My first globular cluster - omega Centauri

[Pierre_C]

Here is my first image of a globular cluster using a DSLR through a refractor on a tracking mount – omega Centauri.

I am very excited by this image, as I have been fascinated by globular clusters since seeing my first vague cotton wool dot in the sky using binoculars. My fascination continued with seeing larger cotton wool spots through a 4” refractor and then sparking Dandelion seed heads through an 8” refractor. I am amazed by how much more detail and depth just a 30 second exposure reveals.

I would appreciate any comments on how to improve my technique.

In particular, can you tell me what the black dots and dashes are near the periphery of the image might be? The larger black circles on the right of the image appear in other images in the same position. I suspect they may be related to the telescope’s objective lens or the old camera sensor – there is only air in between. Any thoughts on what to do about them?

Thanks.

• Single image, ISO 800, 30s
• Celestron AZ102 refractor
• EQ3 with RA motor drive
• Nikon D7000 DSLR
• NX Studio

[Cyberman (Rob)]

Pierre, that looks pretty good to me for a single frame. Focus and tracking seem ok. Next step is to take a lot of light frames like this (about 120x 30sec frames to start) and stack them in ASTAP or deep sky stacker. These are free programs. I like ASTAP better. You also need to take calibraton frames ie. dark, flat, dark flat and perhaps bias frames to add to your stack. Flat frames will remove defects from your optical train such as dust motes on your lens or sensor. Dark frames will correct some problems with your sensor eg. hot pixels. The peripheral dots might be dust motes. The other streaks might be something to do with pixels in the sensor. The calibration frames may correct a lot of these problems. You then process the stacked picture with other programmes. I use Siril and Gimp which are also free. You can also improve tracking with a guide camera and guide scope using the PHD2 program (again free) but I don't know if you can do this with the EQ3 mount. But so far you're on the right track.
Good luck for the rest of your astro journey, Rob.

[Dekker (Derrick)]

Hi Pierre,


The dark dashes in your attached image are definitely JPEG artifacts, which is a side effect of the forums compressing a very large image down to under the attachment size limit (200kB). You can see they form a lower-resolution grid than the image's native resolution.


Don't rely on the forum software to compress your image. I'm not sure what options NX studio has in this regard, but you could use free software like GIMP or Paint.Net to compress the image as a JPEG under the 200kB limit yourself, which would get rid of most of the artifacts at the same resolution.

I prefer Paint.Net for this purpose, since you can adjust the compression parameters and get instant feedback on the final quality and file size.

[Pierre_C]

Thanks for your helpful comments, Rob and Derrick.

[Dennis]

Nice work for your beginning efforts Pierre, much better than my images when I had my L-Plates on.

You picked a good target.

Keep it up, accept any setbacks and smile at any frustration, for it is a rewarding hobby.

Dennis.

[Pierre_C]

Thanks for your support and encouragement, Dennis.