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How to diagnose stars?

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#26 rgsalinger

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Posted 26 July 2019 - 03:22 PM

My bet is that you are dealing with differential flexing. Period. It is very difficult to do what you are trying to do --  guide with a guide scope and a Celestron SCT because the mirror locks are not actually locks, they just make the mirror harder to flop. That means that you've minimized flexure but still have it which is why they look pretty good but just a bit off (IMHO of course).

 

Start taking short exposures and lengthen the exposure until you see that the stars are misshapen. If you're collimation is good, then the stars will be perfectly round at 5 or 10 or 15 seconds. If  Do this after a careful focusing run, at the zenith and after the mirror has had time to cool down.

 

If you have differential flexure then the longer the exposures are the more eccentricity you will find. If you have an optical problem it will show up in short exposure. And, Stelios is correct - you've defocused the star to check collimation but that's a pretty crude test. 

 

If you download a copy of CCD Inspector, I think that there's a 30 day trial period you can get a nice quantitative reading about what's going on including the presence of tilt, collimation error, etc.

 

Rgrds-Ross



#27 JP50515

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Posted 26 July 2019 - 04:01 PM

My bet is that you are dealing with differential flexing. Period. It is very difficult to do what you are trying to do -- guide with a guide scope and a Celestron SCT because the mirror locks are not actually locks, they just make the mirror harder to flop. That means that you've minimized flexure but still have it which is why they look pretty good but just a bit off (IMHO of course).

Start taking short exposures and lengthen the exposure until you see that the stars are misshapen. If you're collimation is good, then the stars will be perfectly round at 5 or 10 or 15 seconds. If Do this after a careful focusing run, at the zenith and after the mirror has had time to cool down.

If you have differential flexure then the longer the exposures are the more eccentricity you will find. If you have an optical problem it will show up in short exposure. And, Stelios is correct - you've defocused the star to check collimation but that's a pretty crude test.

If you download a copy of CCD Inspector, I think that there's a 30 day trial period you can get a nice quantitative reading about what's going on including the presence of tilt, collimation error, etc.

Rgrds-Ross


Ross I don't think you read the thread entirely mate. ;) I stated that the stars are identical at 1s vs 5mins.

I already burned up a trial of ccdinspector on my Newt but will see if I can fanagle another copy

#28 JP50515

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Posted 26 July 2019 - 04:06 PM

You *cannot* collimate accurately when you are that much defocused. Plus you have a heat plume, and even at this amount of defocus the secondary shadow is slightly offset to the right.

Attempt a much tighter focus and look again. You may also want to try metaguide--once you figure out how to set the camera exposure properly, it will show very clearly the direction (and roughly the amount) of mis-collimation.


Just to clarify this image is severely cropped and scaled. In reality I was just to the point of being able to see the disk when I snapped this. How defocused is too defocused?

#29 Chuckwagon

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Posted 26 July 2019 - 04:24 PM

Just to clarify this image is severely cropped and scaled. In reality I was just to the point of being able to see the disk when I snapped this. How defocused is too defocused?

This will give you an idea of the various levels of focus to use to check collimation.

 

http://www.astrophoto.fr/collim.html

 

Though I don't think your issue is collimation, at least not entirely.


Edited by Chuckwagon, 26 July 2019 - 04:26 PM.

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#30 rgsalinger

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Posted 26 July 2019 - 06:49 PM

I'm very sorry that I misread your situation.

 

Even if I had, I would still recommend that you start by removing flexure from the possibilities from consideration by taking short exposures and analyzing them. I use a system that has this problem and some nights we get perfect and I mean perfect stars. Then the next night on the same target (at 10 minutes) it's just a mess. I just went through this on two consecutive imaging runs. And this is a permanent system with absolute encoders. You have to set up each night. I may well be wrong, but if you don't start by eliminating flexure then you can/will go down a lot of blind alleys before you find the cause.

 

BTB - what you say "the same" that's "measure the same" not just "look the same". It's very hard to misjudge things like eccentricity because you have to stretch things to see them and so what you're seeing can be quite different from what's been recorded. 

