Digicam resolution and compression

[AllanJ]

I'm a newcomer to imaging with my NexStar 4SE, but since I don't have a DSLR, or a laptop to take outside to drive a webcam, I am using my digicam - a Canon Powershot A85 which is 4 megapixels, and records in jpeg and avi only) for afocal photos. Having already started to play with avi (Saturn), I'm now trying to learn how to take still shots. I think I have got the hang of various camera-specific settings (max exposure time is 15 sec), but one area that I can't work out is what is the most appropriate image resolution and compression.The Powershot avi movies get by with 640x480 and Registax is happy with these. For stills, I have so far tried using the camera's "Large" (2272x1704) resolution and "Superfine (High Quality)" least compression - i.e. the biggest files, rather than the Small (640x480) resolution. So am I right in thinking that the 4 megapixels of the A85 are recorded approximately one to one in the Large resolution, but would be cumulative in the cells per image pixel in the Small resolution [that is, 4 million divided by (640x480)= 13 and a bit]. Intuitively, I think that the largest is "best" but I don't know if smaller images would be an advantageous use of the resolution. Also, I don't know what effect the jpeg compression has on an image. Any advice that doesn't require a PhD in physics/optics or whatever will be appreciated - just telling me to get a webcam or DSLR won't do!

Allan

[blinky]

It is best to use the highest quality setting/low compresion option. The lower resolution images dont do anything clever like adding the 4 pixels together into a supert pixel. As far as I know a lower setting in resolution just makes the camera delete the extra pixels.

JPG compression should always be set to the highest quality as this will retain as much of the fine detail as is possible with your camera.

Some folks have taken very high quality pictures with similer camera's to yours so dont be too downhearted about not having a DSLR.

[AllanJ]

Thanks, Craig.

Downhearted at not having a DSLR? Me? Never - I'm having too much fun with my existing kit.

Allan

[rawhead]

Downhearted at not having a DSLR? Me? Never - I'm having too much fun with my existing kit.

Allan

Just looked up your M13, it's amazing what can be achieved without taking out a 2nd mortgage.....

More pics please

[themos]

There is CHDK, a software enhancement for Canons but unfortunately your model is not supported.

http://chdk.wikia.com/wiki/CHDK

[Whippy]

Allan, I had been using a Powershot A610 for afocal work and I was using the highest quality settings for my shots. Not sure how your model works but after setting your ISO, aperture etc etc I used the timer to give me a 10 second delay after pressing the shutter button (time for any vibrations to die down and step well away!) and give me 10 shots in sucession. Do this as many times as your batteries will last , then you can stack the results in DSS and process in the image software application of your choice.

One thing you may want to look into as well would be a focal reducer of some kind to get a faster focal ratio.

Tony..

[AllanJ]

Tony - thanks for your tips on settings. This is pretty much what I've been doing. I use a 10 sec delay, and have a cable release bracket so there is time to let that go and let it stop swinging (I keep the "heavy" end downwards); then take up to maximum available 15 sec exposure. I also set 8 sec review time in the LCD to see if I can spot the object at all (!), or it drifting in the view. Then I sit and take lots of shots. I'm still learning on this. I took over a hundred last night, but I think most of them are out of focus.

Allan

[Whippy]

Allan, try finding a double star (Albeiro would be a nice easy one atm) and get your focus on that before locating your intended target. If you split the double and get the stars nice and round in the viewfinder, then you're pretty much there. Don't forget to take a few shots of the double too, a good colour contrast double looks great in pics .

Just out of interest, how are you connecting the camera to the scope?

Tony..

[AllanJ]

Tony, thanks again for advice. The camera is connected as follows:

Canon Conversion Lens Adapter LA-DC52C

Scopetronix Digi-T (from Green Witch)

SRB-Griturn cable release bracket (from Pennine Online).

Because the Digi-T fastens onto whichever eyepiece I use, with 3 grubscrews, once I get it focused with that eyepiece then fit the Digi-T, it can get out of focus again, and the camera LCD monitor isn't up to the task of checking focus in detail.

Allan

[Whippy]

Gotcha Allan. You should be able to connect it all together and the focus using the view from the LCD when you're looking at a double star. Even better would be the trapezium in M42 but you might have to wait a while for that!

Tony..

[AllanJ]

Tony - I'll try this next time I'm out.

Ta, Allan

[dph1nm]

I take trial shots to focus with my digicam. It helps I have a helical focusser on one scope but the other is just a standard rack and pinion. I find a rough focus - either with the LCD on a bright star or just from knowing where it was on previous occasions. I then move to a field with lots of stars (e.g. an open cluster) and take, say, 7 shots, of either 8 or 16sec, starting from one side of the rough focus and moving to the other in equal(ish) steps (with the heilcal focusser I just rotate e.g. 45 deg each time). I then take the memory card out of the camer (leaving the camers attached to the scope) and go indoors to look at the shots. I choose which shot has the best focus, go back outside and rotate the focusser back to position that shot was taken at (or as near as I can remember!). I then take one shot to check. If I am not happy I repeat the exercise with finer focussing steps.

I do this at the start of the night and do not change focus after this unless there is a good reason to do so.

I find it a good idea to pay attention to the stars at the edge of the field - I find these are the most sensitive to focussing errors.

NIgelM

[AllanJ]

Nigel - you've given me a good idea there. Not sure my wife will take kindly to me tramping up and downstairs to the computer in the middle of the night, however.

Allan

[Ant]

I used a pentax EI2000 for a long time. This was only a 2.2 mega pixel camera. But it had a good screen. I tended to take images with maximum optical zoom - but use the digital zoom (which didn't change the focus point) to check focus.

I got (what I think) are excellent results. Here are some of them...

I sat hunched up under a branket for about 7 hours for that last one.

I never had any luck with long exposure stuff with my digicam, it went to 16 seconds, but the noise (electronic) was awful. So I never went any further.

Ant

[AllanJ]

Ant - impressive stuff; a bit beyond me at the moment! Optical plus digital zoom focussing sounds useful, thanks.

Allan