Thank you all very very much for the prompt and very helpful responses. There is a reason that the forum and the community have their reputation of being very helpful and tolerant to novices.
Quote: I take it that you're not using a Pentax for which AstroTracer is an option?
I was getting pretty good results at ISO 200 for ten seconds at totality with an f/4.8 optical train, so you can math that for exposure :-).
I have K-5 and now I have a second hand O-GPS1 so will definitely try it next time, though it will be in 2021
for my country.
When I succeed to set the O-GPS I get decent star shapes at least in the middle of the image with HD Pentax-DA 55-300mm @ 300 mm for a 10-15 secs exposure.
Will not be able to practice on the Moon meanwhile as the exposure will be too long when not in eclipse. Or maybe I can try with an ND filter.
The problem is that sometimes I just can't set the O-GPS good enough. Even if I had it with me the last time I would not have had the time to set it so my formula will be used as a backup plan next time.
Quote: Well, you are certainly putting your math (maths) skills to work. But, IMO, you are killing a fly with a bazooka.
I was really upset after the second time.
Actually, the formula becomes quite simple when you replace the pixel size with it's value for a given camera. For K-5 (with a 4.78 microns pixel pitch) it becomes 63.4/F (if moon orbiting velocity is taken into account) so I just need to remember 64/F (63/F will be better but I find 64 easier to remember and divide) and take the closest round shutter speed.
If only the sidereal movement is taken into account then it is 65.7/F (or 66/F rounded).
Quote: However, my own feeling is that to ensure a truly sharp image we should expose for no more than half of that.
Quote: Half a pixel will be sharper.
That's very good to know, could be useful for other situations with fast object movement.
Quote: You just need to remember that on the equator the stars move at a speed of 15"/s so a 400 mm lens with a 5 micron pixel size will have a 1/12 s exposure limit, each pixel having a 2.6" fov so a 1.3" tolerance.
So, I need to take half the calculated exposure time by my formula for the sharpest possible result and I can use the formula as it is (or even increase the exposure a bit) if I don't need single pixel level sharpness.
Quote: Note that 1.3" tolerance is most of the time sharper than sky seeing will allow (at least up here in Quebec) so you don't need to adjust for smaller pixel size.
Does this mean that there is no much point of using more than F=400mm lens or it is only valid for calculating the exposure? I have in mind DSO, not just the Moon.
Quote: During totality tracking can be done with astrotracer if you have one. By turning off "Timed exposure" you will have control of how many seconds you want to expose.
Quote: To be clear, the only advantage to turning off timed-exposure mode in this scenario is that it allows exposures of less than ten seconds, which is the minimum in timed-exposure mode. But if ten seconds is too much just reduce the ISO and or increase the f-ratio. Other than allowing exposures of less than ten seconds, I see no advantage to turning off timed exposure.
But turning off timed exposure does have a disadvantage. Since you have to press the shutter button a second time to end the exposure, a remote (IR or wired) is a practical necessity to avoid introducing vibration (even if only at the end of the exposure).
My K-5 allows me to set 1, 2 , 3, 4 ... secs of timed exposure.
I have an IR remote and my mobile has an IR port so could be used as an emergency backup remote, though glowing screen in the night is not the best but the battery of my remote just died once during a session despite being relatively fresh. Even with timed exposure I prefer to use the remote with the mirror-up.
It occurs to me now that I missed something but I can not figure it out. My camera has an AA filter.
Does it mean that for best results I could still use a bit longer exposure because the image will be blurred a bit anyway or it means that I should stick to the most firm requirement for a max "1/2 pixel motion blur"?