Help with Stacking (Registax)?

[MacDogg]

I hate the Yahoo! Group setup and how their discussion board works so I was hoping someone here might give me a little help. I took about 250 two second shots of Lulin last night and on each frame I cannot see the comet, but I can see the three start that surround it so I know I have the right part of the sky.

After Registax processed 50 images I couldn't see anything but I used auto-contrast in PS CS3 I can see it is there. It is just below and to the left of the bright star.

Do I just not have enough exposure to see it in the stacked photo? I tried to use all the images in Registax but it kept losing the alignment points. Do I need to add a completely black image into the jpg's I import?

If I used a four second exposure it started seeing trails in the stars so that is why I used two seconds.

Here is the picture from Registax, cropped to a smaller size.
Any comments and help is appreciated.

[Nighthound]

It's a nice effort but you're simply not stacking enough data per image to help the cause. It's important to remember that while stacking improves the signal to noise ratio, it cannot create signal. Increasing your ISO setting will help some but at such short exposure it'll be minimal. I can't tell specifics from your file but I'm assuming this was at over 100mm FL if you're getting movement at 2 seconds. Shortening the FL will help extend exposures as well but at the loss of scale. You could look into building a barn door tracker if a tracking mount isn't in the budget right now. Tracking in Sidereal will make all the difference.

[MacDogg]

Thanks, Nighthound.
Focal Length on the CR2 files say 155mm and ISO was 400.
I'll see if I can get more images into Registax. The problem here is when the stars move from top/left to bottom/right and starts over when I move the camera, with the stars back in the top/left, it doesn't grab the right alignment points.

[troypiggo]

My take in layman's terms: by "not enough data", Steve is saying that there isn't enough photons actually hitting your camera's sensor in the 2sec exposure time. Stacking hundreds of images together where only a few photons on a few of the images, there's still not enough to get a clear enough image.

Increasing the ISO will make your sensor more sensitive to light, but you still need the shutter to be open long enough for the photons to actually hit it. Particularly Lulin - I haven't imaged it, but it seems to be quite faint from what images I have seen.

Without the tracking gear, and unless you want to widen up the field of view to reduce the apparent trailing, you'll need to up the ISO a lot and the penalty will be more noise unfortunately.

[Adrena1in]

Exactly, not enough exposure time per frame. You could stack a million 2 second exposures, and you won't get the sort of data you'll see from a single 1 minute exposure. I personally don't understand why not. I mean, those photons are travelling through space, and I would think that opening the shutter for a minute will capture exactly the same data as opening the shutter sixty times for 1 second each time. But sadly it doesn't work that way.

For 155mm focal lengths you really are going to need to sort of some sort of tracking. Either a Barn Door Tracker (http://en.wikipedia.org/wiki/Barn_door_tracker), a cheap mount like an EQ1, or splash out on an expensive mount which you can continue to use when your passion for Astro Photography really takes hold of your bank balance. ;)

[hollis_f]

Exactly, not enough exposure time per frame. You could stack a million 2 second exposures, and you won't get the sort of data you'll see from a single 1 minute exposure. I personally don't understand why not. I mean, those photons are travelling through space, and I would think that opening the shutter for a minute will capture exactly the same data as opening the shutter sixty times for 1 second each time. But sadly it doesn't work that way.
I very much doubt that the sensors on the CMOS are sensitive enough to pick up single photons - or even hundreds of them. There must be some threshold number before that particular photosite registers a 'hit'. If the shutter speed is too low to reach that threshold during the exposure time then that photosite will register as zero, or black. Adding together a load of zeros will just result in zero.

If exposure time is limited by trailing and aperture is limited by the lens then the only option is to up the ISO and put up with the extra noise.

[MacDogg]

Thanks for the info, everyone.


For 155mm focal lengths you really are going to need to sort of some sort of tracking. Either a Barn Door Tracker (http://en.wikipedia.org/wiki/Barn_door_tracker), a cheap mount like an EQ1, or splash out on an expensive mount which you can continue to use when your passion for Astro Photography really takes hold of your bank balance. ;)

I see on Amazon they have these three pieces of hardware for about $205. I have to double check the thread on the XTi is a 1/4-20 but it looks like the right mount size.

Orion EQ-1 Equatorial Mount
Orion EQ-1M Electronic Drive System
Orion 1/4-20 Adapter for EQ1 Mount

I almost wish now that I didn't puchase this XT8 Orion Dob telescope and went with a motorized one right from the beginning. The price at the local shop (now closed :( ) at $400 about 3 years ago was too good to pass up.

[Adrena1in]

I have to double check the thread on the XTi is a 1/4-20 but it looks like the right mount size.

I'm sure it is...that's just the standard thread for pretty much all cameras and tripods, isn't it?