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Does this look like off collimation, or mirror flop?

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#1 Ballyhoo

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Posted 15 July 2019 - 11:06 AM

Hello all,

 

Last night I wanted to do some planetary but just to see how the Meade 8" ACF would perform, I guided w Phd 120 sec subs.  I checked the collimation prior using Deneb, and I wished I would have kept a sub of that, but what it would have shown is a seemingly well rounded collimation star.  I have learned that can be deceptive, because I was using a 17mm Ethos, it likely was under-powered for a really accurate collimation.  In any case, is this sub tell tale of OOC?  Certainly there is a lot of moon-light but I am not thinking that is what causes the sub to be looking like this. I used a bahtinov mask and I am fairly certain that focus was good.

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#2 Don W

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Posted 15 July 2019 - 11:12 AM

I'm going to disagree that the focus was good. Seems a bit soft to me.



#3 cfosterstars

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Posted 15 July 2019 - 11:18 AM

Hello all,

 

Last night I wanted to do some planetary but just to see how the Meade 8" ACF would perform, I guided w Phd 120 sec subs.  I checked the collimation prior using Deneb, and I wished I would have kept a sub of that, but what it would have shown is a seemingly well rounded collimation star.  I have learned that can be deceptive, because I was using a 17mm Ethos, it likely was under-powered for a really accurate collimation.  In any case, is this sub tell tale of OOC?  Certainly there is a lot of moon-light but I am not thinking that is what causes the sub to be looking like this. I used a bahtinov mask and I am fairly certain that focus was good.

Unless you were near or crossing the meridian, it is not mirror flop. Did you use the mirror lock on the mirror focus? That should eliminate mirror flop basically completely. I agree with the Don. Your focus looks off also. Show your defocused star pattern so that we can look at the collumation. This dose not look like a collimation issue to me. It could be, but the defocused star method works reasonably well. It could also be camera tilt. Did you verify that your camera was tight and did not slip?



#4 Ishtim

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Posted 15 July 2019 - 11:19 AM

I'm going to disagree that the focus was good. Seems a bit soft to me.

+1  

 

Stars all seem to be elongated in the same direction, that could be caused by several (or a combo of many) things.   More info is certainly needed to provide more than a guess...


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#5 scadvice

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Posted 15 July 2019 - 12:05 PM

My guess is soft focus also. Something may have moved after you used the Bahtinov mask.   Long shot could be dew on the lens or sensor... though I don't think so.



#6 WadeH237

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Posted 15 July 2019 - 12:18 PM

How long was the exposure?  It looks like a combination of soft focus and poor tracking.

 

Regarding the Bahtinov mask, I'm going to go outside of conventional wisdom:  I don't care for them.  I have used them, and I agree that they can help you to get close.  But (except for the case of software analysis of the diffraction pattern) they are far too subjective for me.  It's not like they can give you a yes/no answer as to whether you are in focus or not.  As you get closer to focus, it becomes harder to see mis-focus in that pattern.  Without careful use, it is far too easy to believe that you are in focus, when you actually just close, but not in the critical focus zone.

 

Here's an experiment to try:  Use the Bahtinov mask and get the best focus you can.  Then, turn the fine focus knob an 8th of a turn.  Does the Bahtinov mask show you being out of focus?  Because at this point, you are probably out of focus (on most systems).

 

If you are imaging, the best way to get the best focus is by picking a star at the center of the field and doing a repeated subframe capture of it with exposures that are short enough to avoid saturating it.  Then get your imaging software to show you the FWHM of that star and let it take 4 or 5 exposures.  The FWHM will probably be changing a bit, and that's ok.  You are looking for the trend.  It also helps to zoom the subframe to at least 400% so that you can get a good look at the star and see its profile.  Then, turn the focus knob and let it take 4 or 5 more exposures.  You are looking at three things:  Did the zoomed image of the star get smaller or larger?  Did any faint stars appear that weren't there before (or did any faint stars that were there before disappear)?  And did the FWHM number trend get higher or lower?

