DSLR long lens - any good for astrophotography?

[Leedsgreen]

Dear All,

I have been fortunate to 'borrow' an old Canon zoom lens from my dad , which reaches to a FL of 300mm. Given that there is a scale factor of 1.6x on my Canon 450D, I imagine this will work at effectively 480mm (and with the fact that the camera has 12 megapixels, means there will be room for cropping). Placed on a good tracking mount, can anyone tell me how this set up would work with say DSO like M31? Would lack of aperture affect the depth and resolution of the image, just like normal, or would the sharp optics of the lens make a good difference? I suppose what I'm asking is, how would this compare to a 5" APO refractor or even larger well colliminated

Thanks for any advice,

Nick

[Merlin66]

Should work reasonably well. What's the fastest f ratio available on the lens?

Setting to 200mm f3.5 (or better) will give some interesting DSO results. Camera on ISO 200 and 2-3 min subs.

[Leedsgreen]

Mmmmm, not to sure without it at hand but I don't think it is that fast. Certanly over f5 at 300mm but probably quicker, as you note, at 200mm. If the tracking on the mount is poor and limited to 5-10s exposure, is there little one can do, even at ISO 1600?

Thanks again,

Nick

[steppenwolf]

I use a Canon 200mm 'L' Series lens for wide field astro imaging and it works OK but despite its excellent optics for normal use, it does not fair that well astronomically despite having some so called 'UD' glass elements. Most normal camera lenses produce a fair amount of chromatic aberration but having said that, they can take some really good wide field image not available to ordinary telescopes so well worth experimanting with.

[skye at night]

You will get the FOV of a 480, but not necessarily the focal length,,

Steve

[Merlin66]

Going to ISO 1600 doesn't help. Justs builds up the noise signal. Stick to ISO 200 or ISO 400.

More subs at shorter exposures, then stacked, will help.

Back in the Film days, we used Zuiko 200mm f4 lenses with 20 min exposures to get some very good shots.

[Leedsgreen]

Thanks for advice everyone, I wonder, are better results likely to be obtained from a (cheapish) 4" or 5" achromatic refractor?

[skye at night]

I wouldnt rule out the lens since you have it...

You said, reaches 300mmm so presumably not a prime... these are better for AP work....

Still worth a go, advantages are you can make settings adjustments on the camera as you go, maybe autofocus on a star if fast enough optics...

I am presuming not too fast a lens, but even the best 'L' (canon pro lenses) need stopped down a notch or so to reduce coma...

i used a 135mm L f2 lens to some success (before acquiring a bigger brother) but it had to be stopped down to f4 to reduce coma to minimal or none....

Thanks

Steve

[Sparrow]

I'm hardly an expert but my advice would be - if you've got it, have a go with it - try different ISOs - Whilst I don't disagree with Merlin66 about more noise at ISO 1600, plenty of subs will reduce the noise substantially - try a compromise at 800 ISO.

A the end of the day it's a digital camera so it's not like your wasting film - so give it a try

[themos]

I plan to focus on M31 with a 300mm f/4.5 prime lens, it probably has about 66mm of aperture. Gives me about half a thumb at arms length field-of-view, 4x2.5 degrees, roughly.

[laser_jock99]

When using zooms for astro-photography you need to be aware of 'zoom creep' i.e the lens changing focal length during exposure. Most likely to happen if the lens is pointing straight up (or down). Prime lenses of course don't suffer this and also have simpler lens designs less likely to suffer from abberations.

[skye at night]

300mm is a good length for m31...

just check a few early shots for coma and stop down if not happy..

Steve

[Leedsgreen]

All,

Thanks very much for helpful tips (particulalrly with zoom creep, which I know can be a problem). When I manage to give it a try, I'll let you know the results!

Nick