Proper "rule" for pinpoint stars with M43

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?

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