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This will be my last image this season. The moon is all too bright atm, and astro darkness ends in a few days.
This image was a real challenge, both in acquisition and in processing. I haven't completely tamed the MN190 yet, collimation is still a bit off. Visually it looks ok, but when I start imaging, I get star shapes that suggest that collimation is still way out. First I thought that it might be the addition of an oag to the imaging train that was the cause of it all, but I've now come to the conclusion that it actually is collimation.
Anyways, the details:
MN190 on AZ-EQ6 ZWO ASI174MM-Cool at -20 C with ZWO filters Guiding was a mixture of ST80 guidescope, OAG (the new ZWO version), and finder guider (9x50) with PHD2 L. 92 x 2 minutes, gain 20, collected April 15 under a 70 % moon R, G: 30 x 4 minutes each, collected March 31 and during two nights in April B: 22 x 4 minutes, collected March 31 and during one night in April Processed in PixInsight
Larger version is here:
If anyone wants to have a go at LRGB processing, here's the raw data. Only crop and DBE applied. Have fun!
A few weeks ago I decided to take my TOA150 off the mount for a while to give the Celestron Edge 14 a proper try-out. The refractor is beautifully straightforward to use - it "just works", has a wide field which makes plate solving easy, and a reasonably trouble-free focuser. I anticipated some problems moving to a long focal length mirror-shift SCT.
I've fitted the Edge with a Lakeside motor focuser - rather than going the Crayford route I've put it on the normal primary mirror focus knob. I expected this to throw up challenges due to mirror shift, but after some experimentation with SGP settings I found the right combination of focus direction and backlash compensation which generates good V-curves and little net image shift between the beginning and the end of the focus run. I can definitely recommend the solution which Peter at Lakeside has come up with for this fitting.
I'm using the OTA at native F/11 (3910 mm focal length), with an Atik 460ex CCD and Lodestar guider via an OAG on a Mesu-200 mount. Plate solving was an issue to begin with - the 460ex imaging area is quite small, and the system is slow. I've found that adopting 4 x 4 binning and 15 sec exposures for plate solving captures, usually works, and I can now centre a target in the middle of the frame from any part of the sky in 2 or max 3 plate solve cycles.
By far the biggest challenge with the entire thing so far has been finding and keeping guidestars - in the wide field of the refractor it's hardly ever a problem, but with the 14" it's a real challenge for some targets because the area of sky captured by the Lodestar is so small. Very often there aren't any usefully bright ones available in the 4-8 second guide exposure times I'm using in PHD2, and now and again I've hit problems in which the guide star is lost because it has moved out of the guide square in PHD2 after a focus run - although I seem to have managed to tune most of that out now (I really wish there was a setting in SGP to perform a re-centre after auto-focus - that would solve lots of problems).
Guiding at f/11 has posed no problem at all - really flat PHD2 curves. The "Target" view in PHD2 has shown clustering of points within the 0.5" circle most of the time this week. Collimation is probably not brilliant, and I've found having to think about that business to be a bit of a pain compared to the trouble-free experience of the refractor - not that it's difficult, just that it's something you have to do.
At the beginning I spent a night jumping around from object to object, marvelling at how targets that are tiny in the refractor field of view suddenly fill the frame. But eventually I settled on NGC 2683 as the target for a proper first attempt at imaging with this combination.
Result shown below. Exposures: 26 x L, 10 x R, 9 x G, 9 x B subs - all 600 sec with 2 x 2 binning.
It's clearly far from perfect - I've tried not to introduce too much noise in processing but there's definitely noise there with mottled background being a real issue - though that's my processing not the instrument. I also have a problem in L with a couple of dust bunnies that aren't flat fielding out properly - either the filter wheel is not returning to exactly the same position, or I've had a small shift in the orientation of the camera. Need to check collimation as well.
I think it's promising although there's more work to be done on the image and on the setup. However, by coincidence I'm posting on the same evening as @wimvb who has posted an image of the same target taken with an MN190 and 460ex, and the level of detail in Wim's image compares very well indeed with mine, despite the difference in the aperture and plate scale of the combination used - make of that what you will. I have my own views on why, and ultimately it's the refractor which will be mounted for most of the time - but the 14" certainly offers a lot of different options for imaging and I think it's worth working to tame the challenges which the design introduces.