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M31 with a Sigma 70-300mm lens

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#1 joelimite

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Posted 03 November 2013 - 03:06 PM

I've owned a Sigma 70-300mm lens for a few months now, but I've never used it for astrophotography. Last night, I had a go at M31. Unfortunately, I had some serious dew problems, and I wasn't able to collect nearly as much data as I intended. Overall, though, I'm pretty satisfied with the result.

Here are the technical specs:
Taken with a Sigma AF 70-300mm f/4-5.6 APO DG at 200mm, Canon T3i DSLR, and Celestron Advanced VX mount. Consists of 35 light and 35 dark frames, each a 90-second exposure at ISO 800, and 21 flat frames, stacked in DeepSkyStacker and processed in Photoshop.

Posted Image
High-resolution version here: http://www.flickr.co...in/photostream/

#2 epdreher

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Posted 03 November 2013 - 03:23 PM

Very nice work, sir!

#3 CounterWeight

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Posted 03 November 2013 - 03:33 PM

Looks very good to me, but a lot of issues show on larger scale with the stars.

#4 joelimite

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Posted 03 November 2013 - 03:37 PM

Thanks, epdreher and Jim. Yeah, I'm not sure if the elongated stars are due to the lens or tracking issues. I'd intended to take 60 or so exposures and use only the best of those, but the lens severely dewed up after 35 frames, so I went ahead and stacked them all, even those with less-than-perfect stars. I need to come up with a solution to manage dew with this lens.

#5 Wouter D'hoye

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Posted 04 November 2013 - 03:51 AM

Hi, I tend to agree with Jim. great image at reduced scale, but stars are less than perfect on original scale. I think the issues are partially due to the lens itself. Very few zoom lenses are stellar performers. Dew can be resolved by making a simple dew heater. that wraps around the lens.

Kind regards,

Wouter.

#6 joelimite

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Posted 04 November 2013 - 08:23 AM

Thanks for the comments. This image was taken at f/5. I'm going to stop it down, say f/6.3, and see if I get better performance. That should improve its performance on stars, correct?

#7 Raginar

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Posted 04 November 2013 - 08:28 AM

What they said! Good luck.

#8 Thirteen

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Posted 04 November 2013 - 09:49 AM

No expert here, but I have some experience with this which may help. I tried some imaging with my 18-270 tamaron zoom. I wanted the reach and it is my longest lens. I found with the barrel fully extended and unsupported there was enough flex to give my stars the same shape as I see here. At least, I felt it was some sort of misalignment in the optics. It is something I have not seen in normal daylight photography. I tried stopping down also but it just never eliminated the problem. I came to the realization I should stick with prime lenses....something that folks reiterate here all the time. And....if you search, it seems that not much look better than the 200 f/2.8L.

That being said, your processing, acquisition and overall look is great.

#9 joelimite

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Posted 04 November 2013 - 10:37 PM

Thanks, Thirteen! I'd love to get the Canon 200mm f/2.8L someday but can't afford it for a while. (Too much other astrogear I want to get first.) In the meantime, I'm going to try 150mm and f/6.3 or slower to see if I can get better stars. I saw a couple shots with the same lens at f/9 and the stars looked great.

#10 kbev

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Posted 05 November 2013 - 02:20 AM

Very nice image, makes me wish this sudden storm system passing thru the area would go away so I could give it a try.

On the star issues what f-stop where you shooting with? If you were shooting wide open or close to it then you might want to try going down a stop or two to see if it resolves your problems. Doing AP with a regular camera lens is a great way to see all the aberrations that don't normally present themselves in terrestrial photography thanks to all of those point-source lights in the sky :p

#11 joelimite

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Posted 05 November 2013 - 10:42 AM

Thanks, Kevin. This was at f/5. I'm going to stop the aperture down a stop or two next time to see if I can eliminate some of the abberations.






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