Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 10/20/2023 - 19:45
I went back and watched the AAVSO Webinar on using DSLRs and the presenter, Dr. Barbara Harris, talked about her experience using a Canon 400D (IIRC) and the 18-55mm zoom kit lens to take images of bright stars with no tracking. I have some questions additional questions to ask anyone in the community who ever used camera lenses for photometric work.
- How long in focal length or how wide of a field of view is too much? Is there a limit to how area is covered by the camera?
- According Dr. Harris, you need at least 10 seconds of exposure time to get accurate DSLR measurements? What is the longest focal length you ever used without any tracking at all.
- Even at low focal lengths, there is some visible elongation of the stars. I tried to take images last night with my nifty-fifty and was still getting stars that looked like tic-taks after taking a 10-second exposure unless I defocused by quite a bit, which seem to make all of the lens aberrations that much more visible, which leads to the next question.
- What lenses do you use? Did you ditch the stock lens because the results weren't suited for photometric measurements and what lens did you buy?
- And finally, did you have to stop down your lens to correct for aberrations and how much can be tolerated without distorting the photometric data?
I may be doing a bit of "arm waving", but will give it a go!
1) 10sec minimum exposure means total time for stacked images. So stacking 10 or 15 1 sec images should correct lack of tracking issues.
2) For individual exposures, I would use the rule of 400. Divide 400 by fl of lens for max single image time in sec that doesn't show significant trailing.
2.A) A little tracking error is OK if you can make measurement circles sufficiently large to capture the entire chicklet shaped star
3) Do you need the corners sharp?? Are there check or comp stars there?? If not, don't stop down. Or stop down 1 stop to allow focus "slop".. Aberated stars should fit within measurement circle.
4) Regarding focal length use any fl that allows star images sufficiently far apart for measurement.. If they are too "bunched up", its difficult to measure.
In general I'd say use what ya got. Try it out! Learn to calibrate images with at least darks and flats. This is also good for Deep Sky imaging with your camera. I got several killer images of comet Neowise with a 180mm f2.8 tele on my Canon 6D sitting on a tripod using stacked 2 and 3 sec exposures.
If you don't give it a go, you'll never know. It's a learning process. My present images are 1000000% better than my first.
And let us know how you do! Since we can't post images here, I suggest posting your images for critique on Cloudy Nights Scientific Amateur Astronomy forum. Some of us look there regularly..
Peter
BPEC.
Hi, I’m not suggesting that you spend money yet, but I can give thumbs up to the Rokinon 135mm F2. Being a prime, it has a bit more tolerance to defocusing before getting “kidney bean/donut” looking stars and as mentioned it gives enough separation to allow for picking your variable and comp/check stars with good confidence. The F2 -F2.8 on this lens gives very good stars near centre, F4 giving telescope like performance with tracking if you ever decide to add that later. Great price/performance ratio!
I would like to add two points:
a) consider giving used vintage lenses a try. Back in the days of chemical photography, some really good lenses were made that you can now buy on ebay for really small money. You'll need an adapter ring to make them fit to your modern DSLR, and they won't have some fancy features of modern lenses but you need none of those anyway for astronomy/photometry (no auto-focus, only manual aperture stopping, no image stabilization, no chip to talk to the camera ... bla bla). Quality lenses for the Pentax-K-mount were sold in huge quantities in the 70s and 80s and fit snuggly to Canons with a relatively cheap adapter ring (but don't buy the cheapest on the market). Just as an example, look at this here: https://www.pentaxforums.com/lensreviews/SMC-Pentax-M-50mm-F1.4-Lens.html and compare the price to a modern 50mm, F1.4 lens. And it's great for portraits anyway.
b) once you have fully explored the options without tracking, you might want to move on to longer focal lengths. I really like those mechanical tracking thingies like the Omegon Mini Track LX 2 / LX3
I agree that the manual focus Samyang lenses have excellent price/performnace ratio, but for 135mm focal length you will probably want to have tracking.
CS
HBE