Proper "rule" for pinpoint stars with M43

boxerman wrote:

OutsideTheMatrix wrote:

Just wanted to file an update- I am back to using 10 sec exposures at 14mm, the colors at 10 sec seem to be much better than the ones at 13 sec, I'm not sure why. I recently did a test of exposures between 6 sec and 13 sec and 10 sec came in first, followed by 8 sec and then 13 sec, 6 sec came in last. Also since I am stacking, 10 sec exposures much easier to determine total exposure for than 13 sec exposures are

You don't say f-stop or ISO, so I'm uncertain what your exposure actually was. Here's what I know from experience. With brilliantly clear skies (Kalahari desert), the following exposure will JUST about reach saturation for the brightest stars.

f/ 2.0, ISO 200, T = 30 secs.

If you expose more than that, you're into blowing out starts. That's not a big issue most cases, but if you want the colors of stars, you'd best stay below that. On the other hand, if you're photographing astro structure other than stars (nebulae, etc.), then you can go with a much greater exposure. There's no hope to have the same exposure cover both stars and other astronomical structure.

What software are you using for stacking?

I came up with f/3.5 ISO 1000 and 10 seconds shutter speed (that's just about exactly 1 stop below your values when summed and compared. Then again I am also dealing with light pollution here.) Regardless of the other numbers, reduced DR above that ISO rears its ugly head in paler stars. It's still fine up to ISO 1600 (and you can still see some star colors at ISO 3200) but star colors peak at ISO 1000.

I'm using Deep Sky Stacker for stacking and a program called Sequator to enhance dim stars above the noise floor. Both are free.

One other thing I noted is that when the moon is +/- 3 days from full its light overpowers the stars and they all appear to be white regardless of the settings used. That might also be because I was using Auto WB.  Once you're 4 days away from a full moon or more then the star colors come back (this translates to a moon that is 3 hours or less after moonrise or 3 hours or less before moonset.)

Question- to expose for both star colors and faint nebulae can I just use exposure bracketing and blend multiple exposures into the same frame? I wonder if stacking is good for that or if I need to use a different type of blending- perhaps HDR blending? I see that option in Sequator.

As an aside, what is the degree separation between Bellatrix (in Orion) and Sirius? I want to find the right focal length that gives me the tightest view of this area.

I'm sure you can figure this out from a star chart. I've never done this, so it'd take me a while to figure out. Surprisingly, I googled "angular separation between Bellatrix and Sirius" and got some hits! This one seems to say it's about 7 1/2 degrees, but I did not read carefully enough to be sure:

https://books.google.com/books?id=EVdLAAAAMAAJ&pg=PA26&lpg=PA26&dq=Bellatrix+to+sirius+angular+separation&source=bl&ots=ukT6ESCwFQ&sig=ehofH3Lpk4dbIZuAtZYlfD6LNOM&hl=en&sa=X&ved=0ahUKEwjRip7hsLjQAhVFxmMKHZqSD4AQ6AEIJjAB#v=onepage&q=Bellatrix%20to%20sirius%20angular%20separation&f=false

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The BoxerMan

I loaded up Starry Night and got a radically different result- 30.4 degrees angular separation and then ran that in a focal length calculator and came up with 20mm as the appropriate M43 focal length to use.

Oh that explains it- the link you presented states that the distance between Betelgeuse and Bellatrix is 7.5 degrees.