Camera to 80ED Refractor

Hi all, I've just got my Celestron 80ED delivered, and has now been put together, but I have one question : I plan on doing some astrophotography with this, I have both a t-ring and a t-adapter which couple together nicely. The t-ring mounts on to my 300D easily.

I notice that I can put my t-ring / t-adapter combination straight into the 1.25 inch barrel adapter and attach the camera. But also I find i don't have to use the t-adapter at all, I can easily screw the t-ring on it's own into the 2inch barrel adapter after removing the 1.25 inch barrel adapter.

Which is best for astrophotography? I know focus would be an issue for starters, what about vignetting? Just asking what's best as I have'nt had a chance to take any shots yet.....

Thanks, Keith..

Keith - try both setups - 1.2 and 2" - see which works best. I'd say you could probably do it during the day.

If you have minor vignetting, you can take flat-fields to remove it. If its more serious, then its er, more serious...

Give it a lash and see which gives better results.

Look forward to seeing some more lovely wide-fields from you!

Cheers

Dave

Thanks Dave!

If you have minor vignetting, you can take flat-fields to remove it.

How is this done exactly?
Keith..

Oh no, exactly?

OK - this is what worked for me... Take a clean white t-shirt and place it over the end of the OTA. You can secure it with an elastic band, sellotape, string etc. Doesn't have to be tight but avoid wrinkles.

Make sure you have a fair thickness of cotton - at least two layers.

On a dull day you can point the assembly at a uniform piece of ground or a wall (I used the lawn). On a bright sunny day, you're better of waiting till the sun starts to go down.

Take a bunch of shots (say 10 for luck). For CCD's the recommended exposure time is to the point of about 50% saturation of the chip. For the DSLR I'd just make sure the image didn't look over or underexposed. I guess a neutral (50%) grey look would be fine.

If you have severe vignetting, you'll see that immediately - like looking into a tunnel. If its minor, it may not be even be noticeable. You may also notice dust donuts - don;t worry - that's fine.

You should then either average or median combine the 10 shots using software. This creates your master flat. If you want to go the whole hog (which in your case you needn't) you could take flat darks and subtract those from the master flat. And you could throw in some bias frames for luck too.

You then subtract you master flat from all your subsequent images. This will remove all but the worst optical-train anomolies. Voila.

Beware - if you change the opotical train, you'll need to shoot more flats. 'Change' includes moving the imager, using filters, cleaning the optics etc etc. But if you don't disassemble the optical train the master flat will last a good few weeks (when the dust donuts will appear in ever increasing numbers).

Don't be disheartened - its sounds complex but only takes a few minutes. The results are well worth it and WILL make a difference to your images.

HTH

Dave

PS - avoid using your favourite hard rock band t-shirt or all your images will have Metallica all over them (well, in reverse anyhow)

Keith,
My feeling on this (and experianced this first hand) is that it is better to have as wide a barrel as possible leading up to a camera such as the 300D, I get more vignetting if I used the 1.25" barrel.