CG-5 GT Calibration

[jmdharkness]

Hi,

I have a celestron 80ed on a CG-5 GT mount. A few nights ago I set up and took a few 1-2 min exposures of the Orion Nebula, which turned out ok, heres an example of just a single exposure no stacking etc... Orion Nebula (M42, NGC 1976) | Flickr - Photo Sharing!, however, I attempted to redo this image tonight putting in a bit more effort, going out of the city etc, but after performing the calibration and polar allignment steps, I got trails of stars.

I did the usual, 2 star alignment followed by a few calbiration stars, then polar alignment and lastly star alignment one more time but didnt seem to track as well tonight. Was just wondering if there is any advice or tips to better align the scope, or possible reasons why this may have occured.

Thanks A lot,

John

[brantuk]

I would guess your polar alignment is out a bit (assuming all the lat/long and time/site settings are correct). That's usually the culprit for star trailing.

HTH

[jmdharkness]

Thanks, I'll have another go tonight if it's clear and try do the polar alignment a bit better. Maybe I was using too wide an angle eye piece for accurate alignment

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[brantuk]

Yup - I find something like a 10mm eyepiece will improve accuracy. What I do is start around 20mm-25mm and gradually refine it with higher power eyepieces. I find a decent 8mm-24mm zoom eyepiece very useful for this purpose.

It's also great for refining alignment stars and goto accuracy. But polar aligning is the important one to reduce star trailing. Also use something like "Polar Finder" by Jason Dale very useful (and free):

Free Polar Alignment Software Download Polar FinderScope by Dr. Dale Jason Version 2.04

The other way to ensure the scope stays on target of course is with guiding - essential for imaging - but of course that's a whole other subject

[Pauly]

Hi John. Out of interest, are you polar aligning through the peep hole in the mount or are you using the polar align function on the handset? I have fitted a polar alignment eyepiece in the mount and use that method. I start by leveling the tripod, polar align, balance the tube then star align. Normally works ok. One thing I have found is that the 5GT is a bit fussy about the power source being used. If you are not providing enough current it will work but tracking is compromised. I have a 12v battery pack and a 1A rated a/c supply both of which seem adequate for it to swing an 8" Newt around.

Hope this helps.

[x6gas]

As usual, excellent advice from Brantuk...

The best and most useful piece of kit that I ever bought is an illuminated crosshair reticule eyepiece. Mine is 12mm and it really takes the guess work out of the Celestron alignment routine. I centre the first star in the finder scope then use a 32mm to centre and then switch to the illuminated EP (I realign the finderscope on the first star to check that it's bang-on.) After that I can often just use the illuminated EP, but if the star is not in the field of view I just swap in the 32 mm - don't bother refocusing, just centre the de-focused star and then back to the illuminated EP.

I use a really cheap 32mm plossl and the optics on the illuminated EP aren't award winning but that doesn't matter for alignment.

The other possibility it that your power supply and or power cable aren't up to the job. When you had good tracking were you using mains power by any chance? Switching to a portable supply might be the culprit...

Best wishes,

[x6gas]

Whoops - post crossed with Pauly...

[Pauly]

Nice to have my findings confirmed though

[jmdharkness]

Wow a lot of advice there to work off thanks. Will let yous know how it works out. Funnily enough I was on portable power with the star trails, hopefully that's not it the power supply wasn't cheap :-(. Thanks all.

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