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Noob Collimation question - Celestron Evolution 8

astrophotography beginner Celestron collimation
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#1 rotoviper

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Posted 25 October 2020 - 02:04 PM

Howdy folks.   I'm a noob and have caught the astrophotography bug recently.   I have a Celestron Evo 8 and have been having some trouble with sharpening focus on all objects; terrestrial, stars, moon, clusters, planets, etc.   Some of the problem is clearly thermal and seeing issues with turbulence, but it can't explain why every single object is slightly out of focus.   At first, I suspected fat fingers on the manual focuser so I bought the Celestron focus motor for super precise and no-hands focus.   Didn't help.  

 

Next, I suspected collimation problems, so last night I locked in on a mag 2 or 3 star near zenith (forgot which star exactly).  I noticed that when I de-focused "in", I got a weird oblong or egg-ish shape on the "upper left" of the of the diffraction pattern, when I de-focused "out", the pattern looks spot on circular.   My understanding is that the diffraction pattern should be the same shape independent of which way I de-focus (see figure 9 in the celestron collimation guide).  

 

My rig was temperature stable as far as I know, and this problem reproduced using a 10mm eye piece as well as with my ZWO ASI224mc cam over the course of an hour.   I attached are a couple shots and here is an avi showing the problem.   Any ideas why I am seeing different shapes of diffraction based on focus direction?   I don't want to mess with the collimation screws if this isn't due to mirror alignment.

 

Thanks in advance for any ideas, insight, wisdom!



#2 rotoviper

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Posted 25 October 2020 - 02:06 PM

The png's didn't attach in my post, "focus in" png, "focus out" png



#3 wrnchhead

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Posted 25 October 2020 - 02:12 PM

Your collimating looks close, there are definitely tube currents going on. The pattern that switches from 2 o clock to 8 o clock looks like classic heat plume out of the baffle tube. The subject of SCT acclimation is full of opinions, and mine is that I prefer an insulated tube because generally that makes the scope immediately functional if the temperature delta between inside and outside is not huge.
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#4 wrnchhead

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Posted 25 October 2020 - 02:15 PM

Also, the amount of defocus shown the most in that video is too high for collimation, you want to get it down to the airy pattern (the rings) which will be much smaller.

#5 rotoviper

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Posted 25 October 2020 - 02:15 PM

Why do you suspect that it reproduces every time when I focus in vs out?  Shouldn't I see the same aberration independent of de-focus direction?



#6 fcathell

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Posted 25 October 2020 - 02:16 PM

I don't see any serious issues here, however, you need to get closer to focus so only a few diffraction rings show on either side of focus. The "focus out" would seem to indicate some vignetting on one side (flattened area), OR, the image of the star was not exactly centered.  There also may be a bit of residual heat inside the tube but is it minor. Get a pic of the in and out of focus pattern closer to focus and a more critical analysis can be made.

 

Frank


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#7 wrnchhead

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Posted 25 October 2020 - 02:17 PM

My personal belief is that it is thermal. More acclimation time would probably relieve the issue. Something else, the magnification you get from 2032mm of focal length is high, so your atmospheric seeing is probably also dominating your focused image quality.

#8 rotoviper

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Posted 25 October 2020 - 02:23 PM

Frank - check out 13s and 18s in the AVI I sent, shows both directions of focus and tighter rings.   What do you think?

 

Wrnchhead - I'm leaning this way, it is probably the simplest fix :-).   I'm in Seattle area and we just got our first freezing temps last night, and the temperature cruised from 49 to about 34 over about 2 hours when I was viewing, so the tube probably never settled.



#9 wrnchhead

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Posted 25 October 2020 - 02:31 PM

Same here in Kansas. Supposed to be in the 20s for the next three days, what?! Was 82 two days ago! (I’m moving to Seattle in 18 months)

Another thing you might do is get the tube outside way ahead of your viewing session to try to give it every advantage for acclamation before you start viewing.

Also like fcathell said, your collimation and thermals aren’t too bad, just a guess but when you do actually have to adjust something it will most likely be very minor. I’m fairly certain that (like everyone else) 85% of your deviation from perfect is due to the atmosphere, which is unfortunately out of our control. And when you are new, you don’t know where to point your finger when an issue comes up. In your case here, my money is on the seeing.

Edited by wrnchhead, 25 October 2020 - 02:31 PM.


#10 rotoviper

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Posted 25 October 2020 - 02:45 PM

Thanks!   I really appreciate the insight.     I'll get the rig outside earlier today to stabilize, once I can place it out of the sunlight (yes, there is sun, in Seattle!)


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#11 Migwan

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Posted 25 October 2020 - 03:57 PM

Your pics have to be 500kb or less.  You can reduce them in paint by percentage.

 

Looks like you have some thermals, but with a temperature drop like that, that's not surprising.  I'd have to see a star test with the scope not so near to zenith to call it a heat plume.   In your avi there is at times a point that is not centered.  Can't see it at other times.  The flattened side not being on both sides of focus is a little worrisome, but not a cause to panic.  Might be related to mirror shift, but not sure.

 

As others indicated, you should collimate / star test at much closer to focus.  Look to put the point in the center.  Better yet, on a night of good seeing, star test in focus at very high powers centering the airy disc in the first diffraction ring.  Something like the 7 seen HERE.   You might be able to do this with your 10mm, but if you can get hold of a 2x barlow that would even be better.   The higher the power, the easier it is to see the diffraction rings.  Also true by using less bright stars.  

 

Collimation is best done at around 60-75° as your targets won't all be near zenith.  Errai (mag 3.2) in Cephus would be a good candidate this time of year, but might be too bright with only the 10mm. 

 

Link     Link

 

 Good luck

 

jd


Edited by Migwan, 25 October 2020 - 03:57 PM.



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