Another M42 thread, but my first with my scope!

Hey all,

Managed to get in a few hours last night on M42, my first proper session with it ... I know there's a million posts on here for it, but keen to hear any constructive feedback or pointers on shooting and/or processing. Used PHD for polar alignment, then Orion StarShoot to autoguide. Tried several exposures to get the focus and colour right, settled on 1min subs, as 2 and 5min subs were just slightly oval shaped stars, so guess I need to work on my alignment some more I suspect. Shorter subs left me worried I wouldn't get enough detail though feel 1min is blowing out the center core too much.

Tried a few different processing methods in DSS, then in Photoshop. This first one is 23x 1min lights, with 10x 1min dark subs, no cropping (black point is a little too dark):

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Well for starters this is the most photographed Neb we know of . And you did very well starting out ! I do see the Trap is blown out but that is hard to keep from doing that . Read this section from Jerry lodriguss website and you can easily learn how to take shorter subs for the Trap and copy and paste for final results . Look forward to more of your images .

http://www.astropix.co​m/HTML/J_DIGIT/COMP2.H​TM

That's a fine start. Well done!

As Ron suggested, although M42 is an extremely popular target, it is a bit tricky because of the extreme brightness around the trapezium asterism. Best results definitely involve exposure blending of some sort.

To get more of the nebulosity, I'd definitely be going longer than 1 min subs. You should be able to push that out to 2, 4 mins, even longer. Around the trap you could shorten down to 30 or 15 secs.

You'll need to get your polar alignment down pat, but that's just practice. These stars look nice.

Awesome, thanks for the advice guys ... will try M42 next clear night I get at 15s, 30s, 60s, 2min, 4min and see how I go in PP. Polar Alignment is tricky, and I blame my position rather than lack of experience, hahaha ... am boxed in by neighbour's houses, so only get about 30 degrees from the horizon and up, swinging from sort of NE through N to NW, any tips on PA with limited views?

- Daniel

Forgot to ask ... below are my subs at 5min, where my PA is showing up as out of whack, but just wondering though, are these subs too washed out? Is this what you'd call light pollution? Still trying to learn what to look for in brightnesses, etc, as I don't want to expose too long that light poluution causes me to lose details.

Single full frame image, at 5min:

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Yes those are normal washouts when taking such long exposures . Thats the reason you take flats and darks , etc.... . When stacking it takes out noise and brightness .

You get the best bang for your buck if you expose to get the in-camera histogram peak completely off the left axis and up to about a third across. If you can't get round stars at that exposure length, drop the sub time to where they are round and shoot more subs. You can fix noise but you can't fix blur.
For polar alignment, look at EQAlign. It suggests ideal stars but works with whatever you select so you can choose stars you know and can see.

Steve.

I could be wrong, but those elongated stars don't look like polar alignment issue to me. They're radial inwards, while the right hand stars are ok. If it was guiding I'd expect them to be tangential around the guide star.

I'd be looking into your field flattener and/or perhaps the sensor is not directly flat against the focus plane?

About polar alignment, do you know how to drift align? Is that what you mean when you said you used PHD? ie turn off DEC guiding and watch the graphs for movement?

Hey Ron and Steve, thanks for confirming the washout-ness to expect, will keep a closer eye on the DSLRs histogram.

Steve, the tip about a third across, I guess you mean the peak of the midtones? I had a look at EqAlign but struggled to get it talking to my scope, the software itself seems awesome and makes sense, will take another look at it, thanks again!

Hey Troypiggo, I don't have a field flattener, just using my 50D into the ED80 with the 2 inch T-adaptor. Yes, turned off DEC guiding and watched the red (dY?) both when watching stars on the meridian and near the eastern horizon.

Thanks again guys!

- Daniel

Gotchya. I think you'll find your polar alignment isn't too bad, it's actually mis-shaped stars from the ED80. Might need to look into a flattener.

I was thinking the same thing Troy. It's odd that it is only on the left side. Very nice for your first try Daniel. I got a flattener for my 80 ED it also add a wider FOV. I had to use it to get the Rosette Nebula all in the frame.

legoman_iac wrote in post #13643997
Steve, the tip about a third across, I guess you mean the peak of the midtones? I had a look at EqAlign but struggled to get it talking to my scope, the software itself seems awesome and makes sense, will take another look at it, thanks again!

I'm not sure that "peak of the midtones" is the best description in an astro context. In most astrophotos, most of the pixels are the dark sky so the majority of the histogram is the sky. As you expose for longer, the sky becomes brighter so the majority of the histogram moves towards the right. Once the left side of the histogram drops back to zero, you're no longer gaining anything by exposing for longer but you do start to eat up your dynamic range. However, the histogram you see is after the camera has converted it to jpeg and on a tiny little display so you want to be sure that it really has returned to zero, so the easy way to do that is to get the histogram over to about a third to half way. Half way works out to something like a 3% loss of dynamic range so is a reasonable compromise.

For EQAlign, you probably need to use the POTH Hub so that two ASCOM things can talk to your telescope.
Having said that, it works really well without being connected to your telescope (which is how I use it).