Canon L lens vs Refractor for astrophotography?

[mbaddah]

Hi

Was wondering in terms of astrophotography, how much difference is there between the Canon L series lenses and a refractor ? I'm mainly looking at the 70-200mm F4 IS lens in comparison to ED80/ED100?

If they are very similar might opt for Canon lenses, can serve dual purpose for me in terms of photography :) Although can't use the Canon lens as a refractor (unfortunately) :(

Your help would be greatly appreciated thanks.

[bojan]

Price!
Canon lens will be much, much more expensive, for the same result (on stars).

[dugnsuz]

Sold my ED80 in order to buy the 70-200mm f4L so hopefully can help...

Purchased my L lens 2nd hand for approx $650, so roughly same ball park price as ED80

I would disagree that both systems give the same results on the stars though. The L lens is very well corrected, resulting in pinpoint stars to the edge of the frame. The ED80 suffers from field curvature which needs to be remedied by using a flattener/reducer which is an additional $200 cost.
Alternatively, you may need to crop out the offending stars.

The 600mm focal length of the ED80 will let you get up close and personal with your DSOs much more so than with the lens at 200mm. I used a 1.4x extender with the 70-200 to push the focal length to 280mm f5.6 for some shots.
Here are a couple of Eta shots, first at 280mm with the 70-200mm, 2nd done with ED80. Don't judge performance on the Eta lens pic as I this was done with subs I think were not 100% focussed (excuses excuses!!)
http://s327.photobucket.com/albums/k461/doug-robertson/?action=view&current=8.jpg
http://s327.photobucket.com/albums/k461/doug-robertson/?action=view&current=9.jpg
You can see the field curvature in the 2nd pic quite clearly

...and with the lens you can take killer daytime pics too...
http://www.iceinspace.com.au/forum/attachment_browse.php?a=43831

All the best with your decision
Doug:thumbsup:

[mbaddah]

Thanks guys for the response. Doug those pictures were beautiful, you are right though the shot taken from the 70-200 is definitely more corrected, I'm surprised! You have made my decision even more difficult now :(

Wouldn't the 200mm + 1.4x = 448mm on a 1.6x Crop body?

[dannat]

mbaddah - even though the cameras crop to 1.6X - they are providing the fov of the extended f/l - but they are not actually magnifying to that focal length - ie you are seeing 200mm f/l picture on the capture size of a 280mm f/l lens.

[bojan]

200mm is 200mm...

Focal reducer reduces this FL by a certain factor (1.4x for example), this is then 142mm.

Forget about crop factor. It only shows how much the field of view is reduced, compared with standard 36x24mm frame (because sensor is smaller in those cameras, usually 22.2x14.8mm).

[RB]

The Canon 1.4x extender that Doug is talking about is not a reducer, it increases the FL of the 200mm to 280mm.

[dugnsuz]

Not really!
Buy the lens:P
Doug:thumbsup:

[bojan]

Ah OK..
Then it will be 280mm.
But the comment on crop factor remains :-)

[mbaddah]

Bummer about the 1.6x crop factor not taking effect :(

I guess the ED80 would have 3X the magnification than as the 70-200mm...

Doug: Is that enough reach or will most objects appear quite small? Would hate it if I had to crop to get object to scale to decent size for viewing.

Thanks guys :thumbsup:

[bojan]

It is easy to calculate FOV for given focal length and sensor size..
For example, 300mm on Canon 400D has 254x170 arc minutes field of view..
Enter those values into CdC Eyepiece/CCD field and voila.. you will see exactly what will be on your image.

[dugnsuz]

Take a look at my photobucket page link in signature below.
Pics name: Moon, Eta Carina,Jewel Box, Orion Nebula, Horsehead, Rosette, 47 Tucanae and Tarantula Neb were all taken with the ED80 with, in most cases, minimal cropping.
The scope is great for larger nebulae - astronomical greatest hits like the Orion Nebula for example, but the results you might get on smaller objects such as certain galaxies and planetary nebulae might be disappointing!? Longer FL would be needed for these objects.

All personal taste at the end of the day - a widefield image of a few galaxies would look amazing after all...so being creative in composition will further one's enjoyment and use of a small scope like the ED80.
There's never a one size fits all scope in astronomy!
Doug:thumbsup:

[iceman]

Depends what type of objects you want to image.

Very short focal length like a lens is great for widefield vistas and some large extended nebulas will be great but still generally small on a DSLR chip.

Short focal length like an ED80 (especially with an (almost mandatory focal reducer/field flattener) is great for Orion, Tarantula, Eta and other large nebulas, but ultimately disappointing on galaxies and planetaries and some globular clusters and open clusters - just because they end up being quite small on the DSLR chip.

Long focal length (like a 8-10" newt or SCT) are great for getting in close on galaxies and planetary nebulas, globs etc, but place greater demand on guiding and you just can't get a widefield view if you wanted it.

Check my Deep Space Gallery (http://www.iceinspace.com.au/mygallery/index.php?cat=6) pages to see how Deep Space objects look on an image using an ED80 with 0.8x WO field flattener/reducer (with the exception of the widefield category, where they were taken with a camera and lens piggybacked - not through the ED80).

[mbaddah]

You guys have certainly broadened my horizons and i'll need to think hard about what to purchase. Most like I'll be purchasing a HEQ5 Pro mount (which I assume can cater for both ED80 and Dob up to 10"?).

I am leaning towards the ED80 (possibly even an ED102) and a piggy back system for my 400D, and over time I'm sure i'll come ot know what images I'd need to cater for most. Will add 8" tube down the line :)

Appreciate all the help :thumbsup::D

[iceman]

Can your budget extend to the EQ6? There really is quite a difference between the size of the two.. bigger is better!

[dugnsuz]

Yeah I know, bojan put me right on this a while back.
Way I get it straight in my mind is by a simple thought experiment...
I've got a Canon 5D (full frame sensor) and a 40D (1.6x crop sensor) side by side, both with 200mm lenses. I take identical images of the full moon - on both cameras the size of the moon is the same size, but the image from the 40D has less sky around it!
On the 40D, the Field of view (not magnification of object) is "AS IF" I had taken it with a 320mm lens on the 5D.

Does that make sense or have I confused you?
Hope not
Doug:thumbsup:

ps...if you're planning to put a 10" and an ed80/100 +dslr and associated hardware on a HEQ5...get an EQ6!!!