 

If you put some shorties in a drop box and share them with me, I'll measure them in CCDI and in Pixinsight and tell you if I'm right or if I have to eat another of my hats. Three so far this year.

 

Rgrds-Ross 



#31 Stelios

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Posted 26 July 2019 - 09:57 PM

As someone who's been through both stages of denial, I can tell you with some certainty that *nobody* wants a problem with star shape in an SCT to be collimation or differential flexure. And also that the problem often *is* collimation and/or differential flexure.

 

As Ross said, why not eliminate those issues? Waste an hour or so on a good seeing night collimating at increasingly closer focus (that will pay off regardless). And (if the problem or a fraction of it remains) get an inexpensive OAG and try guiding with it in a star-rich region (so you don't need to upgrade the ASI120 yet).

 

If the problem persists, at least you know to focus elsewhere. It's the classic debugging process. 


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#32 JP50515

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Posted 28 July 2019 - 05:41 PM

As someone who's been through both stages of denial, I can tell you with some certainty that *nobody* wants a problem with star shape in an SCT to be collimation or differential flexure. And also that the problem often *is* collimation and/or differential flexure.

 

As Ross said, why not eliminate those issues? Waste an hour or so on a good seeing night collimating at increasingly closer focus (that will pay off regardless). And (if the problem or a fraction of it remains) get an inexpensive OAG and try guiding with it in a star-rich region (so you don't need to upgrade the ASI120 yet).

 

If the problem persists, at least you know to focus elsewhere. It's the classic debugging process. 

I took this advice last night and really dialed in the collimation at the appropriate focus levels this time. Just to make sure I'm not off base...the collimation *should* appear to shift as you move the star out of the center of FOV, yeah?

I also tried a number of other things. 

1. Rotating entire imaging train - Stars still pull on image in same cardinal direction on the image, even though target was rotated 90 degrees.

I feel like that's a pretty big clue as to where the trouble lies. This means the issue is somewhere in my imaging train, right? 

2. I replaced my janky T2-T2 with a new one as I thought maybe the messed up old one was not letting the EFW and camera seat properly. 

 

No change with this. 



My next test is going to be to see if these issue replicate without the EFW, and if still there, then the reducer. 

Not entirely sure what my options are if it does happen to be caused by the reducer though...



#33 Stelios

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Posted 28 July 2019 - 05:55 PM

I have one more item to offer. 

 

Heat plumes.

 

I noticed in my own imaging yesterday, that I was having really weird-shaped stars (and inconsistent over the image). My first reaction was "I must have screwed up the collimation..." (I've been trucking the scope back-and-forth a lot to my dark site). But I was too tired, and I just went to bed and let it rip.

 

Later I reviewed the images, and saw to my surprise that I had about 1 hour of poor frames, followed by much better ones, ending in perfectly shaped ones. And it was not a bloated stars hiding the defects issues--HFR's went down as the eccentricity improved.

 

Based on my overall many years with SCT's, I suspect this is due to heat plumes. Temperature was dropping rather rapidly, going from 76 at start of imaging to 59 when things steadied. An hour earlier it was as high as 90. I remember how much that can affect images visually (including the defocused donut, which looks pinched on one side with a ray of light slicing through half of it), and I believe that what I was experiencing--and what *you* may be experiencing--is the photographic equivalent. 



#34 JP50515

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Posted 29 July 2019 - 09:40 AM

I have one more item to offer. 

 

Heat plumes.

 

I noticed in my own imaging yesterday, that I was having really weird-shaped stars (and inconsistent over the image). My first reaction was "I must have screwed up the collimation..." (I've been trucking the scope back-and-forth a lot to my dark site). But I was too tired, and I just went to bed and let it rip.

 

Later I reviewed the images, and saw to my surprise that I had about 1 hour of poor frames, followed by much better ones, ending in perfectly shaped ones. And it was not a bloated stars hiding the defects issues--HFR's went down as the eccentricity improved.

