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Advice regarding First Attempt


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Hi,

I have had a Celestron CG-5 Go-To mount for about ten years now but never attempted imaging with it. The mount has been boxed upstairs for a couple of years unused. Following some recent success imaging with an Alt-Az mount I set up the CG-5 mount last night and attached my SkyWatcher Startravel 102mm refractor and Canon 600D DSLR. I carried out a auto three star alignment, hand controller polar routine and a fresh auto three star alignment. I slewed to Vega and took a series of exposures ranging from 30 seconds to 120 seconds. Looking at the images on the pc I could see trailing in the images. I had hoped someone with experience could look at the following images and advise. 

120 seconds-

IMG_3524a.jpg

105 seconds-

IMG_3523a.jpg

90 seconds-

IMG_3522a.jpg

75 seconds-

IMG_3521a.jpg

60 seconds-

IMG_3520a.jpg

45 seconds-

IMG_3519a.jpg

30 seconds-

IMG_3518a.jpg

Really hoping someone can assist me here.

Cheers,
Steve

 

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The 30 second exposure looks pretty good but there is coma increasing towards the edges of the field, which is what I'd expect to see with a Startravel 102 as it's not really an imaging scope. With a little cropping I think you'll get some decent results if you point at it towards some open or globular clusters, or a few of the brightest nebulae (M42, M27, M57).

Getting exposures over a minute at 500mm focal length is always going to be tricky without guiding, I'm not quite sure how far this combination of mount and scope could be pushed. Camera lenses work well for unguided imaging due to faster focal ratios and shorter focal lengths allowing longer exposures. For example, this was taken with a 1960s vintage 135mm lens and modded DSLR:

15147147820_89347b3d5e_c.jpg

Hope that's some help.

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You can't really expect too much of this mount unguided. I don't know how well you polar aligned but the best way is to do a drift alignment to check. This will get the best out of the mount ungudied.

The other thing is balance, which makes a big diffeence. Running very slightly east heavy can help. This means the worm wheel is always pushing on the wheel.

Olly

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Thanks Knight of Clear Skies and Olly for your very welcome comments. This was very much a first attempt and I was most intrigued at the shape and cause of the trails, they seemed to alternate first one way like a letter n (90 secs) then a u shape (105 seconds) and 120 an n shape again. From what you say I should not expect that much from the mount without going into guiding. My Synscan Alt-Az mount can give 60 second exposures with no trailing in a high percentage of frames and I was very interested to compare native performance. It sounds there could be a combination of causes compounded by my inexperience and some avenues to explore such as improving polar alignment and balancing the tube appropriately. I will explore drift aligning but my views are quite restricted to the South by nearby housing. 

Thanks once again for your help and guidance.

Cheers,
Steve

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Judging from the shape of the trails (120 s), my first thought was like Olly already suggested: balancing.

With my EQ3 mount, I found that I can have good tracking on one side of the meridian, but not the other, and I've always suspected balance to be the cause.

Other causes you shouldn't rule out are wind and tripod instability.

By aligning your camera with the RA and Dec directions, you should be able to determine if most of the trailing is along Dec or RA.

Aligning the camera is quite easy: just put a bright star in one corner. Then move RA and rotate the camera until the bright star moves parallell to one edge of the viewfinder.

Take a long exposure to see if tracking was good or poor in dec or RA.

Just my € 0.02

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Hi Wim,

Thanks for your post and comments, I do appreciate the help given. I'm intrigued how the strange track gets created when the ota is unbalanced, is it showing the individual cogs slipping under load, that could explain why sometimes the tracks are u shaped and other times n shaped? Fascinating. There was no wind when the images were made and the mount tripod seemed sturdy.

Best regards,

Steve

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