New project

cristian abarca · #1 24-01-2006, 05:21 PM

Hi guys I have a question that I'm having trouble getting answered. Does anyone know of a site where I can get details of a modern Herschelian telescope? I have been in contact with someone at my astronomy club who will help me build it, but he has to find the plans and details . He built a 6 inch about 20 yrs ago. I want to build a 10 inch. I've already built a 10 inch F7 Newtonian so I don't want to build another one. I need something more challenging. (I've already made another 10 inch F5 mirror). I want to read up on it. I'm a little bit obsessed with mirror/telescope making, some would say very obsessed . All I know is that it is an off axis telescope. The light hits the primary which is at an angle then bounces off and through two lenses that are located under the secondary then onto the focuser. There is no spider.

Regards Cristian

33South (Chris) · #2 25-01-2006, 12:37 AM

I found a few lines and diagrams on the original design in my Amateur Astronomers Handbook

"The problem with this design wherby the mirror was tilted at an angle to remove the need for a diagonal is that in order to reduce off axis astigmatism to acceptable limits (ie below the resolving power) the focal ratio has to made made very long."
For an 8" need have a focal ratio of f30 - 20ft focal length.

Larger the diameter larger the focal ratio, dont know values for a 10"

There were also some designs for a folded version.

This has been abandoned as an impractical design

Probably doesnt help but I thought it was interesting

cristian abarca · #3 25-01-2006, 06:50 AM

Thanks for that. The way to make it a practical design is to put 2 corrector lenses in it. This helps correct something, I'm not sure what and keeps it within acceptable lengths. It's an unusual and interesting design, that's why it's got me so intrigued. It will be very interesting to show when I build it.

Regards cristian

Roger Davis · #4 25-01-2006, 11:53 AM

Rather than kill the idea, get a bit more impetus from the following:
http://bobmay.astronomy.net/johnpierce/piercetop.htm
Great advice on herschelian scopes from the 30's.
No reason why you shouldn't try.

cristian abarca · #5 25-01-2006, 04:48 PM

Thanks for the link Roger. I have no intention of shelving the idea. I'm just waiting for Arthur Coombs to get back to me on the plans and details. I just want to research it while he finds all the material.

Regards Cristian

Roger Davis · #6 25-01-2006, 05:32 PM

Good old Arfur!
Spoke to him yesterday and caught up on his observing experience at -20°F, yes minus.
Oh well, back to the medial telescope ...