Last Night's Efforts

[bluescope]

It's been a few months since I've even had my scope out and last night I tried to remember how to use it again. These are not great shots by any means but as I was up 'till 6am doing them so I thought I would share to make it all worthwhile ...... I feel like sleeping for a couple of days right now. However I enjoyed the chilly air hanging over Perth last night after all the hot weather we've had this Summer past. I much prefer the cooler months for the clarity of the sky etc. I'm also looking forward to the end of daylight saving in a couple of weeks.

All taken with ST2000XCM, 200mm Newt, EQ6 Pro, Baader MPC, Baader UV/IR
Most were 30x25sec exposures except for M83 and M8 I think. I processed M8 to bring out core detail and I'm pretty sure all the images are clipped because of the amount of processing I have to do to overcome my horrendous light pollution problems as I have described in prior postings.

:thumbsup:

[jase]

Good effort Steve. Pleased to see you back at it. I feel you need to get that ST2k working for you a little better. Subs are too short. Are you guiding yet?

While light pollution can be a major issue, try to work with it. Go long on your subs and get acquainted with handling gradients in your image processing routine. There are many methods to neutralise/desaturate the background. Of course, imaging from dark pristine skies is always very rewarding, you can equally achieve great results from the burbs with patience.

Keep at it. Look forward to seeing more in the future.

Cheers

[Peter Ward]

Steve,

Just a few tweaks and we'll have you pumping out awsome imagery.

You need to obtain better flats. I recommend median twilight sky flats at about 30% full well capacity.

The camera self guides, so why such short sub exposures? 5 minutes is my bare minimum, just make sure you have corresponding darks.

Spend a bit more time getting accurate focus, and DDP RGB combine will do wonders for the contrast.

Hope all that makes sense!

Peter

[bluescope]

Thanks Jase ..... I know the subs are short and that's a big part of the poor quality images. I'm still using track and acumulate mode in CCDOP's which I think uses a guide star ( I have to select one before imaging begins ). I've tried self guiding mode and it doesn't seem to do what I would expect .... crappy results with bad trailing etc. I wish some of you guys were just around the corner it's a bit hard across the continent.

:thumbsup:



Cheers Peter, I get what you mean. I answered Jase regards self guiding. When you say 30% full well flats do you achieve that by adjusting exposure time ?

:thumbsup:

[Tamtarn]

Steve I don't know much about the ST2000XCM.

The camera is supposed to self guide but do you need to polar align, is that the problem. As Jase said you need longer subs and you have been doing very short exposures for a long long time, including when you were using the DSLR without progressing further.

Hopefully Jase can help

David

[jase]

I'll jump in with some flat info (Peter can interrupt as required);

Flats are not going to help with light pollution gradients. Such gradients are typically dynamic and difficult to deal with. You’ll simply need to hone your image process skills to address these. I don’t like dealing with gradients and yet to hear of an imager that does. They are, unfortunately, a fact of imaging unless you’ve got some pristine dark skies available.

Not sure of your experience with flats, but what you’re trying to do is evenly illuminate the telescope aperture so it will fill the camera pixel well depths to around 20%-30% saturation. I can elaborate more on this if desired.

Now, I’d recommend starting off with a lightbox or t-shirt flats. I started with t-shirt flats which I found satisfactory for what I wanted to achieve, but have since moved to sky flats. Sky flats take a little more work, but with the right software it can simplify things. Firstly, when taking sky flats you need to point the telescope where there is minimum gradient visible. This immediately excludes anything low in the horizon. The ideal position is known as the solar circle which is near zenth, offset toward the anti solar horizon by 0 to 40 degrees. It is the best source of even illumination. This information is detailed from our good friends at NASA. I would recommend you and others considering sky flats download the PDF document from this link: http://adswww.harvard.edu/cgi-bin/np...&nosetcookie=1 (http://adswww.harvard.edu/cgi-bin/nph-bib_query?1996PASP..108..944C&db_key=AST&nosetcookie=1)
Once you’ve got the telescope pointing in the right area of the sky, the next challenge you face is the ever changing luminosity. At dawn, the sky is getting brighter and dusk, darker. Visually we don’t detect the changes quickly but a camera does. Now, if your camera exposure times are fixed you’ll have problems compensating for the varying brightness levels. Therefore you need to use software that will take an image, download it, analysis the background sky brightness. If the analysis indicates the sky hasn’t reached the saturation threshold you define, increase the exposure time until it does. Conversely decrease the exposure time if it’s over the saturation threshold. This is the main advantage of lightbox or t-shirt flats - the luminosity doesn’t change making it easier. I use a MaximDL plugin call Sky Flat Assistant - http://winfij.homeip.net/maximdl/skyflats.html. There is plenty of other software around that can help you out with the changing luminosity should you choose to go the sky flat track. I’ve blinked both t-shirt flat and sky flat and found the latter gave me a better result, but this has solely been my own experience.

