I'm making a home brew Hartman Mask. Once focused on a distant object such as the moon is infinity constant? If not is there a way to figure the infinity line of a particular scope?

Thanks
Bill

__________________
Bill
tampapi@gmail.com
www.tampapi.com

Celestron C130
Orion 100MM F6
ETX90
Celestron Astro Binos

Stars make a better focus target than the moon. Though an isolated sunny mountain peak works much like a star. Those aren't always around however so I use a star. I don't follow what you mean by "infinity line". Just focus a star down to one point. 3rd of 4th magnitude stars work well for 1" holes or triangles. Then remove the mask and move to the object you are imaging.

Rick

I have been professional photographer for many years. No weddings or birthday parties, just evidence and crime scenes. That being said, I don't know squat about astrophotography. I do know I'm loving the heck out it, and have not really shot anything extraordinary.

When doing terrestrial photography there is a point in focus where where the infinity line or point starts. From that point on the focus it the same. This point is greatly affected by aperture. If this works with astrophotography, (and I have no idea if the same rules apply), than it would simplify focusing. A planet at 4 million miles and a star at 5 light years would still be focused at infinity.

It would be neat if someone made a lens cap with an iris. Point it at the moon, crank it down to f22, and have at it.

Thanks for the input

Bill

__________________
Bill
tampapi@gmail.com
www.tampapi.com

Celestron C130
Orion 100MM F6
ETX90
Celestron Astro Binos

That idea is fine for camera lenses (though most good ones will suffer some resolution loss at f/22 compared to a couple stops down from wide open) but lousy for astronomy. Stopping down a telescope reduces the resolution which is directly proportional to the aperture. So if the scope was f/6 to start with if you stop it down to f/12 you cut the resolution in half. Not at all what you want to do. Now the atmosphere has some say in this. Still with today's techniques that doesn't come into play with planetary photography until the scope is about a half meter in diameter.

In astronomy we want as much resolution as possible and that happens, if the optics are reasonably good and most are today, when used wide open. Also that shortens the exposure time giving the atmosphere less chance of blurring the image.

Rick

Yeah, if you're diffraction limited, which you aren't likely to be in an amateur telescope from the ground. The bigger issue with stopping down your telescope is the light loss from the smaller entrance aperture.

I find the loss of resolution up to about 16". Over that it is only a brightness gain but since few use scopes over 16" I stand by my statement. Stopping down any of my scopes 14" LX200R, 10" f/8, 10" f/5, 6" f/12, 6" f/4 and several smaller all reduces resolution besides light. Only if the optics aren't up to snuff does stopping down not hurt and can sometimes even help. And my seeing isn't great. But even then there are those instants of great seeing you won't see in a stopped down scope.

Rick

Rick

Fair enough. I guess a 6 inch telescope has a diffraction limit pretty close to an arcsecond. I'm just very used to working with much larger telescopes.