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8SE image jitter doing AP

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#1 timm2021

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Posted 08 March 2021 - 02:09 PM

I am an AP beginner and spent last night photographing the Orion Nebula using a stock 8SE with a focus reducer lens (F/6.3) and a Sony A6300. I aligned with SkyPortal on Mars, Betelgeuse, and Rigel, did GOTO back to Betelgeuse and update alignment, GOTO to Alnitak, add alignment, and GOTO Orion Nebula and add alignment. I put the camera on a timer of 20 second exposures at ISO 400 starting an exposure at 25 second intervals with 50 such exposures. Over the span of this 50 x 25 seconds the nebula drifted a little under a third of the FOV, some times down and left and sometime down and right (visually). (I use UP and RIGHT to align following the instructions in Michael Swanson's book.)

 

I assume this amount of drift while far from perfect is pretty standard for this equipment. Doing visual observing I would probably not notice it.

 

The big problem is that while stars are round in less than half the shots, they are varying degrees of oblong in the rest. So finally my question is whether this is just the limits of my equipment. Because I used a timer I never touched anything, and there wasn't much wind. I set up on asphalt as I had vibration problems on the deck. Is there a variability to the tracking or can this mount just not do better?

 

Thanks. 



#2 coinboy1

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Posted 08 March 2021 - 02:21 PM

Yep 15-20sec exposure max for an 8” SCT with an alt-az SE mount. I would stick with 10-15 sec next time and try ISO 800-1600. With a higher end alt-az mount you can go longer like the LX200 or CPC mounts but still a max of 30 seconds until field rotation creeps in. Even higher end alt-az mounts like the Planewave L350 can go hours with a field derotator.

#3 Lasko

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Posted 08 March 2021 - 03:44 PM

That has been my experience as well, at 15 sec there are about 50% rejects.  At 10 seconds it is much better and 12 works at time.  I figure it is better to use 8s with almost no rejects rather than 12 with 10-20%.

 

As far as it drifting, that seems unavoidable and during a 20 or 30 minute capture I will nudge it back the right way if needed.  You will trash a few frames moving it but after that everything is fine.  Having the mount aligned as accurately as possible helps with the drift but you will still get some rotation.



#4 speedster

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Posted 08 March 2021 - 04:46 PM

Welcome!  Chips in the Sony A6000 series have least noise at 1600 ISO.  You can bump up the ISO and reduce exposure time.  Example 8SE, reducer, Sony A6000:

 

web M42sm.jpg



#5 Gamewarden

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Posted 08 March 2021 - 04:53 PM

Here is my take with the 6 se reducer and Zwo 294 uncooled camera.

 

 

Stack_26frames_156s_WithDisplayStretch Orion.jpg


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#6 timm2021

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Posted 08 March 2021 - 07:54 PM

Thanks to all. My first outing was an attempt on M37 where I bracketed 8, 15, and 30 seconds at ISO 1600, before getting the focal reducer. 30 had a pretty low yield, but 15 was better than last night, and the 8s got good scores in DSS. After reading about photons and sensors I was concerned that just boosting ISO was not getting enough light. Next clear night I'll go back to 1600 and maybe 8 seconds since I have picked up a stop with the focal reducer.



#7 rnyboy

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Posted 12 March 2021 - 10:39 AM

With my 6se and 385mc sensor I do a lot of stuff at 5s or 10s max exposure per frame at a high gain of 450 while dark correcting when livestacking in SharpCap.  When using a UHC or other narrowband filter the star bloat from the high gain gets reduced quite a bit.  The other night I was livestacking Thor's Helmet, Bode's galaxy, and Leo Quartet as well as a few other objects at ~f/3.3 (combo f/6.3 with cheap 0.5x reducer) and never dropped a frame with up to 45 min total exposures.  I've read that the se mounts make a tracking correction every 30 s so short exposures are best to eliminate the star distortions caused by too long of exposures per frame.

 

Also, not a big deal considering the imaging results people get, but the C8 is pushing the SE mount as far as weight capacity goes and one of the reasons I got a 6se, besides the lower price, was it left more head room for the weight of addons.   Total exposures are about 2x longer but you can do a heck of a lot with reducers and a smaller sensor.  For your A6300 the f/6.3 is your only real choice for filling the image frame and gaining about a 2.5x faster exposure with overall field flattening.  If you don't mind a smaller image and losing imaging capabilities in the outside region of your camera frame from the vignetting you could still use a 0.5x reducer and decrease exposure times even further, but with no designed in field flattening.  It's all a tradeoff in that you're just packing a fixed number of photons into a smaller area on the sensor for faster image acquisitions but trading off image resolution in the process.  Very few free lunches in astronomy unfortunately.  (Those cheap, ~$30, 0.5x reducers are pretty much the same piece of glass used to make the objective lenses in cheap binoculars only mounted in a 1.25" spacer tube .  A few people even make DIY 0.5x reducers from old binoculars.) 

 

With my 6se I've done quite a few 45 min total exposures for smaller DSOs when well centered and drifting is under control.  The field rotation is part of the deal when using alt/az mounts.  I very often can crop that out if the object is only filling up less than about 2/3 of the frame with very little or no drifting.

 

Maybe you already know this but for the bright stuff, like solar system objects or the Trapezium region in the Orion Nebula, video captures at 60 fps can work quite well for stacking and post processing in free software like Registax 6 and such.

 

One other thing... If your star distortions are along the altitude axis your scope may be unbalanced and continuously slowly drifting up or down while doing the image captures.  This could be happening in your case because you mentioned the drifting is right or left but always also down.


Edited by rnyboy, 12 March 2021 - 11:07 AM.


#8 rnyboy

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Posted 12 March 2021 - 01:57 PM

Hi Gamewarden.   Love that increased FOV.  I've been thinking on and off about moving up to a 294 because at f/6.3 it would give me about 50% more FOV than I can get at f/3.2 with my 385mc.  What conditions did you use for the capture as far as gain, total exposure, and sub exposure times go?


Edited by rnyboy, 12 March 2021 - 02:07 PM.


#9 Gamewarden

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Posted 13 March 2021 - 06:32 PM

Gain was about 300 at 8 second subs and for about 2 mins. I really don't see much improvement on anything over 2 mins. With my other camera the 224 I was struggling with finding objects and thinking I should be doing plate solves. With this camera the FOV is so large I have yet not had the object well within the FOV so given up with plate solving for now anyway. Smaller objects like planetary nebula etc do look small though with the 294 so I am experimenting with the ROI in Sharpcap. Really like this camera so far though. Right now I am getting 0.7 for my reduction on my Celestron reducer so going to try to get to 0.63 and see if it makes any difference.

It is the uncooled version.

 

Mark


Edited by Gamewarden, 13 March 2021 - 06:35 PM.



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