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Reducing Secondary

Started by wpaolini, 06/20/2006 08:20AM
Posted 06/20/2006 08:20AM Opening Post
I have an 8" f/5 primary that I'd like to use to build a scope optimized for more planetary use by reducing the secondary size. Other than just reducing the secondary for a reduced illumination field, are there any techniques such as putting lenses before the secondary to lengthen the light cone so a smaller secondary could be used? If anyone can point me to links for various methods to accomplish a reduced secondary, please let me know.

Posted 06/20/2006 09:01AM | Edited 06/20/2006 09:01AM #1
If you have a program like Newt and know that you want to get the focal plane as close to the secondary as possible, ie a low profile focuser, then I think you have covered your bases.

Lens in front of the secondary are for the birds, or make that Jones-Birds... Well not quite but almost...

Posted 06/20/2006 07:48PM #2
Yes, this should work. Essentially you want to mount a high-quality Barlow in front of the diagonal. If you simply use the lenses say from a 1.25" Barlow however, the benefit is small -- you could get by with a 1.25"
diagonal anyway. You'd need to find a set of small high-quality lenses for the Barlow.

Another idea is to put the focal plane near (but not at) the diagonal, and then use a relay lens to re-image the focal plane outside the tube. This is easier because the lens assembly is not in the light path. You'd be looking at something like a high-end 50mm F/2 macro camera lens. Lens must be very good or will dominate the aberrations.

Easiest, of course, is to make F/8 or F/10 scope instead.
Posted 06/22/2006 03:21PM #3
I would go for a 8" f8 or higher and you may want to try your hand at grinding the mirror yourself.
Nothing really beats a long F/L scope for planets.

Posted 06/23/2006 06:47PM | Edited 07/02/2006 12:41PM #4

I built a 10" F/15 planet killer with a built-in barlow back in "94". The primary was an F/5 Pirate Optics mirror that was refigured by Alan Raycraft. It was a planet killer for sure 8) The secondary obstruction was about 13% and the fully illuminated field (100%)was 1/2 degree. I witnessed the Jupiter impacts with this scope. Flat out awesome! I had it at the 94 RTMC meet. It won an award that year(for the mount). Dr. Don Parker came over to look through it and was impressed with what he saw (and he said so in my observing log).

Note: It's not an easy project. You will have to do a little machining to get your desired result. My housing for the optics came from a Celestron Comet Catcher Jr which was modified to suit. The lens is an Edmund Scientific negative achromat with a focal length of -109mm and a diameter of 26.4mm. Part number NT4R-782. If you run some simple formulas you will notice that a standard barlow that's designed to be used in a focuser will not work for what you want to do. Their focal lengths are too short.

The difference in performance going from F/5 to an F/15 is amazing if you're into high power viewing. It's nice to use "average" medium focus eyepieces to get great high power views (15mm=254x). Be warned though, the resulting instrument will be a special purpose scope. Lunar, planetary, multiple star observing were my intentions when I made mine. Don't worry about "color". The effect is no different then when you use a standard barlow in your focuser.

Even though I no longer have the 10" I did save the "special secondary". Check the pic. I can post the optical layout if you're interested grin


Attached Image:

klawson's attachment for post 104807


40 year old restored C14 with recirculating cooling w/Skywatcher DOB GOTO Conversion (CPC14)

10" F/15 thermally managed ATM Coulter Cassegrain mounted on a Celestron C14 wedge to GEM conversion

"Electro-DOB" mounted Sky-Watcher Evostar 150mm/1200mm ED Refractor

"Rescued" Craig's List Orion 120mm/1000mm Refractor