 

Based on my overall many years with SCT's, I suspect this is due to heat plumes. Temperature was dropping rather rapidly, going from 76 at start of imaging to 59 when things steadied. An hour earlier it was as high as 90. I remember how much that can affect images visually (including the defocused donut, which looks pinched on one side with a ray of light slicing through half of it), and I believe that what I was experiencing--and what *you* may be experiencing--is the photographic equivalent. 

This is an interesting analysis. I can totally comprehend this happening...however here's where I diverge regarding my setup. 

I'm stationed in a permanent obsy, so my scope is always acclimated to ambient temp when I start a run. Now this obviously doesn't account for dramatic temperature drops during the night, but I have seen this issue persist on both incredibly stable nights, and fluctuating ones. 

While checking my collimation the other night I noticed that I had chosen a good night for it as the telescope was not fighting any discernible hot air pockets or heat differentials. 


I really think the big hint for me was that the stars still stretched in the same cardinal direction on the image, even after rotating the entire train 90 degrees. 
I am beginning to wonder if my EFW may have just a slight camber to it or if the internal wheel is slightly tilted. 

My next step is going to be to pull the EFW off and see if I get the same problem with just the camera natively capturing images. If I do....the brand new focal reducer will get yanked and I will be looking at a completely naked + native rig....if I Still see the issue at that point...well then I'm stumped. 


I know you're big on the OAG concept here, but we're seeing this at 1sec exposures so I really can't attribute flex as the culprit. 


I do however wonder if the fan on the ASI may be a culprit here. Perhaps that will be my very first place to look...turning the cooler off. 



#35 rgsalinger

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Posted 29 July 2019 - 09:57 AM

When you move away from the center of a properly collimated SCT the stars will not look as good. If memory serves me they get fatter on the side away from the center. If that's what's happening then I wouldn't worry about it. It is necessary that the mirror be cooled to ambient, though or all bets are off. 

 

Here's the thing about rotating the image train. What you want to do is to see if the irregularity stays in the same direction (RA or DEC) with regards to the systems orientation to the sky. If it does then the problem is IN FRONT of the camera. If it does not then the problem is in what you rotated. (I remain mistrustful of just looking at subs as these are always (???) presented with the long side horizontal rather than oriented as DEC/RA. I hope I've got this right; I think that I do.) 

 

Apparently you are not now getting poor stars at short exposures? That's the acid test for both guiding and differential flexure problems. Collimation is a fine idea but that only rules out differential flexure if you now get round stars at short exposures and didn't get them before you collimated the scope. 

 

To check for tilt, just refocus in/out a bunch of times taking short exposures. At some point some stars must be in focus somewhere in the frame. Or you can just measure FWHM across the frame the way that CCDI does but by hand. If you find that there's a gradient of elongations then you know that you have tilt.

 

Rgrds-Ross 



#36 JP50515

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Posted 29 July 2019 - 10:15 AM

When you move away from the center of a properly collimated SCT the stars will not look as good. If memory serves me they get fatter on the side away from the center. If that's what's happening then I wouldn't worry about it. It is necessary that the mirror be cooled to ambient, though or all bets are off. 

 

Here's the thing about rotating the image train. What you want to do is to see if the irregularity stays in the same direction (RA or DEC) with regards to the systems orientation to the sky. If it does then the problem is IN FRONT of the camera. If it does not then the problem is in what you rotated. (I remain mistrustful of just looking at subs as these are always (???) presented with the long side horizontal rather than oriented as DEC/RA. I hope I've got this right; I think that I do.) 

 

Apparently you are not now getting poor stars at short exposures? That's the acid test for both guiding and differential flexure problems. Collimation is a fine idea but that only rules out differential flexure if you now get round stars at short exposures and didn't get them before you collimated the scope. 

 

To check for tilt, just refocus in/out a bunch of times taking short exposures. At some point some stars must be in focus somewhere in the frame. Or you can just measure FWHM across the frame the way that CCDI does but by hand. If you find that there's a gradient of elongations then you know that you have tilt.

 

Rgrds-Ross 

Hi Ross,

This is not my situation. I am still getting slightly streaking stars, the same at 1 sec or 5 mins. 

