To change secondary size or not???

I have a 10" f/4.5 dob that currently has a 2.6" secondary mirror. I am contemplating changing to a 2.1" secondary. The attached photo shows the current (top) performance and dimensions with the 2.6" secondary, and the performance and dimensions if I were to change only the secondary (bottom). The reduction in the obstruction (from 26% to 21%) is a plus (more contrast) but I'm not sure what to expect visually by the angular field of view of the 75% and 100% illuminated area. They are significantly smaller in size with the 2.1".

While not dedicated to planetary viewing, I enjoy the planets and spend time on them when they are in good position but I spend most of my time on DSO's. If it makes a difference, the scope has a 2" focuser but all my eyepieces are of the 1.25" format. (24mm Pan, 13 and 9 Nagler, 5mm TAK)

Would changing from a 2.6" to 2.1" secondary on this scope be of benefit or should I leave it as is?

Thanks,

Mark

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A few years ago I changed the secondary in my 12.5 f/6 from a 2.6 to a 2.14. I recall doing considerable research on what was "appropriate" for a fully illuminated field. As I recall, the target to shoot for with a fully illuminated field was something between 0.3° and 0.5°. Your choices are quite widely separated with the fully illuminated field being either 1.0° or 0.2°.

Without having any real experience seeing a dropoff in illumination on any scope, I would still be hesitant to reduce the fully illuminated field to 0.2°. So if I was choosing, I would probably stay with the 2.6 inch secondary size. If this was a scope on a driven mount, then I might be willing to try the smaller secondary.

Gary Weber

Don't know where Newt's numbers come from, but
the data indicates ~8.5" diagonal-to-focus separation. This would result in ~0.25" field of
full illumination diameter, and ~76% illumination
12mm off-axis (all you need to worry about with 1.25" ep barrel).

As for the contrast effect, 26% obstruction results
in 0.87 contrast degradation factor, and 21% in 0.91
contrast degradation factor. This is comparable to
1/5 wave p-v and 1/6.1 wave p-v of spherical aberration, respectively. Not much of a difference
practically, but it could be as a part of an effort to minimize all error sources inherent to the system.

Vlad

Hello:
Try the following experiment. Make a cardboard disk about 3.1" in diameter, with a hole so you can stick it on the backside of your spider. The next time the seeing is good, put this on the back of your spider to increase your central obstruction by 5%. See if you can honestly see a difference. If you can, then I'd say moving to the smaller diagonal is worth it. If no, keep what you have and save the money. I recently chose to increase the secondary of my 12.5" f/6 from 2.1" to 2.6" to get added field of view (the central obstruction is small in either case).

Cheers
Mike Connelley

Thanks for all the replies and the link to the Mel Bartles secondary calculator. I have attached a photo of the results using both mirror sizes.

The columns show a list of off-axis radius, the ilumination for this circle and the light loss in magnitudes. If the illumination falls below 70%, that line is shown in red.

The Tele Vue website lists the 24mm Panoptic as having a field stop of 27mm which would be a radius of 13.5mm. Looking at these results, does that mean that I would still have over 75% illumination across the whole field with that eyepiece and a 2.1" secondary? (roughly 95% illumination with the 2.6" secondary)

With this additional information, are there any other thoughts on the benefits of (or not) switching to a 2.1" secondary on this scope?

Thanks,

Mark

P.S. I'm learning from the feedback and hopefully others will benefit too.

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