Alex Gorstan
member
Reged: 05/26/09
Posts: 25
Loc: Owings Mills, MD
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Hello All,
This was my first serious attempt at astrophotography. The clouds and humidity weren't cooperating at all so I only had time to shoot my light frames. However, I was getting very inconsistent results with tracking. I could shoot some 90 second exposure's with nice round stars, and then other times, I couldn't even shoot 30 seconds without trails. I'm worried a gear may be slipping or something else might be wrong with my CGEM. But then again, i'm a n00b so maybe this is par for the course.
Mount: CGEM
Scope: C925
Alignment: "2 star" with 3 calibration stars and then a "Polar Align"
Here are some zoomed out shots of my results:
30 Second Exposures:
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60 Second Exposures:
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90 Second Exposures:
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120 second exposure:
Thanks in advance, wise forum gurus! 
-Alex
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CGEM 925
2" EP Kit
12x50 Nikon ActionMaster
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s58y
Postmaster
Reged: 12/12/04
Posts: 5510
Loc: Eastern NY
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Are you autoguiding?
At least some of your subs look like they're coming out good, though.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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QueueCT
super member
Reged: 06/25/08
Posts: 100
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Alex--
I experienced the same thing with a CGEM during my first attempts. I found that I either had to limit my exposure times and throw out about half my subs or autoguide. I went for autoguiding and haven't had a problem since.
John
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Alex Gorstan
member
Reged: 05/26/09
Posts: 25
Loc: Owings Mills, MD
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Thanks Guys,
Yeah, I'm not autoguiding. Sounds like I may have to deal with this until I can get an autoguiding setup.
I'm going to give it one more effort soon and make sure everything is balanced as best as I can get it to see if that makes a difference.
I'm especially curious since this isn't in every frame, about 50% of the time. Any idea what causes this? Is this periodic error?
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CGEM 925
2" EP Kit
12x50 Nikon ActionMaster
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s58y
Postmaster
Reged: 12/12/04
Posts: 5510
Loc: Eastern NY
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Quote:
I'm especially curious since this isn't in every frame, about 50% of the time. Any idea what causes this? Is this periodic error?
How long is the worm period for your mount? It may be that there are 90-second periods during the worm cycle where the periodic error is pretty flat, and other 30-second periods where the error changes rapidly.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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Alex Gorstan
member
Reged: 05/26/09
Posts: 25
Loc: Owings Mills, MD
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The manual says it takes 8 minutes for the worm gear to make one revolution.
The scope also has periodic error correction. I could try setting that up, although i think it requires that I buy a reticle eyepiece to configure it.. could be cheaper and more painless to get an off-axis guider and cheap ccd for guiding.
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CGEM 925
2" EP Kit
12x50 Nikon ActionMaster
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John Miele
professor emeritus
Reged: 05/29/05
Posts: 659
Loc: North Alabama
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I noticed your streaks are not uniform. They have a bright spot at the start and end with a dimmer trail in between. Almost like you are tracking correctly, then jumping a bit and then tracking correctly again...not sure what that means though.  Have you tried balancing the mount so it is East heavy?
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C11/Atlas Mount/Denk II BV
SV102ED/AT Voyager
DSI IIc, DSI II Pro
ST-80, Mallincam Hyper Plus B/W Video Camera
Edited by John Miele (09/23/09 06:53 PM)
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drksky
super member
Reged: 09/01/09
Posts: 115
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I was going to say something right off about the dotted streaks. It looks like something that I experience with my CG-5 when it's not balanced so the worm is pushing against the spur. Essentially, it bounces, which looks like what's happening with the OP.
Considering the choice of targets, M-57, which is directly overhead this time of year, could be the mount is being balanced prior to M-57 crossing the meridian, and the mount isn't being flipped right away.
I'm new, though, so I could be totally BSing...
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Tony C
AT8RC
C6-R
CG-5 ASGT
ST80 w/StarShoot Autoguider
Canon EOS 450D (Stock)
The Drinking Bird
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s58y
Postmaster
Reged: 12/12/04
Posts: 5510
Loc: Eastern NY
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Quote:
The scope also has periodic error correction. I could try setting that up, although i think it requires that I buy a reticle eyepiece to configure it.. could be cheaper and more painless to get an off-axis guider and cheap ccd for guiding.
Perhaps the easiest way to set the PEC correction is with software like PEMPro. I don't know if PEMPro supports the CGEM. Also, you'd need to get an autoguider and software that PEMPro supports.
There may be other software packages that help set up PEC.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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KevinUK
Vendor (DSLR-AstroMod)
Reged: 08/22/07
Posts: 720
Loc: N 51'53 W 00'25
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Quote:
There may be other software packages that help set up PEC.
If you are using EQMOD ( and we all should be doing that!  ) it has a module called PECPrep which can read input from numerous of other packages (MaximDL, K3CCDTools ) but also PHD + Guidemaster which are both free. I have used it with PHD output to analyse my PEC.
It then generates a VS-PEC (variable speed) out put file which is loaded into EQMOD and can be used to correct the recorded PEC accurately on the fly synchronised with the worm gear positions.
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DSLR AstroMod
DSLR filter removal and replacement packages
http://www.dslrastromod.co.uk
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Tapio
super member
Reged: 09/24/06
Posts: 157
Loc: Tampere, Finland
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60 or 90s exposures are not so bad on this mount if it has periodic error about 20" to 60".
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3190921/Main/3184054
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Tapio
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