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#1 JonM

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Posted 09 May 2013 - 12:52 PM

My mount has started giving me stars that look like little Pac-men (see picture). This happens if I am guiding or not and with any telescope I use. This is an Atlas mount in an observatory (no wind), on a concrete pier with no vibration.

I don't see much if any backlash when I reverse slewing directions even though I have tried not guiding at all to see if guiding was the problem. Any ideas?

#2 wolfman_4_ever

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Posted 09 May 2013 - 06:05 PM

can't really see anything in the image.. it's to small to view.. need to post it somewhere else with the original size.

#3 WesC

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Posted 09 May 2013 - 07:14 PM

I can kinda see it... the round stars are cutout a bit at about the 10-o'clock position. but I am not sure why that would be. mis-collimation perhaps?

#4 rigel123

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Posted 09 May 2013 - 08:38 PM

How long has the scope been outside before you started to take images that showed this on your stars?

#5 JonM

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Posted 10 May 2013 - 02:02 PM

It's not a collimation issue. I get the same results with more than one telescope. You can see the cut out area of the star at the ten o clock position. I know the picture is not the best.

#6 JonM

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Posted 10 May 2013 - 02:03 PM

The scope resides outside in an observatory and is temperature equalized.

#7 rigel123

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Posted 10 May 2013 - 03:08 PM

How long does your exposure have to be before you see the stars showing this problem? It looks more like an optical issue rather than a mount issue. Tracking errors show up as oblong stars, double images, etc. This looks (from what I can see in the small image) like something in the optical train, such as a heat plume, something sticking into the light path, etc.

Edit: I just blew up your image and most of the stars look fine (I don't know how long this sub is) but the really bright ones look almost like there is a "negative" blooming with dark bands going out opposite sides of the stars. From this image I don't see what I would call tracking errors.

#8 JonM

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Posted 10 May 2013 - 03:43 PM

I tried very short exposures of 30 sec and long ones. No difference.

#9 JonM

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Posted 10 May 2013 - 03:58 PM

here's another shot of the kind of stars I am getting. I do use a flip mirror in front of my Camera but I can't imagine there is any cut off of the light path.

#10 Footbag

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Posted 10 May 2013 - 04:49 PM

Do your guidestars look like that?

If you get it in multiple telescopes, but it shows up in short or long, guided or unguided exposures, then you need to rule out everything else.

You need to remove the flip mirror and test. Then you may have to see if something is in the light path in your camera.

#11 Bill W.

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Posted 10 May 2013 - 06:45 PM

I would definitely put my money with your flip mirror if I had to bet. What kind of flip mirror is it?

-Bill

#12 Madratter

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Posted 10 May 2013 - 08:45 PM

Off axis guider intruding into the light path maybe? But yeah, definitely the flip mirror needs to be ruled out.

#13 achronut

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Posted 10 May 2013 - 08:58 PM

On the brightest stars I can see another cutout at 4 o'clock position. Looks like something is in the lightpath.

#14 JonM

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Posted 10 May 2013 - 09:08 PM

Guide stars look fine as far as I can tell using my Orion mini-guider. I will check the flip mirror out as I have a second one I can use. They are both Vixen flip mirrors but, I have used the one I have on there now and have not seen this problem in any of my shots earlier than about three months ago. Could this have anything to do with the shutter on my SBIG ST-8300C??

#15 Bill W.

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Posted 10 May 2013 - 09:37 PM

Take your camera off and operate the shutter. That will answer that question pretty quick... :)

-Bill

#16 Jesus Munoz

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Posted 10 May 2013 - 11:00 PM

Make one shot and examine it. Then rotate 90° your camera and check the result. If the problem persists in the same position then your mount is not guilty.

Remove any optical accesory and left only your scope and your camera (no off-axis guiders, reducers, filters, etc). If the problem persists then your camera is guilty, but if not then add one by one each of your optical accesories to find the intruder.

I hope you solve your problem.

#17 JonM

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Posted 13 May 2013 - 01:28 PM

Well, I tried some shots last night without the flip mirror and the stars looked much better, No clipping. I guess that was the problem but I can't imagine what the problem is with the flip mirror as my chip KAF8300 is not that big and the mirror appears to be lifting out of the way properly. I did not see any problem with the cameras shutter. Thanks for everyone's help with this.

Jon






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