Many beginner questions... :)

[chd]

Hi,

I'm just getting started figuring out my current (new) photography setup, and looking for some general guidelines.

Problems I'm running into so far: One, dynamic range - I'm getting images of things like M31 and M42 where the center is completely blown out before I have any detail in the surrounding areas.

Two, fairly significant vignetting (I think), and my question on this is mostly - is this to be expected? I'm not using flat field exposures at this point, but I will do so if the answer is "this is normal and it's why people use them."

Three, I find that in MaxIm DL I often get a badly-rendered image when viewed at 100% - overly bright, and with what looks almost like a visible pixel grid - but it looks normal viewed at 50%. I assume there's some setting I haven't found yet (I've been using the demo version for about a day) - any pointer to where I should look?

Equipment-wise, I'm using a Celestron CPC 8" SCT with HyperStar lens and an ATIK 428EX CCD camera, so that should make it about f/2.2 or so I think.  (At this point, my equipment is obviously far more capable than I am - working on that. ) It's on an altazimuth goto mount; tracking but no guiding. Given that I'm not guiding, what's the longest exposure I can expect to use without getting smearing of the details? Based on one night of experimenting I seem to get visible trailing after about 20-30 seconds, but if the general feeling is that I should be able to do better I'll work harder at getting everything aligned and leveled as perfectly as I can.

Here are two images (assuming I get them attached properly) showing what I mean. The first, with a little bit of the Flaming Star nebula, shows significant vignetting, although there should be nebula bits across most of that area too - I see a similar effect to a lesser degree on images of M1 though too. This one is a stack of about 30 exposures at 20 seconds, binning=2.

The second is my dynamic range issue with M31 in this case.

Thanks!!

-Chris

[ollypenrice]

You very obviously need flats. The first image shows a bright circle which is pure vignetting. You'll get nowhere at all without them.

Your focal length, in my view, is too short for Bin 2. I'd use unbinned capture.

I'm not sure that levelling and aligning will have much effect. Ultimately the problem will be field rotation arising from the Alt Az setup.

We need to know whether the images you're presenting are linear or stretched. Are you familiar with the difference? A linear image comes straight from the camera and a pixel which received x light appears half as bright on the screen as one which received 2x light at capture. Astrophotographers then stretch (brighten) the faint signal far more aggressively than the bright so as to lift the faint details into visibility while trying not to over saturate the bright. Dynamic range is always a bit tricky on M31 but at slower F ratios (the fastest being F3.9) I've been able to control the core tolerably well in 10 minute subs.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

[chd]

Thanks - these are both stretched to get the best-looking results I could without spending a huge amount of time at it.

I set Bin 2 for that one because it was so faint and I thought "more sensitivity = good" but that also seems to have significant exaggerated the vignetting. So this level of vignetting is pretty much to be expected for this kind of exposure without flats?

How important is it that the flats match the conditions for the lights? At least one recommendation I saw says that they should be pretty exact, which means shooting new ones every time I change targets (which is difficult). If I can get away with shooting a stock bunch and reusing them that would be much easier (and it's probably better than nothing in any case).

Without guiding I'm certain that 10 minute subs aren't going to be in my future. If necessary I can take several series at different exposures and layer them together in photoshop, unless there's a common easier approach.

[Astrosurf]

I'm not good at explaining what I do! Plus I'm knackered and brain-dead today!   Objects with a high dynamic range are very hard and you need quite complicated processing. I used layer masks with M42 and subs of different exposures to capture the trapezium. I isolated the core and other bright areas on M31 when stretching. I'm still very unused to PS though. So here's a link to an interesting thread about processing:

http://stargazerslounge.com/topic/87021-photoshop-astrophotography-tutorials/

I haven't needed to use flats yet, but I used the GradientXTerminator plug-in in PS once or twice which worked well.

http://www.rc-astro.com/resources/GradientXTerminator/

Alexxx

[glowingturnip]

Three, I find that in MaxIm DL I often get a badly-rendered image when viewed at 100% - overly bright, and with what looks almost like a visible pixel grid - but it looks normal viewed at 50%. I assume there's some setting I haven't found yet (I've been using the demo version for about a day) - any pointer to where I should look?

Maxim usually gives a fairly brutal screen stretch when you first open a file - open up the screen stretch window (icon on the tool bar, or under the view menu I think), and it has a dropdown box for different stretches, pick one that looks better.  NB this only changes the way you are viewing the file, it makes no changes to the file until you do a permanent stretch

[chd]

You very obviously need flats. The first image shows a bright circle which is pure vignetting. You'll get nowhere at all without them.

Object lesson taken - I took a small number of rudimentary flats a couple of nights later and used them to re-process the same images I posted, and good lord the difference is night and day! I didn't get any additional detail out of the nebulosity, but the severe vignetting was essentially completely cured.  The flats I took aren't keepers for a number of reasons, but they were good enough to demonstrate the value of them.

The next time I have a clear night I plan to shoot a selection of flats using the twilight sky, and hopefully those should be much better.

I do remember thinking "How much real difference does all this calibration work really make - is this a case of four times as much work for 5% gain, worthwhile only for the real purists?"  Well, now I'm sold.