Opinions on this combo for astrophoto: 7D / Sigma 70-200mm f/2.8 / Sigma tele 2x

Hi there,

I've done astrophotography for a number of years with a simple achromat refractor and the tiny 350D, with very satisfying results. I've since upgraded to the 7D, and I'm thinking of doubling the focal length of my Sigma 70-200mm f/2.8 with the 2x teleconverter. I have heard many good things about this combo, apparently they're designed to work together and the results seem to be good. 400mm on a 1.6 crop sensor, that's very respectable, one can virtually shoot all the Messier catalogue with it. So it IS appealing.

However, all the info I've managed to find refers to regular daylight photography - sports, birding and the such. The praise seems to be based entirely on these sort of conditions. Astrophotography, as you all know, is a whole different matter alltogether: tiny sources of light scattered across the frame, long exposures, field rotation due to errors in tracking, etc.

The market is so twisted in Switzerland that it is virtually impossible for me to rent the teleconverter and play with it for a couple of nights to get a first hand impression. The price is also pretty big for what it is (as expensive as the sigma 50mm 1.4 if you can believe it) I thus come to you, ye wise forumers, with the hope that maybe some of you tried this lens with its 2x tele in astrophotography. I don't care much about chromatic aberration, what really worries me is coma (trail like stars in the corners of the frame) and barrel distorsion.

Some info would be much appreciated, photos would be great.

thank you


[I was unable to find any related post on this matter, so redirect me if I somehow missed it]

I'm surprised no-one's replied to this.

No experience of the lens or TC myself, but I'm sure there are plenty of photos on here from people using Sigmas with TCs, and from what I've seen they're all pretty good. A good lens is nothing without good tracking though...(talking about Astrophotograpy of Deep Space Objects I mean). A tracking mount can be anything from several hundred to several thousand pounds/dollars.

But you're right though, 400mm is a good focal length.

Hi, thanks for the answer - I was a bit puzzled myself, but it seems sigma users don't like astrophotography :P

I have a 3-2 EQ mount, with the AD motor (I somehow managed to burn the other one) that reasonably tracks up to 60s at small focal lengths (400mm).

On another note - this may be waaay too obscure, but here goes - I'm slowly moving towards spectrography... maybe one could help me choose between this:

http://www.astroshop.d​e …phe-dados-a-fente/p,15252

and this:

http://pagesperso-orange.fr/shelyak/fr/l​hires3.html

The difference is about 500 us dollars... and the price of the thing is about three times my budget, but one has to hope

From my experience with my 7D and Canon 70-200mm f/2.8L and 2x extender this past weekend, you should be fairly good to go so long as you have a good mount/drive. I only did a few M objects, but I very clearly - even from a city with mountains just to the west - got M13, M57, M8, M20, and M21. And I got hints at M51, but it almost being behind the mountains, a first quarter moon, and being in a city made that one more difficult.

Hi

thank you for your answer. the extender is in the mail, I'm hoping to get it soon.

took a look at your page, that's some nice photos you have there congrats.

L.

I don't have the Sigma but I do have a Canon 2x for my 500L. I rarely use it for daytime shooting due to image quality and focal ratio compromise but you raise an interesting topic. I'm curious about the 2x and nighttime use, especially for astro work. It's nice to have the option of long and shorter FL with the simple addition or removal of the TC but adding and removing it on a tracking mount could possibly introduce some balance problems and impact tracking accuracy. Any time you add or take away something on the fly it changes weight distribution which can be corrected with some inventive bias weights to counter the change. Just something to consider as you get further into it. There are so many things to get right for accurate tracking and balance is way at the top of the list if you aspire to stretch those exposure times.

If you haven't already I would also highly recommend looking into learning to drift align your mount. Once you've learned this method you'll find it very valuable and well worth the 30 or so minutes it takes to perform. Proper alignment and accurate balance will make your nights out much more productive and insure that you get the most from your 7D and 70-200. Thats' a pretty fast lens, it should do very well at f/2.8 under dark skies.

Hi Nighthound,

Proper balance is really important, I know. However I don't see why you would take the extender off during a photo session, you'd want all frames at the same focal length for proper stacking I believe.

Drift aligning is what I use, as my compass dates from 1955 and it was used to direct artillery fire.

Took a look at your page, noticed you photographed 17-P Holmes. Here's what I got with a 350D on an eq 3-2 with a 80mm f/5 achromat refractor.

HOSTED PHOTO
please log in to view hosted photos in full size.

ecce_lex wrote in post #10427663
Hi Nighthound,

Proper balance is really important, I know. However I don't see why you would take the extender off during a photo session, you'd want all frames at the same focal length for proper stacking I believe.

Drift aligning is what I use, as my compass dates from 1955 and it was used to direct artillery fire.

Took a look at your page, noticed you photographed 17-P Holmes. Here's what I got with a 350D on an eq 3-2 with a 80mm f/5 achromat refractor.

Very nice images.

I was referring to a change to accommodate different scale objects in one night. You would most definitely want all sub exposures identical in FL on a given target.

For the sake of others reading this I thought I'd add this link to a great drift align simulator. It helped me to better wrap my head around the technique.
http://www.petesastrop​hotography.com …gnment.html#dri​ft%23drift