More Alt-az DSO madness M109

[Anweniel]

So I've been at it again, trying to achieve the improbable with the Supatrak. After initial setup I was quite positive about my subs coming through at 45s of exposure. After getting a fair deal more data than I usually do I thought I would try to push the mount a little further.

For some perspective getting 30s exposures is an achievement on a good night sometimes 45s if I am really getting lucky. My best to date was 90secs without trailing and having a useable sub. However I got a bit of a good feeling tonight after the cloud filled day had also brought heavy rain and snow to reveal a beautifully clear night. So I pushed the exp up to 2 mins and to my surprise the results were spot on so I got really excited and knocked out a 5 min exposure.... had a feeling in my pants this wasnt gonna work but when the picture had finally finished it had trailed but only very little, still unusable ofc but surely only a tweak or so away from getting it to work!!

Sorry long story but I finally got the workable exposure time to around the 3 minute mark with a good % of useable subs, I certainly feel that I had achieved something tonight and I was quite smug about it

Im not really certain as to what I have done different/better tonight than usual, the only real change I have made is to get a 2"LP filter (already have a 1.25" that I have used previously for webcam work) and this has helped wonders but doesnt really explain the improved exposure times... definitely I need to keep a closer eye on what is going on because I think if I can get the 5min exposures to work and to get it happening most of the time will really be something special!

Anyhoo cutting to the chase and here is my efforts from tonight, a shot at M109 whcih I believe I have captured along with what I assume to be NGC 3953 and a hint of NGC 4102?

Comments, critique, advice all welcome as ever. Thanks for looking,

Barry

[Anweniel]

I forgot to add details, apologies.

Scope: Megrez 90

Mount: SW Supatrak

Camera: Unmodded 1000D

6x2 min 2x1 min 1x 3min 51x45s Lights @ ISO 800

3x darks

21xLights

21xBias

[Anweniel]

Seems even though the LP filter has helped a lot, I've still overdone it with the processing giving that 'bloom' in the centre. Perhaps Im over stretching the value channel??

Anyway heres a re-process with a more subtle approach to the stretching.

What do you think any better or nothing noteworthy?

[Shibby]

Wow, 3min subs on an AltAz?? That's impressive!

You really need to eliminate the gradient in order to process it properly... stretching the curves is a losing battle otherwise. Doesn't look like you took flats, so you could try GradientXTerminator if you use PS?

Also, I'd try stacking without the 3 darks, as you might find this is making your image more noisy.

I'm not sure about the longer subs... they may be having no effect due to the overwhelming weight of the shorter subs in the stacking algorithm - but I'm not sure about this one. With an image like this, I'd be tempted to stick with a sub length that doesn't require throwing away many subs. 50x3m, for example, will get you loads of detail.

A nice image, by the way - it's thrilling to capture several galaxies in one frame, isn't it?

[lukebl]

As Shibby says: Nice work, particularly for an Alt/AZ.

Three things would improve that image no end. FLATS, FLATS and FLATS! They will enable you to stretch the image further without getting that gradient.

[kenmyers]

Well the advice is like a new language to me but I would like to add... "Great pic"!

[jgs001]

Very good for the mount... From my reading, the limit is suppose to be 2 minute exposures (approx) low in the east and west (before field rotation becomes an issue), but I found the drive inaccuracies of my NexStar to be to problematic at that sub length. Lose the darks, the natural dither caused by AltAz tracking will do a lot for the hot pixels etc... Flats is the key for the gradient... although you want more than Three ...

[Anweniel]

Thanks for the replies guys.

I suspect as you have all confirmed the main issue I have is with the flats, believe it or not I did take 21 of them! (I've been taking the same number everytime not sure why but read somewhere DSS likes odd numbers?) However I must be doing something tragically wrong as I dont seem to be reducing the gradient issue at all and it crops up badly in all my pics.

Only 1 of the 3 minute subs was really useable although to be fair by the time I had realised they were working (after taking several shorter subs) it was getting near time to end the session and having only taken 6 x 3mins 1 useable wasnt a great ratio but a nice surprise to find that it had worked out!

Thanks John also for the nice comments, I think there was a misread somewhere, it was the darks I made only 3 of and yes I agree I need to more next time. Certainly 2 minute exposures were more consistent so that sounds about right although towards the end of the session (when I discovered the longer exposures were coming out ok) the target was not far from Zenith, it was ~ 22.30 when I packed up. SO I am still unsure exactly what I did different?better to normal so I am a little puzzled but pleased!

@Shibby, thanks mate I dont use PS am using GIMP, (reserving all funds for EQ mount) so gradient exterminator isnt an option and I havent had much luck trying the black point method :/

Think I may have an attempt with another target tonight and may redress my falt taking. I know someone advised me the correct exposure length for flats is based on getting them to appear approx halfway along the hist and to get as nice and even illumination as possible across the aperture, unfortunately I am not entirely sure how to approach this as my current method was to just take as quick an exposure as possible with my girlfriends blouse hung over the end of the scope and using the torch of the powertank shining directly in the centre.

And

it's thrilling to capture several galaxies in one frame, isn't it?

It most certainly is! I wasnt sure I would get any of them but i think theres a hint of a third near the bottom of the image

[JimStan]

If you don't have flats, or they don't seem to be working properly for you, you can always try the layers and gaussian blur technique in a program that supports them, such as PhotoShop Elements, or equivalent. Essentially it involves making a layer of the background, blurring out the star images ( use the clone tool to eliminate the brightest star ) with the gaussian blur tool, and then making the layer a "subtraction" layer, and laying it over the background image. The layer will smooth out the gradient and make the background all even toned, if it is done correctly. I do believe the technique is explained in a "sticky" tutorial that you can find elsewhere on the forum. Good luck!

Jim S.

[Anweniel]

Thankyou Jim I'll try some better flats tonight and if I still have issues then I'll give the Gaussian blur technique a read/try. I think it will be pertinent of me to make sure Im doing the flats right I mean at some point I ought to be doing it properly may as well start asap

Amazing what you can do with processing and I appreciate the advice, still probably worth me looking even if the flats improve huh?

[JimStan]

Remember, make your flats resemble a standard "gray card" which reflects about 15% of the light that is striking it. Not too bright, and not too dark. Good luck!

Jim S.