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First prime focus astrophoto + questions

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#1 sneutrino

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Posted 05 December 2012 - 05:24 PM

I recently bought a Pentax K-5 and some prime focus adapters. Here's the first result (taken through ES 6" comet hunter, f/4.8).

ISO 200, 15 seconds.

This was not a 'serious' attempt for these reasons:
- I have a Pentax T-ring and the celestron 1.25" drop-in t-adapter, but I can't achieve focus. I CAN achieve focus if I let the t-adapter loosely sit in the draw tube with the extention tubes removed, but because nothing is screwed in, the camera is sitting at an small angle (the focal plane is at a small angle with respect to the sensor plane). I'm going to call Explore Scientific to see what they recommend.
- Very rough polar alignment by pointing appropriate tripod leg approximately north. (It's not possible to polar align from my 2nd floor apartment balcony). Most, but not all of earth's rotation was removed.
- Image taken near soccer field flood lights, in a red zone.
- collimation was not checked but should be close.

Questions:
1) How do I test for the presence of vignetting? My 1.25" t-adapter looks pretty restrictive, but my sensor is only APS-C sized.
2) Is there any free gradient removal software?

full size: http://www.astrobin.com/26656/

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#2 sneutrino

sneutrino

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Posted 05 December 2012 - 05:32 PM

Here is the orion nebula, with some 'fill light' and increased shadows. That process brings out some nebula detail, and what looks like vignetting. 4 images stacked in Deep Sky Stacker, ISO 200, 10 seconds each.

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#3 NeilMac

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Posted 05 December 2012 - 05:32 PM

Looks like vignetting just from the gradient exposure. Probably will have to zoom in to get rid of it.

#4 SMigol

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Posted 05 December 2012 - 06:12 PM

It seems like you will need to get an extension tube to allow your camera to come to focus. This is not uncommon as many scopes are designed to have a mirror diagonal in place to allow an eyepiece to be used reasonably. Thus, you would need to add the distance back into the optical train to allow the camera to be at the proper focus point.

Regarding your vignette, if you are able, you might consider using a 2 inch diameter extension tube rather than a 1.25 inch to reduce the effect if possible. There are 2 inch to T ring adapters, so that may be something to consider.

To test if you have vignetting, you can quickly check this:
Do a bulb exposure with just your t-ring and 1.25 drawtube in place. Looking at the front of the camera, are you able to move the camera from side to side and see all the CCD? As you move from side to side, does the CCD disappear completely before you can no longer see into the camera? This latter test is like looking down a straw - the longer the straw, the less you can see off-axis. If the tube occludes a lot of the light cone, then you have a vignette issue -- going to a 2 inch adapter setup will help.

Hope this helps!

#5 sneutrino

sneutrino

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Posted 05 December 2012 - 09:40 PM

The Comet Hunter comes with a 2 additional tubes that attached to the draw tube: A short extender tube, and a longer extender tube that has the eyepiece compression ring. Both need to be removed to achieve focus.

Couldn't get in touch with ES today but based on this guys message,
http://forum.timesca...php?f=17&t=7949

I just needed some acetone. I got the compression ring off and attached it directly to the draw tube, but I still can't focus.

The main culprit now is actually the celestron 1.25" T-Mount, which adds an enormous 3.5 cm or so. I will have to research other options.






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