 

You are looking to see the subject star get smaller and brighter (it may get bright enough that you need to lower the exposure time).  You are looking to see increasing numbers of faint stars in the field.  And you are looking to see the FWHM number decrease.

 

As you get closer to focus, you will be making smaller and smaller adjustments to focus on each iteration (kind of like collimation).  I find that about about a minute of this, I can get it to best focus.  At that point, I am making really tiny turns of the fine focus knob, like 1/32nd or 1/64th of a turn.

 

Also, be aware that you'll almost certainly pass the point of best focus (how else could you know that you find the best focus?)  When this happens, if your focus mechanism has backlash - and most do, especially SCTs - then you'll need to make note of the smallest FWHM trend that you did, back up and approach it again.  Make sure that you back up far enough to actually clear the backlash.  This will probably be a much bigger turn than you were making as you approached focus.

 

And finally, it really helps to approach best focus by turning the knob in the direction that works against gravity.  In the case of an SCT, this would be a counter-clockwise turn of the focus knob, which pushed the primary mirror forward in the tube.  For a focuser with a drawtube, just look at the focuser when you adjust it so see which direction lifts the camera up closer to the OTA.


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#7 Ballyhoo

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Posted 15 July 2019 - 01:38 PM

Here is another image, of M57. So, it is typical to lose focus on a Meade C8, ACF, slewing from focus star to object? In my case I went to Deneb to sync and focus, then I went to M 15.  This sub, a previous image from maybe an hour earlier, does not seem so OOF.   Okay and another way to look at it, if the Bahtinov mask is not reliable then, would you still expect to see the object look so very OOF?  I would expect a little soft, but not so bad. I am not the expert. No one mentioned collimation. Or did I miss that. 

 

Sub = 120 sec

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  • bad stars.jpg

Edited by Ballyhoo, 15 July 2019 - 01:39 PM.


#8 Ishtim

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Posted 15 July 2019 - 01:42 PM

Can you post the whole/uncropped image?



#9 Ed Wiley

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Posted 15 July 2019 - 01:47 PM

I might suggest some "Frame and Focus" before you commit to a 120 sec exposure. A well-focused F&F shows the maximum number of faint stars, pin-point, not fuzzy. Yours look rather fuzzy with some guiding error to me. Also, once focused well, try a bunch of 60 second subs and stack them, might help with the guiding. For F&F, I do mine bin 2x2 for 10 seconds, but you can use a longer exposure.

 

Just a thought,

Ed


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#10 cfosterstars

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Posted 15 July 2019 - 01:50 PM

Here is another image, of M57. So, it is typical to lose focus on a Meade C8, ACF, slewing from focus star to object? In my case I went to Deneb to sync and focus, then I went to M 15.  This sub, a previous image from maybe an hour earlier, does not seem so OOF.   Okay and another way to look at it, if the Bahtinov mask is not reliable then, would you still expect to see the object look so very OOF?  I would expect a little soft, but not so bad. I am not the expert. No one mentioned collimation. Or did I miss that. 

 

Sub = 120 sec

Do you tighten the mirror lock after focus before you move? This looks more like tracking errors than anything else. The stars are all elongated in one direction. You could see the same thing with collimation, but tracking is simpler. Collimation is usually quite stable on my Meade 10" ACF. Once I adjust it, it stays put for a long time and only if I am very rough with the scope. For focus, I went to a Starlight Instruments R/P focuser on the optics train and stopped using the mirror focuser completely. I use it only to set the focus roughly with my R/P at the center of its tube draw and then lock the mirror. 



#11 Ballyhoo

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Posted 15 July 2019 - 01:51 PM

Can you post the whole/uncropped image?

I can do that. Thank you

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#12 cfosterstars

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Posted 15 July 2019 - 01:56 PM

If it is collimation, you can also tell by looking at the image on both sides of perfect focus. You stars are more comet shaped. If it is collimation the shape will change on either side of perfect focus. You should be able to tell this by eye and you dont need any numbers to see. If this is columation, the defocused star will clearly show the central obstruction off center. Go to a brighter star and defocus. Collimation is easy to test on a SCT and the easy thing to rule out. For you stars to look like this for collimation error, it would need to be off significantly and it should be easy to see. Unless you bottomed out the collimation screws, it should be rock stable on a meade SCT.  



