A little bit of DIY mirror re figuring.

[RAC]

Yes I hope there are no more out there like mine. What I find amazing is how well it performed in that state!

To think it could take Mars pics like this blows my mind.

Mars, Blue - Green - Red - RGB by Raymond Collecutt, on Flickr

[Ajohn]

The errors just result in some loss  of contrast, That's where Rayleigh comes into things. He decided that 1/4 wave error resulted in an acceptable drop in contrast. In loose terms the max drop is at the scopes mid resolution point. The max resolution point isn't of much use on planets as the contrast is awful anyway, around 7 1/2 % of what is actually there, no problem on stars.

The problem with scopes with mirrors is that the central obstruction can easily have the same effect as a 1/4 wave error and it all stacks up along with seeing conditions.

I wonder if people have problems with Foucault because of the bad leads on the web and setting up. When I started looking around at slitless testers I only found one description of setting up and that didn't mention that the return image from the mirror needs remain stationary on the knife edge as the tester is moved back and forth.  It has to be for the light levels to match in adjacent holes in a mask. That's the same on other forms of the tester. Also no mention of the fact that the theory is based on the knife edge and source being exactly on top of each other so the separation should be kept as small as possible. I've seen details of several where they could easily have been much closer, usually on youtube.

 Actually when I think about setting up the usual slitless, a knife blade half  way across a led also forming the knife edge I'm beginning to think the whole thing is a bit daft. As a for instance if the tester is dead square on to the mirror the return beam will more or less miss the knife edge. The nice thing about a moving knife edge is it doesn't matter exactly where the return beam is. In fact that can accommodate a bit of an axial alignment error.

Anyway I am awaiting a builder to remove and add a wall before I will have some where to work on a mirror but all  things considered I'm going for a tester that can easily be converted to the sort of arrangement I have used before.

I'll probably build a bath interferometer as well as a 2nd check.  This bloke has a couple of decent video's on that subject but I would hold the lens against the cube rather than trying to cement it. That would need to have a shorter focal length for very fast mirrors.

It's worth looking around at other designs and I reckon that camera macro slides could be used to make an xyz stage cheaply. Just need big knobs adding to allow finer adjustment. The parts needed are very cheap from Surplus Shed but the beam splitter mustn't be a polarising type.

John

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[mapstar]

Yes I hope there are no more out there like mine. What I find amazing is how well it performed in that state!

To think it could take Mars pics like this blows my mind.

Mars, Blue - Green - Red - RGB by Raymond Collecutt, on Flickr

Just goes to show even a poorly corrected mirror can still perform.

I look forward to seeing the results of the refigured version with great interest.

Thanks for posting the post images though as it will be a good reference to compare with.

[mapstar]

Yes I hope there are no more out there like mine. What I find amazing is how well it performed in that state!

To think it could take Mars pics like this blows my mind.

Mars, Blue - Green - Red - RGB by Raymond Collecutt, on Flickr

Excellent images too