View Full Version here: : M83 - first (deep sky) light with my 200mm f6 newt
14-06-2012, 12:20 AM
After recently adjusting the mirror position of my 200mm f6 dob to allow prime focus DSLR imaging and now mounted on a NEQ6, i've finally had first light on a deep sky subject - M83. Also trialled the rig in a side-by-side rather than piggy back configuration.
50 x 4min subs @ ISO800, collected over two nights with a modded 400D plus dark, flat and bias frames. Processed in PixInsight (which i'm still coming to grips with).
Thanks for stopping by
14-06-2012, 08:00 AM
Looking again this morning, there looks to me star elongation in the up/down (ie RA) direction. looks like i need to work a bid harder on polar alignment at this longer focal length.
14-06-2012, 06:08 PM
Sounds like a big learning curve, with 8" f/6 and pixinsight!
I Have an 8" f/6 parks mirror in a dob cofiguration too, and want to do the same as you. My main problem is weight, with the parks mirror being thicker and heavier it also has a metal tube which I may have to change for something lighter.
What size secondary are you running?
Thanks for sharing.
14-06-2012, 07:53 PM
The 200 f6 has the stock 50mm secondary (as its built primarily as a visual scope), and gives 100% illumination at a field radius of 0.4" (10.16mm) with 93% illumination in the far corners (13.34mm field radius). So some vignetting, but fortunately flats looked to have kept that under control.
[Re my elongated stars in the RA axis - i think my 'RA Aggressiveness' might have been too high in PHD Guiding (was set at 100). Have lowered it to 80 and will try that out during my next session.]
Nice pic! Can see quite a bit of detail :thumbsup: The stars don't look too elongated to me, but possibly slightly out of focus? The diffraction spikes also seem to suggest that (Though it could be collimation, or me not knowing what im talking about :P)
Here's my first light on a few DSOs just like you I took through my very old 8" f/5 on a EQ5 mount if you wanted to compare :) I used a non-modified Nikon D80, which is really really old and had shocking noise. I also wasn't polar aligned, and used a 400mm Guidescope. I was still a complete noob to post-processing then (as I still am now), and really was struggling with the guiding cause of the poor polar alignments.
My RA settings are 70, and was guiding at 0.05 sec intervals. I found the most important parameter to be in the calibration step size and max durations.
Keep up the good work, it's a great scope to get started in. Just gotta keep at it!
14-06-2012, 09:18 PM
It looks like you are well on your way albeit
maybe with a few guiding & focus problems.
I too have an 8" f6 Newt & you can see my photos
& some tips on my flickr photos.
The weather has been the main problem for the last 6 months.
14-06-2012, 09:19 PM
thanks Meru/Alpal :)
Focus was confirmed with a Bahtinov mask at the beginning of each imaging session and locked in place, however i guess focus must have shifted slightly as the ambient temps dropped. Collimation could also be a factor, as I only have a laser collimation thingy - not the full suite of newtonian collimation tools. Seeing was pretty variable over both imaging sessions for that image - great until about midnight when it hit 100% RH, and then a light fog as the temp dropped from there.
FWIW I also have a 200mm f4 imaging newtonian that i've been working with over the last few months, so i've been able to learn and practice much of the capture and processing workflow (have uploaded some of those shots to my astrobin page). PixInsight and now this second longer focal length scope are just the latest AP toys :) Would love a permanent obs, but I still need to convince my better half :lol:
14-06-2012, 09:24 PM
this is a previous M83 image, with the other 200mm f4 and old processing workflow
14-06-2012, 11:05 PM
Hi Richard - again,
I read your comments.
I found that it wasn't until I used an OAG - top tube rings & saddle plates &
also modified the counterweight bar that I got rid of guiding problems.
Focusing still robs me of better pics sometimes.
I too use a bahtinov mask but if you focus on a bright star in
a part of the sky well away from your target then it may
not be perfect when you're on the target.
One trick is to leave the bahtinov mask on & take a 2 minute ISO 1600
frame of the target itself.
You can then check the diffraction spikes across the whole frame.
Repeat that every hour to check it.
While waiting for the current exposure I now check every frame with
maximum size on the screen ( in .jpg format )
With 5 minutes between frames there is plenty of time to do that on my laptop.
14-06-2012, 11:30 PM
Thanks Allan. Food for thought ;)
15-06-2012, 12:12 AM
That's a good effort. I agree that focus looks a little soft and it would be good to increase exposure time if possible. I use a Skywatcher 8" f/6 Newt and have imaged M83 (http://www.flickr.com/photos/47268787@N05/7265659072/in/photostream/) using my umodded canon 400d. Camera settings for each sub were 10mins @ ISO800. It's a total of just over 3 hrs. Im still not happy with the colour though so I will eventually do a repro. See my flickr account for other examples.
I was fortunate that I didn't have to adjust the primary mirror position to be able to focus using a dslr. My focuser has a 2" to 1.25" adapter and the 1.25" adapter unscrews revealing a male T-thread.
Keep up the good work :thumbsup:
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