#13 WadeH237

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Posted 15 July 2019 - 02:04 PM

So, it is typical to lose focus on a Meade C8, ACF, slewing from focus star to object?

I am only a sample of 1, but my Meade 8 (not C8 - the C stands for Celestron) was an excellent scope and showed virtually no signs of mirror flop or collimation changes with scope position.

 

Well, there was this one time...

 

I did have one dark sky visit where nothing would cooperate.  I was getting 9 arc second stars with significant variability when I re-pointed the scope.  There were two root causes.  The first one was that the secondary mirror became loose from the corrector plate and was moving around.  This is kind of a "zebra" problem (ie. if you see hoof prints, think horses, not zebras).  Since I don't generally go around wiggling the secondary assembly to see if it's loose, it wasn't the first thing that I looked at (or second, or third...)  Once I discovered the problem, I removed the corrector plate so that I could tighten the secondary mirror holder (it's basically threaded so that two pieces are threaded together, sandwiching the corrector plate between them).  Fortunately, this is a pretty easy field repair.

 

So that resolved the variability problem.  The second issue is that it was just a poor imaging site.  It was in a campground in a valley below some nearby cliffs.  The campground hosts were great people, and put us out in an area separated from everyone else so that there was no direct lighting.  But the geography of the site was such that wind would roll off the nearby cliffs, resulting is terrible seeing.  Even after fixing the scope, I could not get better than 6 arc seconds FWHM.  I've not been back to that site...


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#14 Ballyhoo

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Posted 15 July 2019 - 02:05 PM

If it is collimation, you can also tell by looking at the image on both sides of perfect focus. You stars are more comet shaped. If it is collimation the shape will change on either side of perfect focus. You should be able to tell this by eye and you dont need any numbers to see. If this is columation, the defocused star will clearly show the central obstruction off center. Go to a brighter star and defocus. Collimation is easy to test on a SCT and the easy thing to rule out. For you stars to look like this for collimation error, it would need to be off significantly and it should be easy to see. Unless you bottomed out the collimation screws, it should be rock stable on a meade SCT.  yes

yes well for collimation I just used my ASI 294 and I decofused the star on both sides until I could make it look as good as possible. but I think it is more accurate under higher power which is not the method I used. 



#15 Ballyhoo

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Posted 15 July 2019 - 02:08 PM

I am only a sample of 1, but my Meade 8 (not C8 - the C stands for Celestron) was an excellent scope and showed virtually no signs of mirror flop or collimation changes with scope position.

 

Well, there was this one time...

 

I did have one dark sky visit where nothing would cooperate.  I was getting 9 arc second stars with significant variability when I re-pointed the scope.  There were two root causes.  The first one was that the secondary mirror became loose from the corrector plate and was moving around.  This is kind of a "zebra" problem (ie. if you see hoof prints, think horses, not zebras).  Since I don't generally go around wiggling the secondary assembly to see if it's loose, it wasn't the first thing that I looked at (or second, or third...)  Once I discovered the problem, I removed the corrector plate so that I could tighten the secondary mirror holder (it's basically threaded so that two pieces are threaded together, sandwiching the corrector plate between them).  Fortunately, this is a pretty easy field repair.

 

So that resolved the variability problem.  The second issue is that it was just a poor imaging site.  It was in a campground in a valley below some nearby cliffs.  The campground hosts were great people, and put us out in an area separated from everyone else so that there was no direct lighting.  But the geography of the site was such that wind would roll off the nearby cliffs, resulting is terrible seeing.  Even after fixing the scope, I could not get better than 6 arc seconds FWHM.  I've not been back to that site...

I thought the "C" is for Cat. So I have an M8? 



#16 WadeH237

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Posted 15 July 2019 - 02:17 PM

I thought the "C" is for Cat. So I have an M8? 

C8 is an actual Celestron model name.  They have lots of different models that use the same OTA, and it's changed appearance over the years, and not all of them are called "C8", but the community uses C8 as way to refer to any 8" Celestron SCT that is not an EdgeHD.

