Camera Lenses vs ED scopes?

[Robert_T]

Warning, possible "dumb question" coming...

I'm looking at doing some piggy-back DSLR work and moving up to an ED scope. It occurred to me that there's not much difference in focal length between a lot of telephoto lenses and the smaller ED scopes - eg 400-500mm.

Now I know ED scope are good for eliminating CA... but what is the deal with most telephoto lenses... are they effectively like achromatic refractors or does the coating achieve similar CA inhibiting results

cheers,

Rob

[Suzy_A]

I've wondered about this myself.

I have a Canon 400 mm f5.6 lens - basically a 71.5 mm telescope. I have actually made up a eyepiece holder out of a rear lens cap and used the lens as a telescope.

As far as I can see, the main differences are the camera lens extra elements to reduce the actual physical length of the lens - it's only 256 mm long - and so that the focussing is all internal so the total length doesn't change when focussing.

My ED80 is about twice the length - and also was about 1/3 the cost.

I've been meaning to do some tests for CA and so one to compare the two - I'll see if I can do the in the next few daze...

[Suzy_A]

Hmm. Just went outside - it's clouded over in the last hour. Maybe tomorrow...

[dugnsuz (Doug)]

Well, this ones right up my alley so to speak!!

I recently sold my ED80 in order to fund the purchase of a Canon 70-200mm f4L zoom lens and 1.4x extender in order to possess a range of useable wide field focal lengths from 70 to 280mm. Factor in the 1.6x DSLR crop factor and the range is actually 112 to 448mm.

The lens shows minimal CA at f4 - perhaps enhanced by inaccurate focus when I tried. At f5.6 the CA disappears to my eye - without pixel peeping!
ED80 was excellent in the "no CA" department too.

Major area of improvement is that the stars in the lens shots are pinpoints right out to the edge of the field. I haven't needed to crop any of the pics taken with this lens.
The ED80 on the other hand suffered from major curvature at the edge of the field - a problem inherent in the doublet lens design.
I had to crop the images I took with my ED80 quite severely to eliminate this problem. I found that being aware of this prompted me to become more fastidious about composition (a good thing!) - the object HAD to be in the centre of the FOV or else!!!

This field curvature can be fixed by the purchase of a (not inexpensive) field flattener/reducer like the William Optics P-Flat2.

Also the AutoFocus function of the lens can be used to focus on brighter stars with great success - I was surprised that this worked. thanks to our resident Canon lens expert leon for the heads up!

I enjoy widefield imaging - I love to see DSOs framed by constellation stars as close to how I might observe them naked eye (if I had Ha sensitive peepers!!). That's why I went for the lens system.
My plan is to focus(!) on wide stuff with a modded 40D.

I reckon I will purchase another (ED)80mm scope down the line though.
They are fantastic scopes for the cash, but be aware that flattening that field is going to cost another $200.

Couple of Eta pics attached for comparison. The wider 280mm FL pic is not cropped whereas the 600mm FL ED80 is heavily cropped and still shows the stars as seaguls at the edges of the pic. I reckon Eta brings out the best in the ED80 though.

All the best
Doug

[Ian Robinson]

Camera lenses up to 80mm diam are more versatile (you can use them for happy snaps easier).

Above 80mm diam , camera lenses become prohibitively expensive and so ED or Fluorite APOs with field flattener lenses are the way to go. Unless you are a camera lens speed junky.

[Robert_T]

Thanks all, especially Doug for your detailed reply. That answers a lot of the questions swimming round in my head...I might be tending to your way of thinking too. In any case I'll want to get some piggy back work done before attempting longer focal length ed stuff.

cheers,

Rob