8" Refractor Project

[stringscope (Ian)]

Well it is an astrograph really.

The optics are owned by CAS and I understand have been in storage for a couple of decades. I have mounted the optics in a plywood alt/az mount so at least it can be used visually. It was a fun project, I am not sure how practical it is but it does work.

It is not yet complete. I have got it to a point where it is usable and I have then put it on a set of temporary wheels so at leat I can move it around the workshop. 2 items still to go; dew shield and retractable wheels. A little bit of info if you are interested:

Optics supplied by the US Navy Research Labs for use in a "Minitrack" satellite tracking camera.
System was in use by NASA at Orroral Valley until passed onto CAS in the mid 80's.
Camera used photographic glass plates 8"X10" => 11X14 degrees FoV.
Lens system appears to comprise 6 elements that are mounted in a turned steel tube. The objective cell unit is 220mm in diameter, 500mm long and weighs 26kg!!
Apeture is 200mm, focal length is 1000mm and there is an internal adjustable aperture stop allowing for a variable F ratio from F5 to F16.

Given the weight of the objective cell it has been a challenge to ensure the mount and tube are up to supporting the optics.

Given the poor weather we have been having I have only had limited opportunity to use it. From what I have seen so far, really nice for low power wide field views. However, as you push the power up the CA becomes dramatic. As might be expected a OIII filter changes the picture and it becomes tack sharp to the edge.

I can post more images if anyone is interested .

Cheers,

[iceman (Mike)]

Wow Ian, that's an impressive looking construction. Well done!

You going to SPSP again this year?

[netwolf]

Wow an 8" F5 refractor, thats brilliant and so is the workmanship. Could you show some pictures of how you baffled it inside? Is this an achromatic lens if so how good is the color correction if any?

[stringscope (Ian)]

Thanks Guys.

Mike, my plan is to attend SPSP this year, however, due to a family issue, only 50% probability at this stage.

I am not sure about the impressive bit, but it sure is no lightweight. I cannot lift the tube into the cradle on my own with safety.

Fahim, I have attached some additional images.

Effectively the optical assy is a large camera lens (all 26kg of it) and I have just mounted it in a tube with a diagonal mirror and focuser at the bottom end. The lens assy has some internal baffles that you might be able to see in some of the images.

An apocromat it aint!! As a newtonian user for 37 years I find I am quite unused (and sensitive) to chromatic aberation (CA). To my eyes, this scope, unfiltered, exhibits lots and lots of CA When I get around to it I will acquire a violet minus filter and see if that improves things.

I should add this is at F5. As you increase the F ratio via the chromed knob near the focuser the CA as expected reduces. At F10 it is quite good.

Cheers,

[netwolf]

Thanks for the photos, that explains it well. I understand CA is less of a problem with longer F length but at F10 your Tube assemble is going to be 2m right? How does this camera adjust for that or is it using a lens to extend the F length? Like a built in Barlow?

Regards

[stringscope (Ian)]

Fahim,

The F ratio is changed in this case by changing the apeture. I have attached 3 images taken from in front of the scop showing the internal aperture stop at F5 (fully open), F10 (partially closed) and F16 (minimum selectable aperture).

Hope this helps.

Cheers,

[netwolf]

Oh I see, it like a camera lens with an iris shutter. Thanks.