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Sanity Check Needed.

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Hi Folks,

I'm looking for a sanity check for the design of the 222mm f7 telescope I'm attempting to build, every time I look at this thing I get to thinking it's not right and then spend ages trying to convince myself it's OK :?

Would one of you wonderful experienced people give me an opinion on this design that I currently have, it was designed using NEWT V 3.0 and the parameters are as below.

Primary = 222mm

F = 6.94 (measured using the radius of curvature method )

Tube inside diameter = 250mm ( Yep! I know its a bit tight but that's what I was limited too ) :D

Tube thickness = 4mm

Focuser Ht = 82mm

Focuser i.d. =50.9mm

Focuser extra travel = 20mm

Diagonal minor axis =44mm

Focuser to front end of tube = 200mm ( I think I was a little too generous here!)

Mirror face to back of tube = 100mm

Any suggestions for improvements or problems envisaged would be welcomed. I propose to use this thing for visual observing initially but might progress to imaging later.

Thanks for your comments in advance.


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Are you grinding this yourself? What size is the blank? 222 sounds an odd size. Are you sure it's not an 8" blank and the oversize is for the chamfer? Your 44mm secondary will give a very nice 18% CO which means you'll get great contrast, the 1554mm focal length is hefty though and you'll need a decent mount for it, especially for imaging.

I'm sure other more experienced people can give better comments!

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I expect that the main problem lies with the distance between the two mirrors. I chickened out and made mine adjustable as I wasn't going to stick a tape measure on the front of the primary to do the measuring. I managed to be able to get loads of focus travel for using focal reducers etc. but ended up with a big clumsy thing with a focal length thats almost more than I can manage with my teensy weensy camera. For a fixed length, I'd have to go too long as cutting a bit off later is the easier option. I've never had much luck cutting a bit back on. maybe within the 100mm at the bottom you could have some coarse adjustment as many of us curse the lack of in travel of the focuser for imaging.

As for being too generous with the focuser to end of tube distanec, that can be chopped of later I would have thought? Built-in dewshield?

Best of luck with it, and can we have some pictures please?

Captain Chaos

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Hi Geppetto,

The Newt program did say that there was vignetting of the 75% ray but when I looked at the Newt dimensions data it was only 1mm too small to pass the test and I didn't think it would be a problem. :D I think your right about it being a beast, I'll have to make sure I control it rather than it controlling me :whip2:


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