To summarize a bit better...I have "rotated the background sky" by rotating my entire imaging train as a unit, but the stars still streak NE to SW on the image if we say top of the image is N.  Some subs are a bit more N to S, but all stars in all subs are always streaking in the same direction. 

There was marginal change after collimation...stars were a bit smaller FWHM but still streaking. 


Edited by JP50515, 29 July 2019 - 10:16 AM.


#37 TxStars

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Posted 29 July 2019 - 10:52 AM

I would check if the issue is the same for all parts of the sky to completely rule out an issue in the imaging train.

If it changes then the issue is flexure somewhere in the system.

If it does not change check your guide logs to see if there is a lot of corrections in Dec as this could indicate a mount issue..


Edited by TxStars, 29 July 2019 - 10:55 AM.


#38 rgsalinger

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Posted 29 July 2019 - 01:32 PM

If you do not plate solve then all you know is that the stars are oblong. You don't know if the axes of the camera are parallel to the axes of the mount. Still if there are distorted stars across the frame with short exposures and you are using a reducer, remove it. Once you do that you will see if it's causing the problem. That's an easy next step. 

Rgds-Ross



#39 JP50515

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Posted 29 July 2019 - 01:54 PM

If you do not plate solve then all you know is that the stars are oblong. You don't know if the axes of the camera are parallel to the axes of the mount. Still if there are distorted stars across the frame with short exposures and you are using a reducer, remove it. Once you do that you will see if it's causing the problem. That's an easy next step. 

Rgds-Ross

yes, I do plate solve, to acquire location, but haven't solved them post acquisition. 

yes my next steps will be to turn off the camera cooler and test, pull the EFW off and test, then the reducer and test. 

If I make it to removing the reducer and that somehow solves the problem I'm not even sure how I'd go about correcting it. 
 


Edited by JP50515, 29 July 2019 - 01:54 PM.


#40 TxStars

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Posted 29 July 2019 - 04:14 PM

You do not have to remove the reducer to test it, just rotate the camera in relation to the reducer to test it...


Edited by TxStars, 29 July 2019 - 04:14 PM.


#41 JP50515

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Posted 29 July 2019 - 04:19 PM

You do not have to remove the reducer to test it, just rotate the camera in relation to the reducer to test it...

I did this. That's what I was describing a few posts back. 

Stars still stretch in the same CARDINAL direction on the image. Not necessarily in the same direction in relationship to the target. 


So if top of image is North:  At original orientation stars stretch NE to SW ---> Rotate camera 90 degrees clockwise ----> target rotates on screen, but stars still stretch NE to SW. 



#42 TxStars

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Posted 29 July 2019 - 10:19 PM

OK ..

Then I would check other areas of the sky..

Point at some stars to the N/E then N/W then S/E then S/W

Take a couple of short 1-5sec iages in  each area and see if the stars still look the same in each.

If they do then remove the reducer and repeat the images.

If the stars are good after this it is the reducer.

If they are not better it is the main scope.


Edited by TxStars, 29 July 2019 - 10:20 PM.


#43 JP50515

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Posted 30 July 2019 - 01:03 AM

SOLVED

 

 

I'm half tempted to hold a competition right now to see if anyone can guess what was causing my woes. Winner could have all my AP gear. 



Jokes aside...my stretching stars were cause by a build up of mosquito carcasses in the fan on my camera..............


Not even remotely kidding. Mosquitos....dead ones...causing the fan to vibrate. 

 


Edited by JP50515, 30 July 2019 - 01:07 AM.

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#44 TelescopeGreg

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Posted 30 July 2019 - 11:27 AM

Oh.

My.

Lord.

 

Well done!  You get the Astro Sherlock award for the month.

 

And do thank your camera for removing at least a few of the pests from the planet.


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#45 TxStars

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Posted 01 August 2019 - 01:29 AM

****..

In the days of film a buddy and I were out shooting and a firefly got into the dew shield of his schmidt camera.

When the film was developed he found that it had worked wonders on several exposures.

Needless to say it has become something that is checked for on every exposure since.




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