(https://astrob.in/2lk0ly/0/rawthumb/gallery/get.jpg?insecure) (https://astrob.in/full/2lk0ly/0/)
This was taken in one session on 4th April (moon was about 60 degs away).
Telescope was BrightStar 15028 HNT on iOptron CEM25P mount. Camera ZWO 1600 Pro MM. I used a Bhatinov mask pointing at Dubhe to focus for each filter. The focusser has a stepper motor and so I noted the focus point and used it next day for flat frames for each filter.
I concentrated on colour data and added a "pseudo" L layer (sharpened) in processing.
I am very unsure about exposure times, I took 30 each of: Red - 60 seconds, Green - 45 seconds and Blue - 45 seconds.
I would welcome expert advice on this. The telescope is f2.8 (424 mm focal length) and therefore fast, is it better to take fewer, longer exposures or more shorter exposures? I was guided by the histogram and was leaving a fairly large blank space between the black point and the start of the hump. Would it be better to be closer to the black point? I find horrible colour blotches in the dark areas in addition to gradients from LP (which is fairly bad - I live across a playing field from a large Tesco store which has a lit carpark and goods in area). If I were to try an exposure through a colour filter of much more than 3 minutes the image is completely whited out.
My processing is still fairly basic and I hope to graduate to using masks etc soon.
I hope everyone is keeping well and am looking forward to the days when we can resume meetings safely,
John
Can't advise on length of exposures as I use a CCD camera, but I know this target to be faint and I haven't even managed a decent image from a dark location as yet, at least not one I am pleased with.
But I would definitely take Luminance (not a psuedo Lum) with whatever LP filter you are using, and take a lot more of it than colour. It is from here you will get the most data and detail.
Carole
Very nice John
Considering you haven't had much processing experience you've done well.
Your exposures sound a little short, but you'r doing the right thing by looking at the histogram. Will leave advice to the experts!
Noel
Great image! M101 is a tough target but your fast scope has definitely helped.
Is the BrightStar a rebadged version of the SharpStar? In any case it will be similar in design to my Tak Epsilon 180ED which is also a Hyperbolic Newtonian.
As for exposure length, I would judge it on the histogram. Adjust camera gain and length of exposure so the average background level is approximately 5% between the bias level and the saturation level i.e. the histogram peak is well over to the left.
Mark
:oops: I keep calling it BrightStar when I mean to say SharpStar.
Thanks for the advice. I think that the SHARPStar is an "homage" to your TAK, I am pleased with it and looking forward to gaining experience.
At the moment I get bad vignetting, I think it is because of the 1.25" filters. I have just spent some of the money I am not earning at the moment on a set of 31mm unmounted filters (including H alpha) which will fit my filter wheel and give a slightly wider aperture - up to 5 mm. I imagine that the vignetting must distort the histogram produced by the imaging software.
I will certainly shift my histogram towards Bias, I think that I am presently about 15 - 20% of the way towards saturation. It means that I can reduce the Gain setting which is presently Medium (139 but I have no idea whether that has any units).
Regards,
John
Hi John,
That's serious kit you have ! I tent to use Sharpcap to gauge exposure length, that won't help on its own but if you also use the ZWO setting for unity gain when doing this then that should get you near'ish exposure setting wise - I generally round up the suggested exposures to a 'sensible' number' like 30, 60, 90, 120s etc for broad band images.
Hope that helps
Robert
Hi John, with my ASI294 I found reducing the offset to the level suggested by SharpCaps sensor analysis to 4 instead of the default 30 fixed the colour blotches. I would run sensor analysis and see what it says. You don't need a dark sky to do it.