 

I don't know that Meade ever called any of their scopes M8, but I've certainly seen that term used by the community in the forum.  So if you said M8, we'd all know basically what you are talking about.  If you wanted to remove the remaining ambiguity, you'd need to add ACF and the focal ratio, since there are both F/8 and F/10 ACF scopes from Meade.



#17 Ballyhoo

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Posted 15 July 2019 - 02:32 PM

C8 is an actual Celestron model name.  They have lots of different models that use the same OTA, and it's changed appearance over the years, and not all of them are called "C8", but the community uses C8 as way to refer to any 8" Celestron SCT that is not an EdgeHD.

 

I don't know that Meade ever called any of their scopes M8, but I've certainly seen that term used by the community in the forum.  So if you said M8, we'd all know basically what you are talking about.  If you wanted to remove the remaining ambiguity, you'd need to add ACF and the focal ratio, since there are both F/8 and F/10 ACF scopes from Meade.

in any case, I understand I need to nail focus, but I wonder whether the issues I have are related to having my guidescope attached like a finder scope. It seems tight in, but I know many have said OAG is the way to go with  SCT's in general.  Perhaps I should guide this at 60 seconds until I get an OAG.  I am not sure what else I could do with PHd. I re-calibrated and I ran GA. Phd seemed to work fine. I think it was something on the scope side.



#18 cfosterstars

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Posted 15 July 2019 - 02:53 PM

in any case, I understand I need to nail focus, but I wonder whether the issues I have are related to having my guidescope attached like a finder scope. It seems tight in, but I know many have said OAG is the way to go with  SCT's in general.  Perhaps I should guide this at 60 seconds until I get an OAG.  I am not sure what else I could do with PHd. I re-calibrated and I ran GA. Phd seemed to work fine. I think it was something on the scope side.

Post your guiding graph. What is your guider pixel scale? If it is not mounted well, and is flopping around, then that could be your problem. I do not recommend using a finder show for mounting a guide scope especially for an SCT at long focal length.

 

Read these thread on OAGs and guidescopes for my Meade 10" ACF:

 

https://www.cloudyni...vs-guide-scope/

 

and 

 

https://www.cloudyni...e-scope-or-oag/

 

The point is, you can have an EXCELLENT looking PHD2 graph and have terrible imaging. A good guiding graph is a necessary but not sufficient condition for good imaging.


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#19 WadeH237

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Posted 15 July 2019 - 03:37 PM

in any case, I understand I need to nail focus, but I wonder whether the issues I have are related to having my guidescope attached like a finder scope. It seems tight in, but I know many have said OAG is the way to go with  SCT's in general.  Perhaps I should guide this at 60 seconds until I get an OAG.  I am not sure what else I could do with PHd. I re-calibrated and I ran GA. Phd seemed to work fine. I think it was something on the scope side.

Are you locking the mirror on your OTA?  In my experience, the mirror lock on the Meades are quite effective at preventing flop - more so than the Celestron EdgeHD locks.  If your guide scope is rigidly mounted, I would not expect to see that much flexure on a 2 minute exposure - but it's possible.  In particular, take a look at the focuser on the guide scope and how the guide camera is mounted to it.  If you wiggle the guide camera, and there is any movement of the drawtube inside the guide scope focuser, or movement of the camera in the focuser, then it's a problem (even if it's not the particular problem here).

 

As for determining flexure, it's pretty easy to identify.  If your guide logs show excellent guiding (where the definition of "excellent" depends on the image scale of your guide scope versus the image scale of your main scope), but the image in the main scope shows drift, then you have flexure  Actually, all of us have flexure to some degree or other.  The question is whether it's enough to be detectable in the main image or not.  Even an OAG will flex, but the scale will generally be far below the threshold of detection.

 

Now fixing flexure is a whole other beast.  Fixing flexure in a guide scope can be exceedingly frustrating.  That's why most of us switch to OAG (or ONAG) for guiding - especially with SCTs.



