Secondary mirrors - source/quality?

[garymck]

Hi,
I've decided to replace the secondary in my 12" f5 Skywatcher newt to get a bigger illuminated field. It currently only has a 70MM which barely illuminates the center of the field. I'd like to install a 3.5" (88mm).

I'd like to know peoples opinions of GSO secondaries - Andrews lists the 88mm at $99, whilst an Antares will cost around $400......

If I went for the Antares, what wavelength spec, they have anything up to 1/30th wave. Is it necessary to go to this level. My reading seems to suggest that a 15th wave would be fine.

Any other vendors to consider?

The primary in the scope is typical chinese, good, but not outstanding. I may re figure it sometime in the future, but it is fine for imaging.

cheers
Gary

[AstroJunk]

Interesting. I have a 3.5" in my 20" f5 with a beautifully illuminated field visually and only minor vignetting with a full frame dslr.

Antares is the way to go. 1/15th is fine. Go for the 1/30th anyway, I would :rofl:

[garymck]

Just used Mel Bartels Diagonal Calculator:
http://www2.arnes.si/~gljsentvid10/diagonal.htm

Think 3.1" will be big enough....

cheers
Gary

[OzEclipse]

If you really want to know what's going on, put the dimensions and specs into Stellafane's "Newt for the web."

https://stellafane.org/tm/newt-web/newt-web.html

The diagonal size, tube dimensions, back focus and the focusser dimensions all contribute to field illumination. Increasing the diagonal alone may not increase field illumination. This web application does a detailed ray trace.

Diagonal wavefront accuracy
Light reflecting off a 45 degree surface doubles the diagonal's wavefront error. If you have a 1/4 wave primary mirror you probably want at least a 1/8 or 1/10 wave diagonal to match. I think Skywatcher mirrors are spec around 1/10 wave?? Spec and your mirrors actual wavefront error can be different. If 1/10 wave, your diagonal should be 1/20. Make sure both measurements use the same method eg peak to valley, rms so that the accuracies are comparable.

Joe

[glend]

I have a GSO secondary in my carbon strut 10" newt that i built. They are good quality as far as i am concerned.

[billdan]

According to Agena-Astro the GSO secondaries are rated at 1/12 wave RMS.
https://agenaastro.com/gso-elliptical-secondary-mirror-88-mm.html

Quote
"GSO uses high-volume, state-of-the-art, high quality manufacturing and test lines. GSO guarantees diffraction limited performance, but their secondary mirrors typically have a mirror surface quality of 1/12 wave RMS at least, and often better. This very smooth mirror surface results in excellent optical performance with practically no light scatter."

EDIT: Not sure how you convert RMS to Peak - Valley, but if its like AC theory then 1 Volt RMS = 2.828V Peak to Peak, which would make it approx 1/4 wave peak to valley??:shrug: .

Anyway for prime focus imaging which is at the lowest magnification the Newtonian scope can provide, I don't think you need 1/30 wave spec's unless you also intend to use it for high magnification visual.