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PostPosted: Fri Mar 04, 2011 7:30 pm 
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Remember in another thread I was saying I had difficulty getting longer exposures without trailing? Might have been Digz who pointed out my polar scope might be out of alignment. Well... earlier this week I saw a break in the cloud and pointed the 300/2.8 at Orion, and was getting worse trailing at 300mm than I had previously at 400mm. Hmm... tonight is another clear night so I just set up the kit again. This time, I paid more attention to the alignment of Polaris. When rotating the scope to line up the 'dipper, I noticed Polaris taking a walk in the image. Must be quite out of line :( The distance moved rotating the finder scope some 90 degrees was... about double that the distance between true north and Polaris' indicated position. Ouch.

This must have happened when I dropped the finder in the past. Looks like it takes small hex keys to do the direction alignment, but I don't know where my tools are. For now I might just have to limit myself to shorter exposures to limit the trailing. I guess I could also try the drift method in the short term.

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PostPosted: Fri Mar 04, 2011 10:44 pm 
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By trial and error I reduced the trailing to about 1 pixel per minute at 300mm. But by the time I dialled that in, Orion was too low in the sky and was obstructed by trees, even moving up to the belt didn't buy me much more time. As such I didn't really advance my imaging of the nebula, but I got a bit more out of the horsehead region than before. At that point I switched to Pleiades but there wasn't much time for that either before it disappeared over my neighbours roof.

Image
300mm f/2.8 ISO400
4 minute single RAW, ACR, cropped

Image
300mm f/2.8 ISO400
5x4 minute lights, 4 darks, DSS, cropped

The 300mm f/2.8 wide open suffers from severe vignetting which shows up quite badly in images. At best I can use a central composition and crop to reduce the effects. The captured data is often low contrast, with most of the histogram stuck in a thin band. I think I really need to go somewhere darker to push deeper as I think I'm still hitting my local noise floor.

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PostPosted: Sat Mar 05, 2011 3:41 pm 
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Some nice results there. 4min exposures is pretty good going in any event, I wonder how much more you could squeeze out with a properly aligned polar scope? Better hunt out those tools, lol.

Out of interest how much was cropped off the images? I plan to have a go at the Flame and Horse Head but I think my scope has too long a focal length.

If you can increase the amount of darks you take that will help significantly. You want to aim for around 20 or so. Pretty easy to take with 4min subs, I lost the will the live last night doing my 10min darks, got to 8 and gave up, lol.

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PostPosted: Sat Mar 05, 2011 4:00 pm 
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Very roughly, imagine a circle wholly fitting inside the original frame. The crop might be roughly in that ball park, as I was trying to keep within the region of lesser vignetting. As a rough guess, I think 600mm ball park on APS-C might be interesting to fill a frame. Would your scope take much of a focal reducer?

On the polar scope, I noticed this morning the mounting ring was loose, so the scope itself might not be misaligned, only because it was loose it could be pointing anywhere. Still, I'll have to fix that before I check it anyway.

If it was perfectly aligned, the manufactures claim "The TT320X-AG has typical unguided tracking error of around 5 arcseconds peak-to-peak or better over a 5 minute period." How that translates to focal length and pixel density I'm too lazy to calculate right now. There is a capability for adding RA only guiding too.

Right now I'm not sure having many more darks is going to help much. I need to work on getting more decent lights first! With the cold it was killing the batteries in the camera quickly anyway...

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PostPosted: Sat Mar 05, 2011 4:11 pm 
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Cheers mate.

Im not sure how much of a field flattener my scope will take. I do have a 500mm lens which I may try, but just need to figure a way to connect my finder scope to my setup for widefield AP.

Im sure tightening the mounting ring will certainly help on your setup.

Darks will certainly help reduce the noise, and I wouldn't say the lights you have are particularly bad. I know what you say about the cold! I went through 4 batteries last night in 3 hours!!

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PostPosted: Sat Mar 05, 2011 5:50 pm 
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Did you use the 300D mod. for this job? Or are Canons really that IR sensitive? :shock:

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PostPosted: Sat Mar 05, 2011 6:03 pm 
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Erm... where does IR come into this? Photos were taken on unmodified 450D. I might get an astro or wideband conversion on it some day...

First step repair to finder is on the way, a nice dab of superglue to fix the mounting ring. Once that is dry I can then look at alignment again.

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PostPosted: Sat Mar 05, 2011 7:09 pm 
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I thought the Horsehead Nebula was H-a, but with a relatively short exposure I didnt know it could show up!

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PostPosted: Sat Mar 05, 2011 7:16 pm 
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H-a is a visible part of the red spectrum not IR.

The IR filters infront of camera sensors cut out IR which also makes them less sensitive to H-a (if my facts are correct) Doesnt mean H-a cannot be captured only that there is more about than is recorded on a sensor.

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PostPosted: Sat Mar 05, 2011 7:28 pm 
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Okay, thanks! Im a bit confused then, I think :lol:

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PostPosted: Sat Mar 05, 2011 7:29 pm 
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No worries, its complicated stuff!!

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PostPosted: Sat Mar 05, 2011 7:33 pm 
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Oh dear!
You know, it's today for the second night clear outside, and yesterday I shot M42, and my eyes felt almost out of my head when I saw the Fire nebula (called so?) on the pic! But it was on the very edge of the picture, so today I wanted to photograph the Fire Nebula but Im too tired.

Shame.

Nice work though, popo. Keep them comming!

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PostPosted: Sat Mar 05, 2011 8:37 pm 
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Hi Ruben,

The following graph, from this commercial site, to pick just one of several, shows why AA filter removal/replacement can be worth considering:
    Image
The Baader filter is tuned to the whole visible spectrum. The precise shape of the graph will be camera model dependent, of course, and it may be that the graph chosen is from an extreme case in order to sell more conversions but it does make the point very well.

Bob.

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PostPosted: Sat Mar 05, 2011 9:30 pm 
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Thanks for the explaination, Bob!

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PostPosted: Sun Mar 06, 2011 8:16 pm 
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Nice shot popo, and I agree, a 4 minute exposure at 300mm (on cropped frame too, right?) with an unguided mount is pretty good. I have a friend who guides his Astrotrac with the same guider I recently bought - a Borg 50mm with an Orion Starshoot.


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