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Long Exposure Drift Alignment Problem


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#1 midcon07

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Posted 18 April 2008 - 12:26 PM

I have my mount drift aligned to the point that I took a 16min 36sec exposure (it was supposed to be 15 mins but I was watching the ballgame on TV and lost track of time). At any rate there was no star trailing.
The setup was a follows:
LXD75 Mount
Canon Rebel XTi w/ 70-300mm Tamron lens @ 300mm, f6.3 Piggyback on 8" SCT.

The problem is I can get no where near that long an exposure when I remove the lens, use a T Ring and T Ring adapter and mount the camera at prime focus.
Any ideas and/or suggestions as to what the problem may be and how to fix it?

Jerry

#2 PeterKennett

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Posted 18 April 2008 - 09:15 PM

There is a HUGE difference in resolution between 300mm and 2,000mm.  While your Polar Alignment was adequate at the low focal length of the lens, you must be highly accurate when you jump over 1,000.  2,000mm requires extreme accuracy in the Polar Alignment.  This is why most astrophotographers with an SCT use a focal reducer at prime focus, to bring the focal length down by half.

here's what you need to do.  Perform a DRIFT ALIGNMENT and repeat it until you so absolutely NO drift in DECLINATION for at least 10 minutes.  This will let you guide (I assume you are guiding somehow?) and make exposures up to 20 to 30 minutes. 

Every DECLINATION correction you make during guiding is an accumulative ERROR that will result in some amount of blurring (rotational trailing).  You can NOT correct for declination error.  Even if you make highly accurate corrections and keep a guidestar dead center, the corrections themselves are rotating your camera in respect to the stars in your field. 

I bet once you perform a drift alignment you will see lots of declination drifting going on.  I have a good tutorial on Drift Alignment on my website.  There's even a Meade Autostar simulator to let you practice on the computer.

Peter Kennett
www.PetesAstrophotography.com
Peter Kennett
Meade LXD75 SN8, Orion Guidescope, Olympus OM-1
Celestron 15x70 and SkyWindow.  MacOSX
Keep it simple, keep it cheap! Learn all about POLAR ALIGNMENT with my animated Drift Method Tutorial and simulator!!

#3 midcon07

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Posted 19 April 2008 - 09:14 AM

Peter, Thanks for your reply and you make an excellent point about the increase in focal length. However I have performed the drift alignment although I only let it run for 7 minutes on each star with no drift since all the literature I'd read always suggested just 5 minutes. I will run a 10 minute alignment and then run the test exposures again.
The mount is running unguided. I do have a DSI Pro I plan on using for guiding but I haven't progressed that far up the learning curve yet.
I'll check out your site for the tutorial just to double check I'm doing the alignment procedure correctly but I think I am.
Thanks again for your reply.

Jerry

#4 PeterKennett

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Posted 20 April 2008 - 04:16 AM

Ahh.. UNguided!  That is why your long exposures at 2000mm show trailing.  That would be EXPECTED.
Periodic Error (PE) alone will ruin an unguided long exposure at that focal length.  Now add in Atmospheric refraction and slight polar mis-alignment and you MUST guide, with manual (as I do) or Auto.

Peter
Peter Kennett
Meade LXD75 SN8, Orion Guidescope, Olympus OM-1
Celestron 15x70 and SkyWindow.  MacOSX
Keep it simple, keep it cheap! Learn all about POLAR ALIGNMENT with my animated Drift Method Tutorial and simulator!!

#5 midcon07

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Posted 20 April 2008 - 05:10 AM

So the error is always there, it just doesn't manifest itself at the lower focal length. That explains a lot.  Now to tackle the next hurtle which is guiding. Unforfuntaely I've had a back fusion so I don't think I'd be able to handle manual guiding which is what you do. I do have a 70mm, f11 refractor that I may be able to use for guiding. I'll check with the Deep Sky Imaging forum and see if it will do the job. Thanks so much for your help.

Jerry

#6 Mark Sibole

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Posted 20 April 2008 - 06:40 AM

You should be able to use PHD guide with no problems.
You will need a send imager for a guide camera and the ascom driver but it should work no problem.

Mark
Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#7 midcon07

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Posted 21 April 2008 - 09:16 AM

Mark, PHD guide program, ok, I'll track it down. From what I've read a lot of people seem to be using it. I also want to do some planentary imaging so I bought a Philips webcam yesterday, removed the lens, etc, and have it already to go. I've been reading that some people use these for guiding. Which would you recommend, my DSI Pro or the Philips webcam?
One term I don't understand is "send imager". Does that refer to a software program? I have Meade's Evisage, Starry NIght Astrophoto and Maxim DL Lite.

Thanks for your reply,
Jerry




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