Now with all this background information out the way, the best time to do sky flats... Typically when you’ve just hit civil twilight is a good starting point. You’ll quickly judge if its still too light or dark. If its too light, the shortest exposure time your camera is capable of will still deliver a saturation over your well depth threshold. If its too dark, you exposure times will be extremely long to reach the saturation limit. I don’t recommend going over 20 to 30 seconds. You can and people do it, but you’ll need to scale your dark frames accordingly.

One last think to throw into the mix, when taking sky flats you want the telescope to move in different directions ever so slightly. This will place stars that the exposure may capture on to different parts of the chip. When you combine the flat frames, the stars will be flagged as outlier pixels and removed. It doesn’t matter if the stars trail. You definitely don’t want stars in your flats! Again, software can assist you by move the telescope between or during exposures.

I think I posted this elsewhere, but this should get you going.

SO... in short to answer your specific question - Yes, the exposure time will determine the well depth saturation. Its not easy to do this manually as the sky is getting dimmer (dusk) or brighter (dawn). Hence my recommendation to automate it via software.

[jase]

Are you calibrating the guider before kicking off the exposures Steve? The software needs to know the X and Y access in relation to the guide star/mount combo. I've used CCDOps once and found it was doing my head in. Try CCDSoft or MaximDL if you're still having problems. For some reason I recall you posting a thread before regarding guiding issues, something to do with the guider cable or guider port in the mount was to blame. Can't recall. Have these been resolved?

[jase]

One last thing on sky flats (before I shut up), which specifically applies to mono cameras with filters (but possibly a OSC camera with a filter - unlikely). As you take flats frames through each filter (i.e. Ha, R, G, B) plan the sequence based on filter transmission properties. Obviously, a luminance filter is going to let in more light in a 10 second exposure than a Ha filter will (for the same exposure length).

So, for dusk flats, I typically go through Ha first (lowest transmission window), through to luminance last as by that time the sky is likely to be rather dark. (Sequence of Ha, R, G, B, Luminance - in order of transmission window lowest to highest). The reverse would apply if you're taking dawn flats - start with the luminance filter and end with the Ha filter as the Sun is rising (sky brightness increasing). Pretty obvious I guess. Finally, don't forget to dark subtract your flats!

[bluescope]

Thanks for the gen on flats Jase, I'll copy it and read it fully at a later time.

I got a new cable for guiding and as far as I can tell it works i.e. it moves the mount etc.

I like the look of CCDSoft which I got with the camera but it won't work on the notebook I use for imaging as it is not compatible with Vista. It runs on my old tower running Win98SE but I use that in the house for internet and so on. I do have access to another desktop running XP that may be compatible and when I finally convert my shed into a permanent setup I may use that for imaging instead.

I do run the callibrate function befoe imaging, it quite often tells me that there was an invalid movement in the X axis or something like that. Don't know what that means so I continue imaging with track and accumulate to at least get some images that are acceptable.

:thumbsup:

[bluescope]

Hopefully you're right ! I'd like to realize the full potential of the ST2000XCM which is by all accounts is a good camera. I've seen some excellent images from them.

:thumbsup:

[strongmanmike]

Nice set of images there.

Regarding sky flats...you can go down the pain in the "A" route Jase has suggested with sky flats or just get one of these:

http://www.pbase.com/strongmanmike2002/astrohandy_lightring

Absolutely simple to use, repeatable and you don't need to be restricted to dusk or dawn, besides and as I often found, what if it gets cloudy before you can take your sky flats and you must dismantle your imaging settup? - can't get matching flats then huh? :doh:

Sky flats work well for a permanent setup where your equipment is left assembled the same way for extended periods, thenyou can get the matching falts another dawn or dusk.

Just my 2c worth :)

[Zuts]

I just got one, as far as invalid movement is concerned I just hit calibrate again and usually it works. With CCDOPS you need to slew the scope so that a decent guide star is in the middle of the screen before calibrating then it seems to work better.

Paul

[EzyStyles]

WOOHOO!!! Nice going Steve!! you must be rapped. your another late nighter!! gee's, i have seen you keep improving and improving and thats what astrophotography is all about! well done on all your images keep them coming :)

[bluescope]

Thanks Eric, your encouragement is appreciated !





Thanks for the link Mike, I will check it out !


THANKS ALL FOR COMMENTS ..... I DON'T GET ONLINE EVERY DAY SO SORRY FOR TAKING SO LONG TO REPLY !!!!!