Examples of DSLR exposure times for lunar imaging

[JamesF]

I was struggling a few days ago with some images I'd taken of the Moon because they looked really poor after stacking and sharpening. I'd been experimenting, trying to get a better histogram fill for the images by increasing the exposure time, but instead I was getting "double vision" images with ghosting of many features. Eventually I realised that the reason the images looked awful was that the exposure was too long and the atmosphere was distorting subs sufficiently that several different copies of the same features were showing up in the stack.

Last night I deliberately repeated the process using exposures of 1/500th, 1/1000th and 1/1250th all at ISO800 with the 450D and 127 Mak. I took 120 exposures of each, stacked them using the same parameters in Registax v6 having picked the best reference frame I could and then applied the same wavelets settings to each. During the time I captured the images there was no obvious change in the seeing that I could determine. I've taken some crops to illustrate the differences.

First, 1/500th

And 1/1000th:

And 1/1250th

There's a world of difference between the 1/500th second and 1/1000th second images which quite surprised me. Clearly 1/1000th is the better choice of the two there. By 1/1250th however I think the ISO setting is insufficient to make up for the faster shutter speed and whilst they're very similar I think I'd say there's slightly better definition in the 1/1000th image and for the moment at least I think that's what I shall be working with for my lunar images.

James

[Jannis]

Nice comparison.

I usually stay with ~1/600 to ~1/800. Can't remember i've had any problems like you have there. That was with the 550d and 1000mm. i guess focal lenght have a lot to say here. You have 1500mm if i don't remember wrong?

I have, however, had problems with camera shake due to the mirror flipping up. I've noticed that using mirror lockup helped a lot on this.

[JamesF]

Yes, it is 1500mm. I'm using APT for controlling the camera, so that should be taking care of the mirror for me I think.

James

[billhinge]

When using my C11 at f10 (2800mm) I was imaging at iso400 and 1/40s but using a mirrorless camera so it seems strange there is such a big variation in exposure between yours and mine (I just tend to use the camera spot meter). I know when I used a x2 barlow it was quite difficult to get a sharp exposure plus I don't know the scale of your image? Maybe I should try iso800 since I used a little deconvolution on the above, its not as crisp as I wanted

Edit: Doh, just remembered, I didn't use a focusing mask when I took this

[JamesF]

I think that really does illustrate that experimentation is the way to find what works best for any given setup. Given the massive increase in aperture over my 5" Mak I'd expect the C11 to be comfortable at a lower ISO setting, but I'm really surprised that you'd get a clear image at 1/40th of a second exposure time. Are you just using single frames, or stacking?

James

[billhinge]

James

I'm just using single frames, I haven't been successful in stacking lunar images (I'm using a mac and Lynkeos the stacker on a mac doesn't seem to work as well with lunar as planetary)

[JamesF]

Ah, right. That may make it harder to get away with ISO800, I guess. I don't worry about the increased noise so much as I'm stacking a fair number of frames.

James

[butlermike]

Taking lunar shots with just a DSLR and lens on a static tripod, the 'advised' settings are a shutter speed of 1/250 or slightly faster, a sharp aperture of about f11 and a low ISO of 100.

Given that you're tracking then perhaps these don't apply but I don't see why not.

What aperture are you using?

[JamesF]

I'm not using a "lens" at all. The camera body is straight on the back of my 127 Mak. As it happens the scope is just under f/12, but I have a metre and a half of focal length. My experience with exposures even as slow as 1/500th is that atmospheric turbulence in individual frames causes distortion of the final image when I stack perhaps fifty or sixty separate images.

James

[butlermike]

Sorry I should have said I realised that you were using a scope but I was just providing an example of what is used in 'normal' photography.

i still would have thought that a single shot would have been sufficient but perhaps working at that long a focal length is a bit different as I haven't tried that. Good luck!

[JamesF]

I'll dig out a couple of images so you can see what a difference the stacking makes.

James

[JamesF]

Here you go. The first image is a single sub of 1/1000th used to create the image above. The second is the same 1/1000th image as above formed by stacking about sixty frames similar to the first.

Even the last isn't what I'd consider a "finished" image. My intent with this thread was just to get things far enough to show what a difference exposure time can make. I processed it separately, but this image is effectively a crop of a section near the top the image here:

http://stargazerslou...on-20-february/

James