the smaller Mak.  You can reduce this time by active cooling or make the symptoms less visible by thermal wrapping.

 

[Saw your Moderator Action, btw - this topic is appropriate for here, also]

I feel like I'm a newb all over again. What's collimation? Lol. I'm second guessing myself, like a survivor of emotional abuse. I mean, I KNOW what good collimation looks like, and I feel I've got it there. Aside from being able to finish collimation AT focus, I'm able to get a perfectly tight Poisson spot inside one or two diffraction rings. And the scope cooled for over five hours. How much more would it need? Maybe an 11" isn't for me, being an apartment dweller with limited space. As for wrapping the scope, it works for four or five months of the year. The rest of Saskatchewan Winters are too much for an insulated scope. Maybe I SHOULD downsize.


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#7 dcaponeii

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Posted 29 September 2022 - 12:25 PM

I feel like I'm a newb all over again. What's collimation? Lol. I'm second guessing myself, like a survivor of emotional abuse. I mean, I KNOW what good collimation looks like, and I feel I've got it there. Aside from being able to finish collimation AT focus, I'm able to get a perfectly tight Poisson spot inside one or two diffraction rings. And the scope cooled for over five hours. How much more would it need? Maybe an 11" isn't for me, being an apartment dweller with limited space. As for wrapping the scope, it works for four or five months of the year. The rest of Saskatchewan Winters are too much for an insulated scope. Maybe I SHOULD downsize.

Show us the image of one of your star tests at or near focus and there are lots of folks on here that can help analyze it for you.

 

Part of the issue is semantics.  The Poisson Spot (and you had me looking things up to make sure I wasn't going insane!) is the spot formed by parallel rays of light passing a circular object (Like the secondary of your SCT).  In an SCT, that spot forms in the center of an out of focus diffraction pattern at the center of the shadow being cast by your secondary mirror (which is the center of the donut in an out of focus situation).  It has NOTHING to do with the diffraction pattern of an in-focus star.

 

The central bright region of an in-focus star is commonly called the Airy Disk or also referred to as the psf and/or centroid.  The Airy Disk is surrounded by one or more diffraction rings, the number of which depends on the wavelength of light being used and the seeing conditions (see the Pickering scale for examples).  When you refer to the Poisson Spot and 1st and 2nd diffraction rings your are scrambling up basic optics terminology leading this reader to suspect that you don't really understand collimation.  Given that even the image you showed most recently above is of average performance from your C11 at best, I think you can see why the issue is relevant.  Just trying to be helpful as many others on this site were to me over the past two years as I struggled to go from poor to average to good images.  Unfortunately excellent images still elude me.



#8 RedLionNJ

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Posted 29 September 2022 - 01:25 PM

Show us the image of one of your star tests at or near focus and there are lots of folks on here that can help analyze it for you.

 

Part of the issue is semantics.  The Poisson Spot (and you had me looking things up to make sure I wasn't going insane!) is the spot formed by parallel rays of light passing a circular object (Like the secondary of your SCT).  In an SCT, that spot forms in the center of an out of focus diffraction pattern at the center of the shadow being cast by your secondary mirror (which is the center of the donut in an out of focus situation).  It has NOTHING to do with the diffraction pattern of an in-focus star.

 

The central bright region of an in-focus star is commonly called the Airy Disk or also referred to as the psf and/or centroid.  The Airy Disk is surrounded by one or more diffraction rings, the number of which depends on the wavelength of light being used and the seeing conditions (see the Pickering scale for examples).  When you refer to the Poisson Spot and 1st and 2nd diffraction rings your are scrambling up basic optics terminology leading this reader to suspect that you don't really understand collimation.  Given that even the image you showed most recently above is of average performance from your C11 at best, I think you can see why the issue is relevant.  Just trying to be helpful as many others on this site were to me over the past two years as I struggled to go from poor to average to good images.  Unfortunately excellent images still elude me.

To get excellent images, you need everything to come together (seeing, optical quality, collimation, image scale, focus) all at the same time, only if even for a minute or so.  That's pretty rare, in my experience, from most of the world.



#9 dcaponeii

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Posted 29 September 2022 - 09:17 PM

To get excellent images, you need everything to come together (seeing, optical quality, collimation, image scale, focus) all at the same time, only if even for a minute or so. That's pretty rare, in my experience, from most of the world.


I would have agreed with you until this past weekend. Image looked awful in the preview screen but came out really well after processing.

#10 jgraham

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Posted 30 September 2022 - 08:39 AM

That's pretty much what I see with my C11: sparkly spikey stars (and sparkly spikey Jovian moons). If the spikiness is symmetrical then your collimation is probably pretty good. If the spikiness is more in one direction than another (often appearing as little sparkly comets) then the collimation is off along the axis of symmetry of the spikiness. If you move the focus slightly in/out just enough to see the shadow of the secondary you'll be able to see which direction needs to be tweaked. To smooth out the seeing I turn the gain way down and the exposure way up. In SharpCap I'll also do a bit of live stacking. In my case the spikiness is always seeing and the extent of spikiness is a good indication of what kind of imaging session I'm going to have.

 

The devil is always in the details. :)



#11 dcaponeii

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Posted 30 September 2022 - 08:58 AM

That's pretty much what I see with my C11: sparkly spikey stars (and sparkly spikey Jovian moons). If the spikiness is symmetrical then your collimation is probably pretty good. If the spikiness is more in one direction than another (often appearing as little sparkly comets) then the collimation is off along the axis of symmetry of the spikiness. If you move the focus slightly in/out just enough to see the shadow of the secondary you'll be able to see which direction needs to be tweaked. To smooth out the seeing I turn the gain way down and the exposure way up. In SharpCap I'll also do a bit of live stacking. In my case the spikiness is always seeing and the extent of spikiness is a good indication of what kind of imaging session I'm going to have.

The devil is always in the details. :)


In Firecapture the pre-processing averaging feature can help too. 5 frames or so.


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