M1 - the Crab nebula, a first attempt and much to learn

[WaveSoarer]

We're gradually learning our way into some basic deep sky imaging with our relatively straightforward set up. Imaging M1, the Crab nebula, is probably our most challenging object so far. We're using a Nikon D50 mounted at prime focus of a SW 200p and tracking is achieved with the SW dual axis motors on a polar aligned EQ5.

In our previous attempts we stored the images as jpeg files from the start. This time we used the Nikon raw file format and read the images directly into Deep Sky Stacker. This all worked fine and DSS was happy with the format. The first issue we had came with a warning during the processing that we were running low on disc space and I managed to shovel some large directories onto another disc (actually a different partition) while the processing continued. However, the real blocker was an "out of memory" message that popped up. Not surprising we thought as the laptop only has 1 Gb of RAM. This morning we put all the files onto our main PC, which has significantly more memory and disc space. We still had issues with memory but we got around this by not using 2x drizzle and including only the highest scoring sub images in the set. The final image, shown here, had the levels tweaked in photoshop. We used the small set of darks but we haven't yet got as far as worrying about flats.

Compared to previous attempts it's clear that the focusing isn't quite right and maybe it's time to think of a Bahtinov mask to make absolutely sure. The tracking wasn't as good this time either and I now realise that the telescope may have not been ideally balanced on the mount. In retrospect I should have balanced the scope (positioned the tube in the rings) with the SLR in place before lining up on M1 using my 20 mm EP (being a bit out of balance with the EP wouldn't have been as important). I haven't found it possible to move the scope down an inch or so through the rings to rebalance without altering the alignment. The image is composed of 18 x 30 s subs with only 3 darks at an ISO of 800. Having done a little bit of reading about what is ideally required for dark images, and from advice in previous posts, I realise that a greater number of darks are required to reduce noise rather than contribute to it. Ideally the darks should be taken with the camera in as close a condition as it was when the lights were taken in terms of temperature and just before and after the series of lights were taken. I also realise that longer exposures are needed to minimise noise - but that then opens up issues with tracking accuracy and, at least for the foreseeable future, we're quite happy to stick to this rather than have the complications of guiding as we like the simplicity of our current set up. If the tracking etc looks good then we'll certainly experiment with exposure time next.

The main purpose of this image, apart from attempting to capture M1, was to see if we could process the lights with accompanying darks through DSS via the Nikon raw format. That all worked but it is a little concerning that we had issues with computer resources on a fairly small data set. I assume that it's possible to break things down in to smaller sets and stack the resulting tiff files in DSS without corrections applied but this seems a bit clunky. Any tips or thoughts would be greatly appreciated.

[Matt Scunthorpe]

I like that image. Beautiful collection of stars too.

[WaveSoarer]

That's very kind of you Matt. Many thanks.

[Matt Scunthorpe]

I should of also said that i am using the same equipment as you, and when i finally get my ra motor (xmas), if i get a result like that ill be pretty happy.

[WaveSoarer]

Matt, that's excellent. We found that tracking really helps, not just for imaging but also for observing. As you don't need to keep nudging you can concentrate more on what you're seeing and take time to soak things in or pick out the detail. The imaging side is a bonus but I really enjoy the observing side and it is doesn't carry the complication.

[stem1989]

this is a really nice image, congrats!

[boabsta]

Congrats on a nice image

DSS can be an experience at times. I also capture NEF files and havent had any problems with memory but then I got 4gb ram.

Have you tried ticking the Allign rgb button? I done this and the end result had a bit more colour in and didnt look too washed out. I'm sure I dont have to tell you that experimenting with the setting is the key to good images as I have found no 2 images get the same results.

As for capturing the darks, I usualy do this when I am dismantling everything at the end of the night. I just leave the camera on the scope with the end cap on and let it fire away. I usualy go for about 30% of my light frames. So if I got 100 lights, I'll go for 30 darks. Probably overkill but I'm still learning as well

Rab.

[WaveSoarer]

Dear Rab,

Many thanks for the tips. I'll put these to the test at the next opportunity. It could be cloudy again tonitgh though.

all the best

Dave