Building an Off-Axis Newtonian design?

I am thinking about starting a telescope project. I have built a pretty large workbench for the task and am considering possible designs to try out. My goal is to build a decent planetary scope.

Leading the list is the off axis newtonian design. ITE sells one of these. The mirror is placed off axis. The mirror is a chunk from a larger mirror to reduce astigmatism.

Mike Palermiti examined a 5 inch model and says:
http://www.itetelescopes.com/report-oa130.htm

"Simply stated, this telescope is at the top of all classes. Whether it is used for low, medium or high magnification visual studies, there is not a closer telescope system to "perfect" than the off-axis parabolic reflective system. The design is truly revolutionary."

This review was by far the best review he's given for any type instrument. So in looking around for alternatives to the $2,700 price tag for this 5 inch instrument I ran across Carl Anderson's site for a home made off axis parabolic reflector.

http://hometown.aol.com/stressedglass/index.html

How close will this system compare to the ITE version? For those who don't want to go to the above link, basically Carl Anderson made a 6 inch F15 spherical mirror and bent it into a lop-sided parabolic shape. He placed his diagonal on the side of the tube. The mirror is tilted about 3 degrees and the bend corrects the astigmatism.

It seems like an easy way to make an ultimate planetary instrument (I was thinking along the lines of an 8 inch model). Why isn't everybody making these? Would I be wasting my time with this project?

Mark Bernhardt

As for 'why isn't everybody making them' is probably mainly because for most people the hassle involved is out of proportion with the gain achieved. When you talk about bending mirrors at odd angles like 3 degrees and such, not to mention the fun you'd have collimating this thing, well... you're getting into areas of fiddling that most people would rather not fiddle with. Most people like nice, straight 90-degree angles and a simple on-axis lens or mirror to collimate centrally.

There is something to be gained with minimizing the secondary obstruction size -- but there is questionable gain, IMHO, of breaking your back to get it to zero. There is a point of diminishing returns.

Others will have strong feelings on this matter I'm sure, but my opinion is the Ultimate Planetary Instrument, barring a $150,000 monster APO, is an f/8 or so newtonian with a 15% or so secondary obstruction and of course a high quality mirror. Or alternatively, a large-aperture long-focus refractor if you have the facilities for such a thing (notice the cost of the 8" off-axis model is about the same as an 8" D&G OTA, so maybe there's a reason everybody's not making them just from that standpoint too).

Not that it's not a exceptional performer, but I wouldn't call it "perfect" just yet. On the first place, it does have residual coma, which is in proportion to the primary diameter vs. mirror from which it's been cut out. In this case, the smallest possible is ~11" f/3.8. A 45% of the coma of this mirror is at the level of an f/5.6. Not really bothersome, just enough not to be mentioning "perfection".

Another thing is that 99% Strehl. It requires an RMS wavefront error of ~1/60 wave at the focus, or somewhat better. With the PV wavefront error at the focus of 1/6 wave, it gives a PV-to-RMS error ratio of 10:1, almost twice higher than "smoothest" traditional (axial) mirrors. Is an off-axis segment cut out of a well made larger mirror so much smoother than "axial" mirrors of the same size? Sort of intriguing.

I had a long interest in off-axis instruments and looked at any I could find at conventions. On looking through most the owner would make a comment along these lines - "It's not quite collimated." At best, they seemed on par with a good Newtonian of modest focal length with a moderate central obstruction. It seems to me that it would be better to make a top quality 10" f/6 or 7 Newt than an 8" off-axis system, at least if your main goal is a high quality instrument. If you are looking for an interesting project, have at it.

Well back to your original ?, take a look at Jose Sasian's Unobstructed Newtonian (S&T March, 1991, page 120). The primary is a 6" f/12 paraboloid and the slightly concave oblate-ellipsoidal secondary is placed outside the incoming column of light. Resultant F# is f/11 for the overall system. The design can be scaled for other aperature's. The unusual secondary is similar to that used in Yolo telescopes, another consideration (S&T, August, 1988, page 198). Ron