I had not used it for a long time and never with a guided mount. So I tried the Sigma 70-200 once more - and I think for the last time.

Though the lens it is excellent concerning CA and speed it is not possible to get the stars sharp, even with a microfocuser and stopped down. All the stars - even the less brighter ones show a blur and smear to the left that cannot be eliminated. The dim galaxies are less affected and so the test resulted in an image I really like: The central part of the Virgo Cluster with a lovely bow of galaxies. It is a stack of 100 Pictures of 120 seconds, F4.5 ISO 800. The Color was lost somewhere during the processing - I don't know where....


The Astrometry evaluation if the image names so many objects that the screen is green. Therefore I named the most prominent galaxies but there are hundreds more



I will furtheron use the SMC Pentax-M 200/F4.0 that shows a bit of fringing but results in a sharper impression.


Edit: Forgot to say that I tried another method of stacking with DeepSkStacker: Within DSS I divided the reference picture into 6 separate parts that I stacked with the 2xdrizzle option. Then I stiched a panorama of the 6 single images. So the final size is 9.400 x 6.300 pixels and the Image shows much more Detail than the normal stacked one. For the image above I extracted a part with 4177 x 6269 pixels to get most of the galaxies into a smaller frame. The numeration was evaluated with astrometry.net and texted manually in Photoshop.

Impressive! Nice result.

Thank you!

impressive indeed !
I just cannot imagine the amount of work, PP and gear behind such a nice image... also If I would get this result with a sigma 70-200 i'd be very happy!
Wondering what could be done with my fa 250-600mm...

Nice work, Pete. Re. the blurry stars: an absolutely essential step of any stacking session is to wean out the blurred or otherwise crappy frames before you do the stack. Otherwise they will be integrated into the stack and mess it all up. It is often the case that there is a periodic error in the tracking mount which introduces a squiggle or jump in a frame that must be tossed. Even something as simple as coughing next to the equipment will induce enough vibration to be noticeable. So if you have not been doing this, please do so. And be vicious in your selection. You only want to use as perfect frames as possible.

Thank you!

It's not that I am unhappy - I like the overall result. But I am disappointed because of the blurry stars. I always had then in past experiments with the lens but had hoped to get rid of the problem using a microfocuser and an autoguided mount.

The gear I use is not an overload I think. I use standard lenses, a small cheap mount (Skywatcher Star Adventurer) with a tiny autoguider (Lacerta MGEN II) that works easy and very effective. I think I could reach > 10 minutes exposure with a 200 mm lens without trailing stars.

Most of the PP can be done schematic and most of the work is done by the stacking software. I let the program calculate while I am away from keyboard. The "creative" part of PP in Photoshop normally needs half an hour or so. This picture was a project for a whole evening because I produced sectioned images with 2x drizzle that had to be calculated, stiched and then processed in Photoshop.

Concernig your thought of using the fa 250-600 I would say that it is a bit too slow and too long for the Virgo Cluster. And you would need a tracking mount that gives you undistorted 2 minutes exposure or so at 250 mm to catch enough light at reasonable ISOs in your series .

Yes, I agree with you concerning possible reasons for blurry stars. But I did my best in the past to eliminate them as good as possible. I had carefully sorted out crappy frames and the variance in the remaining pictures I finally used was small. The stars all show some kind of distortion flares pointing to the left and downwards - in each of the pictures and at each star (maybe ist a bit better at the center of the images).

The periodic error of the mount was my problem in the past. I was limited to about 60 seconds exposure at 400 mm with about 40% pictures to discard as a resullt of PE. Thats why I invested into the MGEN II that even works fantastic with my mini-mount Star Adventurer. With a good polar alignment I have no visible trailing within the limits of exposure time that are given by light pollution.The 2 minutes exposures of this image with 200 mm did not lead to any trailing.

To get rid of vibrations and other imperfections resulting from using a tripod in the past I have the mount fixed to a massive steel shaft screwed to a massive balcony pillar. So for 200 mm Setup the only noteworthy vibration could have com from wind gusts but it was a quiet night.

In the end I conclude that the lens is to blame - it does not tolerate pointlights. But it is a great fast lens for sports and indoor photograpy and I will keep it in my camera bag. To be sure which lens to use when it comes to 200 mm wide field astro images I will compare it directly with my SMC Pentax-M 200/4 next time.

I use a Pentax-M 200mm f4 and a Pentax-M 135mm f3.5. Both are optically excellent. Other lenses I have tried suffered form coma.

The Pentax-M 200/4.0 is fine if focused very good, same as the 135/3.5 I also have. Out of focus mine show heavy color fringing.


This is one I found today that I made with the 200/4.0 (costed 75$ or so) on the Star Adventurer about 2 years ago (F1:5.6, Iso 1.600, 91x120 s). At that time I had not yet understood the possibilities Photoshop offers for PP. That is why all the stars are fat and white and the galaxy is dim and washed out. I wish I had saved the subs (I don't know why the hell I have not ) to test what would be possible today.