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Short Exposure AP on Orion Atlas

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#1 OneRedShoe

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Posted 16 September 2013 - 12:27 PM

I picked up a nice 9.25 SCT and needed a better mount than my CG5. There was a used Orion Atlas that had been hypertuned (I forgot the company that did the tuning) in the area, so I picked it up. It has been 6 months or so and I have only been able to use it 2 or 3 times do to weather and schedule conflicts. I want to be able to do astrophotograhy so I set the mount up over the weekend. My first step was to try out short exposure, unguided. With my C8, I could get nice sharp images for 30 to 60 seconds with no issues. With the Atlas and the 9.25 I am seeing large streaks. When I would expose with the c8 for too long the streaks would be uniform. With the Atlas I am seeing blobs. For short exposures it almost looks like double stars and with large exposures there are several blobs. It almost seems like the mount is sticking and not tracking smoothly. Below should be a 100% crop from a 20 second image. This was an attempt at M57, which was almost at zenith.

Am I crazy for trying to do short exposure at f10 on a C9.25 and expecting better results?

Should I be worried about the "blobs" in the streaks?

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#2 OneRedShoe

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Posted 16 September 2013 - 12:27 PM

I moved to M51 with was low on the horizon to see if things improved. This is a 100% crop on 50 second exposure.

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#3 orlyandico

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Posted 16 September 2013 - 03:30 PM

well if your polar alignment isn't perfect you will get star trailing. and then you'll get periodic error.

but if the C8 @ 2000mm was ok, I can't see how the C9.25 @ 2350mm would be significantly worse.

but that said i wouldn't expect unguided for longer than a minute. this is an atlas after all..

#4 Falcon-

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Posted 16 September 2013 - 05:53 PM

I wonder if your RA balance was either too perfect or slightly west-heavy. If that was the case the mount could be bouncing back and forth through the RA backlash as RA motor tries to keep up with the scope's RA motion. A very slightly East-heavy balance would solve that problem.

That would be a possible explanation for the blob portion of the string-of-blobs streaks you are showing. The streaking itself would likely just be a matter of polar alignment.

#5 OneRedShoe

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Posted 16 September 2013 - 08:02 PM

I tried to make sure it was spot on. I didn't realize it could be "too" perfect. So if I am standing behind the mount (where the eyepiece/camera is) the RA will be moving counter-clockwise, so I want the balance to be slightly heavier on the west side so the end that is moving "up" would be lighter?

Does that sound correct?

#6 orlyandico

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Posted 16 September 2013 - 08:11 PM

it should be going uphill rather than downhill.

hence if the scope is on the east side of the meridian, it has to be scope-heavy. and if it's on the west, it should be counterweight heavy.

#7 Phil Sherman

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Posted 16 September 2013 - 11:39 PM

It looks like you have at least two issues that need to be dealt with. The 50 second exposure shows at least four bright spots for each star. The trailed streaks in that image should be from polar misalignment but the "clumping" of photons into the bright spots is indicative of a short term, probably around 5-8 second, oscillation between two positions.

As recommended, you need to keep the mount a little heavy on the E side of the pier. If you release the RA clutch, whatever is on the E side should start sinking towards the ground.

Imaging with a scope that has a 2400mm focal length requires a very accurate polar alignment. When I align my Atlas using the polar scope, I can get only 20-30 second images with a 1000mm focal length scope. Anything longer requires a drift alignment.

A visual drift alignment can easily take an hour or longer to do. If you use your camera to do it, each alignment check takes only a little over a minute. To check your azimuth polar alignment; do the following:
1. Point the scope just W of the meridian at the celestial equator.
2. Set the slew rate to 1x sidereal.
3. set the camera for a 70 second exposure.
4. Have the mount tracking at sidereal rate.
5. Start the exposure and do the following for the indicated times
Time Action
00-05 normal tracking
05-35 slew East
35-70 slew West

Look at the image. The bright star(s) are from the tracked portion of the exposure. The trails are from the slewed portions of the image. If you're perfectly polar aligned, the return trail will pass through the center of the star. If not, the two trails will form a V. The size of the opening of the V at the tracked position of the star is a representation of the amount of drift that occurred. The relationship of the two trails indicates which way the mount needs to be adjusted to correct the misalignment. If your first adjustment makes the V wider, you moved the mount in the wrong direction.

Repeat this process around 30 degrees above the E or W horizon (still on the celestial equator) to drift check the altitude adjustment.

The worm period on the Atlas mount is over eight minutes. There should be little if any periodic error in a 20 second image and most of a set of 50 second images should not show periodic error.

Phil

#8 orlyandico

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Posted 17 September 2013 - 12:05 AM

i personally use pempro to do my drift alignments... very handy.

#9 OneRedShoe

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Posted 17 September 2013 - 01:04 PM

Thanks for the hints. It is beautful out now and is supposed to be a wonderful evening. I hope to give it another try tonight.

I have been trying to avoid brining a laptop into the field so I will try the camera method and see how it goes.

Thanks again for the help.

#10 OneRedShoe

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Posted 24 October 2013 - 01:01 PM

Weather and calendar conflicts have prevented me from making much progress, but I have tried again. This time I hooked up an ST80 piggyback on the C9.25 and used the NexGuide autoguider. This has helped to clean things up, but I just can not get a steady picture. Here are 2 examples.

These are both 1 minute and 25 second auto-guided exposures with a crop at 100% image resolution. The first one shows stars that are square and you can just make out individual points at each corner.

The second image has rounder stars, but then there are smaller doubles at right angles. I had the camera just about perpendicular to the axis of the Atlas. So "up" in the image should be RA and "right" should be DEC.

It appears that the mount is just jumping around in both axis. The RA was balanced East heavy and the DEC was balanced with no preference to either side.

I am interested in your thoughts as I would really like to get this working properly.

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#11 OneRedShoe

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Posted 24 October 2013 - 01:02 PM

Here is the image with the right angle doubles.

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