DIY refractor for solar projection questions

[Rhushikesh-Canisminor]

Hello!

My friend and I are trying to make a DIY refractor based telescope which will be used only for solar projection as our personal monsoon project. We don't have any specific plan yet but roughly this is the idea : suppose an 80mm achromat f/10 (or similar) with a zoom eyepiece attached to a Sun funnel. (Or similar)

My major concern is the heat build up in the scope. I use my 90mm f/10 refractor for solar projection and within 15 min I can sense the heat built up inside.

Any ideas or suggestions on this? Are there any specific coatings which can work for heat reduction? Any thoughts are welcome!

P.S. I am from Mumbai region and summer temperature does rise up to 40+ degree Celsius. Also,I don't want to have a telescope smaller than 70-80mm aperture.

[moriniboy]

The obvious answer seems to be a solar film cell on the front of the objective to keep the energy out of the tube, but the solar funnel may not work as well.

The only other way is to limit the time spent looking at the sun to reduce the heat build up, but i'm sure you will have assessed this already.

Will your mount be driven?

[Chriske]

Sungun..!

I made one years ago, works fantastic...!

[Peter Drew]

I don't think you can use any additional filtering for white light projection. Once the Sun's image is projected to a useful size, say 15cm the light will have been significantly diluted, solar safety film will not pass enough light for projection purposes. Ambient temperatures will be nowhere like the focused solar heat so will not be a factor, it only affects items that have to be temperature controlled like Quarks and some cameras. Heat in the tube for white light projection is something you have to live with, care should be taken to make sure that nothing gets close to the eyepiece where the main heat concentration is. It would be prudent to limit the exposure time as much as possible as already suggested. 

[Moonshane]

Would a 2" Baader 35nm ha filter or a uv/ir cut filter before the eyepiece (perhaps on an extension tube or Baader fine tuning ring to put it well forward of the focus point) work?  This would reduce heat substantially.

[Peter Drew]

Basic white light solar projection has been done for centuries before without major problems, surely nothing much has changed?.  

[Chriske]

9 hours ago, Peter Drew said:

I don't think you can use any additional filtering for white light projection. Once the Sun's image is projected to a useful size, say 15cm the light will have been significantly diluted, solar safety film will not pass enough light for projection purposes. Ambient temperatures will be nowhere like the focused solar heat so will not be a factor, it only affects items that have to be temperature controlled like Quarks and some cameras. Heat in the tube for white light projection is something you have to live with, care should be taken to make sure that nothing gets close to the eyepiece where the main heat concentration is. It would be prudent to limit the exposure time as much as possible as already suggested. 

What Peter said is correct indeed. No filtering is acquired at all to project the sun onto a screen. It is also wise NOT to use a complex eyepiece as it will be damaged by the heat of the sun. I used a Huygens eyepiece, it could handle the intense heat very well. I projected the sun onto a opaque piece of paper. The sun had a diameter of about 1'(300mm) on my screen. It was rather dim but I could observe the sunspots very well.

[Ben the Ignorant]

You could replace the tube by an open frame like this one:

https://www.cloudynights.com/topic/455127-istar-asteria-tcr-204-8-r35/

[Rhushikesh-Canisminor]

Thanks for all updates!

Actually the idea came in my mind after looking at Sun spotter scope design. I am not a great fan of using film or glass filter for white light as I have seen things going wrong. Projection method is really good if I am having a large number of audience and it's a pain to close the telescope after every few minutes because of heat.

Sun gun design uses only 60mm. The UV/IR filter suggestion by moonshane is kinda interesting. Idea was to have a refractor on motor driven mount and if I am having a good sunspot then I will use the zoom eyepiece to observe it.

On similar note,I have a small doubt.
Maybe everyone is having a 60mm lenses with high f ratio because the longer f ratio will increase the image size and reduce the heat problem?