#20 Ballyhoo

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Posted 15 July 2019 - 04:01 PM

Are you locking the mirror on your OTA?  In my experience, the mirror lock on the Meades are quite effective at preventing flop - more so than the Celestron EdgeHD locks.  If your guide scope is rigidly mounted, I would not expect to see that much flexure on a 2 minute exposure - but it's possible.  In particular, take a look at the focuser on the guide scope and how the guide camera is mounted to it.  If you wiggle the guide camera, and there is any movement of the drawtube inside the guide scope focuser, or movement of the camera in the focuser, then it's a problem (even if it's not the particular problem here).

 

As for determining flexure, it's pretty easy to identify.  If your guide logs show excellent guiding (where the definition of "excellent" depends on the image scale of your guide scope versus the image scale of your main scope), but the image in the main scope shows drift, then you have flexure  Actually, all of us have flexure to some degree or other.  The question is whether it's enough to be detectable in the main image or not.  Even an OAG will flex, but the scale will generally be far below the threshold of detection.

 

Now fixing flexure is a whole other beast.  Fixing flexure in a guide scope can be exceedingly frustrating.  That's why most of us switch to OAG (or ONAG) for guiding - especially with SCTs.

How do you recommend I go about locking my mirror? 



#21 cfosterstars

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Posted 15 July 2019 - 04:16 PM

How do you recommend I go about locking my mirror? 

there is a knob on the back of the OTA labeled  "mirror lock" on my Meade 10" F/10 ACF OTA. You just turning it to tighten and lock the movement of the mirror. Just be sure to loosen it again before you try and focus with the mirror focus.

 

IMG 2670
 
You can see the mirror lock know on the back of my OTA directly above the mirror focuser. My mirror focuser has been upgraded with a feathertouch with motor.

 


Edited by cfosterstars, 15 July 2019 - 04:36 PM.


#22 AstroChampion

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Posted 15 July 2019 - 04:41 PM

I’m not the original poster but I noticed something with my Meade LX200 that I haven’t noticed before. Whenever I pass the focus point clockwise by the slightest turn, ( like 1/64th). As soon as I go counterclockwise the image shifts horribly out of place and I have to turn focus counterclockwise like 2 1/2 rotations, then when I start narrowing focus clockwise the image shifts back to center before focusing. Could something be loose? I read above someone mentioned backlash but I don’t remember it being this bad. Collimation looks good from my amateur eye...

#23 WadeH237

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Posted 15 July 2019 - 04:47 PM

How do you recommend I go about locking my mirror? 

After you get focus, turn the lock knob...



#24 cfosterstars

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Posted 15 July 2019 - 05:15 PM

I’m not the original poster but I noticed something with my Meade LX200 that I haven’t noticed before. Whenever I pass the focus point clockwise by the slightest turn, ( like 1/64th). As soon as I go counterclockwise the image shifts horribly out of place and I have to turn focus counterclockwise like 2 1/2 rotations, then when I start narrowing focus clockwise the image shifts back to center before focusing. Could something be loose? I read above someone mentioned backlash but I don’t remember it being this bad. Collimation looks good from my amateur eye...

Unless it is the latest F/8 OTAs they all suffer from severe backlash for the mirror focuser and a large degree of mirror shift. There is nothing that you can really do to fix this. I have thrown a lot of money at it and there is no good upgrade. The newer OTAs (F/8) have a completely different mirror cell focus mechanism that (from what I hear) makes this much better (I dont know if it fixes it). You a can try moving the mirror across the full focus range several times to redistribute the grease on the focuser. You can by the peterson engineering focuser upgrade. You can replace the focuser with a feather touch focuser and motor. I have tried all three with only very marginal improvement. 

 

As I stated above, I added a very expensive StarLight Instrument 3" R/P focuser to my optics train and that works, but it is not cheap. You can also buy cheaper focusers that might do as well for less.


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#25 Ballyhoo

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Posted 15 July 2019 - 05:42 PM

There is nothing on the back of my 8" Meade ACF other than a focusing knob. So does this mean that this OTA is only useful for planetary?

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  • SCT ACF.jpg

Edited by Ballyhoo, 15 July 2019 - 05:46 PM.



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