Photographing Foucault tests?

[alphgeek]

Hi All,

I've just joined today, even though I've been aware of this forum since it started. Great to see such an active astro community right here in Australia!

I'm getting ready to pick up my mirror grinding project again after a couple of years. I am making an 8" F6 paraboloid (my first) for fitting into some sort of Newtonian down the track.

I put the project away after getting discouraged for two reasons: First, I got a very minor scratch on up-till-then pristine mirror after 50 hours of polishing (obviously I am easily discouraged!). Second, I was having a lot of trouble with the Foucault test. While I understand the principles, I find it very hard to see the shadows using a mask.

To overcome the latter issue, I was trying to take photos of the Foucault tests, either stills or videos, with little success. I have seen some very nice photos of Foucault test results both here and on other ATM forums.

My question is, how are people getting such clear and large photos or videos of their tests? I took a few snaps with my digital camera but its small lens resulted in tiny images.

Is it just a question of using a better camera like some sort of digital SLR with a more powerful lens? ROC for the mirror is something like 2,700 mm from memory so I need a camera / lens combination that will fill the camera's CCD at that range?

Kind Regards,

Justin

[[1ponders]]

G'day Justin and :welcome: to the forum. Not being a atmer I'm afraid I can't be much help, but thought I'd welcome you and bump this thread back up near the top so it doesn't get lost. I'm sure you aren't the only one out there that would be interested in this topic.

[Orion]

Hi Justin and welcome.

I was just wondering if your light source for the Foucault test is moving or stationary?

[alphgeek]

Hi Paul and Ed, and thanks for the warm welcomes.

My Foucault tester is a copy of the slitless Stellafane design:

http://stellafane.org/atm/atm_foucault_tester/atm_tester_plans_1.htm

and has a moving light source. It's rough and ready but it works!

I have hogged and ground 8" and 12" blanks by hand but I am using a Mirror-o-Matic (http://www.mirror-o-matic.com/) for this most recent effort. After hogging the 12 inch I got lazy. This mirror is the first I have got to final polishing stage.

I'm using 8" fine annealed Pyrex blanks that I was lucky enough to get from James Mulherin at Optical Mechanics in the US. James is a great guy and his work is first rate, as anyone lucky enough to have a recent Obsession dob might know.

My grand plan is to make a lightweight dob from laser-cut stainless, either a truss design or, if I get really ambitious, a string scope. Then I want to finish the 12" at F5 and maybe go onto an 8" binoscope or possible a more advanced optical design. But first things first!

Regards,

Justin

[DaveGee]

Hi Justin,

My tester is arlar Texereau. I just hold my el-cheapo digital camera up to the tester and watch the screen and take the photo.

The image on the left is a 10"f6 that has terrible roughness due to a hard (cold) pitch tool taken in June. I just couldn't get the tool to behave. The image on the right is of the same optic from the same tool but used in December. That means warmer and softer pitch.

Regards
DaveGault

[Rod]

Hi Justin,

Do you have a ronchi screen? I don't use the foucault test until I'm ready to parabolise. I find the ronchi test much easier and quicker for testing the surface is spherical and the edge is not turned.

I sympathise with the difficulty using the zonal mask. I found it very confusing until a friend showed me what to do. Now it seems quite straightforward - if a little tedious. Do you know anyone who has made a good mirror who can take you through the process?

The other option is to take the mirror to spherical, build the mount and then test the unaluminised surface on a star, using the ronchi or foucault tests which are then null tests (ie straight lines on ronchi = a parobola). You can also use the star test to very critically evaluate the mirror.

I would not worry about a few scratches. They are usually cosmetic blemishes that have virtually no effect on the image.

Hope that helps,

Rod.