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M16 with the Pillars of Creation

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#26 BlackSir

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Posted 27 October 2015 - 10:16 PM

Cool image, John!

 

Second, I am battling a strange phenomenon that I haven't quite figured out.  If I look at each of my 3-minute subs, the stars are pretty close to pinpoint round and yet if I rapidly scroll through my whole set of subs, the image walks across the sensor.  I only pause for about 2 seconds between the subs so I am completely baffled as to why I can have sharp star images and still have image walk.  It is exactly what you want for dithering…but I'm not doing it!  Whoa.  I've measured the walk and computed how out of round the stars should appear and (at least on the one data set I measured) it doesn't jive.  The stars should be slightly oblong.  I'm still scratching my head over what in the world could be causing the scope, the sensor, the optics, or anything else to shift a little between each two second frame.  I'll have to go back and look at the camera for this data set to see how much the set moved but I bet it did.

 

I'm puzzled with the exact same issue with my NexGuide. Have you figured out what was happening?



#27 jhayes_tucson

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Posted 27 October 2015 - 11:58 PM

BlackSir,

No, I never did figure that one out to my satisfaction.  I did go back and do a couple of things that might provide a clue.  First, I noticed that I was pretty much limited to 3-4 minute subs before a substantial number of them showed significant guiding errors.  Second, I went through my whole system and locked everything in place very tightly.  With mirror locks (which as I recall, I added to that scope a bit later) I could get the system to stabilize a bit better but never perfectly.  I always lost some of the data to image drift.  The drift was due to the following:

1) Poor mechanical rigidity in the mounting of the guide scope,

2) Mechanical flexure between the guide scope and the optical axis of the imaging scope, and 

3) Motion of the primary mirror in the C14 (due to mirror "flop".)

 

So I concluded that I probably had image drift that probably came in fits and starts so that some of my images were good and others terrible.  I was never able to get a good measurement of the problem so theat conclusion was mostly conjecture.

 

Even though I got some good results with the C14 using a guide scope, I could never fix the mirror flop issue and I eventually settled on guiding through the scope with an ONAG system.  That pretty much fixes (nearly) everything and makes really long exposures possible.  The NexGuide still works pretty well on my guide scope (which I still use with my HyperStar adapter) but it can't compare with the Lodestar X2 on my ONAG running PHD2.

 

So, my suggestions are to check all the mechanics and to consider getting a better system to guide through the scope.

 

John



#28 BlackSir

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Posted 28 October 2015 - 01:01 AM

I also struggling with getting my C8 to guide properly with off-axis NexGuide. I have some good results, but can't reproduce them consistently. Will keep trying  :)

 

I guess the reason for creepy but round stars (I do see them in all my images) might be scope mirror flop cased by camera mirror flip.



#29 jhayes_tucson

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Posted 28 October 2015 - 01:13 AM

The NexGuide actually works pretty well--if you have the newest version.  The "new" version has the Sony CMOS chip and the manual says "version 2" somewhere in small print.   I use it on a Vixen VMC90L which has a flip mirror.  I can find a star and put it right in the middle of the guider.  Then I adjust the intensity to get a good image using the +/- keys on the control pad.  Mirror flop could be a problem but you should be able to watch the back of the NexGuide to tell if it's guiding properly.  I was never able to get it to work with an AOG.  The display is just too crummy.  You might have more success with a better guide camera.  Let me know how it goes and I'll do my best to help you out.

 

John



#30 jhayes_tucson

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Posted 28 October 2015 - 01:13 AM

PS   This is a bit off topic so we might want to start a new thread or you can PM me.

 

John




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