Solar Projection Questions

[Arcturus2013]

How can you tell if a telescope, eyepiece etc is suitable for projection?

How long can an unfiltered telescope be pointed directly at the Sun? Before the risk of damaging the scope?

[Stu]

16 minutes ago, Arcturus2013 said:

How can you tell if a telescope, eyepiece etc is suitable for projection?

How long can an unfiltered telescope be pointed directly at the Sun? Before the risk of damaging the scope?

It needs to be a refractor, not a newt, SCT, Mak or other sort of compound scope. Avoid Fracs with plastic internal parts or focuser which might melt! There shouldn’t be too much trouble then, projecting the sun for quite a while. Biggest problem is likely to be the cloud!

[Gfamily]

My understanding is that refractors are the only scopes that are suitable, as glass has more or less 100% transparency for the IR light that is going to cause problems. 

Reflectors and Catadioptric systems are more likely to be damaged because reflecting surfaces may only reflect about 95%, and if the light has been brought close to focus, the 5% that's absorbed can be pretty intense. 

However, this assumes glass throughout, if the light goes through any plastic optical elements close to the focal point, there is the likelihood of damage being caused. 

I've used 50mm binoculars for projection for tens of minutes at a time without any problem, so I would expect anything up to 70 or 80 mm should  be OK for reasonable length periods - though maybe make a cover for the objective to pop back on between observations  

Edited June 9, 2021 by Gfamily

[Cosmic Geoff]

A telescope can be assumed to be potentially suitable subject to certain warnings:

Any plastic parts eg components of eyepieces will melt if subjected to concentrated heat. ( I have a couple of eyepieces with melted retaining rings, and an eyepiece cap with a hole mented through it to prove this)

Multi-element eyepieces may be damaged by the heat (maybe the cement holding compound lenses together is vulnerable).

I would be wary of using a telescope of greater than 4" or 5" aperture.

Finderscopes are a potential danger and should be capped. 

Projection seems to be deprecated these days in favour of using a full=aperture filter made of special filter sheet (which you won't have time to buy now). This is safer provided that the filter is secure. Or a Herschel wedge (q.v) plus a filter.

[Stu]

This is no different to using a refractor with a Herschel wedge from the scope’s perspective ie the full light and heat of the sun is entering the scope and going through to the focuser. Provided the eyepiece has no plastic parts it again should be ok, but simpler designs are likely preferable.

[Cosmic Geoff]

3 minutes ago, Gfamily said:

My understanding is that refractors are the only scopes that are suitable, as glass has more or less 100% transparency for the IR light that is going to cause problems. 

Reflectors and Catadioptric systems are more likely to be damaged because a reflecting surfaces may only reflect about 95%, and if the light has been brought close to focus, that can be pretty intense. 

I note your comment, but I have not heard of any Mak or SCT suffering damage when used in this way.  I frequently did projections with my 127mm Mak without apparent damage (except to a cheap eyepiece with plastic parts 🙂.)  But a full aperture Baader film filter seems a better option, and that's what I use now.

[Peter Drew]

You can use almost any telescope at any aperture provided that the entrance aperture, full or restricted, is completely covered by solar film of glass solar filter.  This gives a better image than projection and is safe to the components and the observer.  The slight drawback is that this just serves one individual at a time unless a camera is attached and the image fed on to a monitor screen.  Small Maks and SCT's can be used for traditional projection but you need to be aware that some secondaries and some secondary baffles are glued on and could separate if overheated.    🙂 

[Arcturus2013]

I probably should have mentioned that I have used Baader film, though what concerns me is that it's easily scratched. My thinking was it might be easier not to have to replace the film and I know this is debatable  but the idea of a non-direct view of the Sun generally sounds less risky to me. I think I might try and look for a cheep used refractor. I'm not sure how easily I'd be able to tell if the internal parts, focuser or eyepieces are problematic.

[Gfamily]

2 hours ago, Arcturus2013 said:

I probably should have mentioned that I have used Baader film, though what concerns me is that it's easily scratched. My thinking was it might be easier not to have to replace the film and I know this is debatable  but the idea of a non-direct view of the Sun generally sounds less risky to me. I think I might try and look for a cheep used refractor. I'm not sure how easily I'd be able to tell if the internal parts, focuser or eyepieces are problematic.

with regard to scratches on the Baader film, my view is that our eye has an aperture of several mm^2, and even if there is a bit of damage to a solar filter, it would need to be really obvious to let more light through than our eye already allows. 

Yes, I appreciate that we don't spend a lot of time looking directly at the Sun in normal times, but we often have it visible to some extent and it doesn't irreperably damage our vision - so small scratches and pinpricks on the solar foil won't have an immediate and catastrophic effect. 

Certainly, hold the filter up to the Sun to check for major damage or thinning before mounting it on the scope, but don't worry too much if you can't see anything obviously wrong. 

Edited June 9, 2021 by Gfamily

[Alan64]

15 hours ago, Arcturus2013 said:

I probably should have mentioned that I have used Baader film, though what concerns me is that it's easily scratched. My thinking was it might be easier not to have to replace the film and I know this is debatable  but the idea of a non-direct view of the Sun generally sounds less risky to me. I think I might try and look for a cheep used refractor. I'm not sure how easily I'd be able to tell if the internal parts, focuser or eyepieces are problematic.

Here's the front of the manual of my first telescope, a 60mm f/11 achromat, and illustrating its projection-screen at the rear of the telescope(look sharp, as I don't have a larger image at hand), which I did use on more than one occasion...

The reason I'm showing you that is simple: get an old refractor like that one.  That one is from the early 1970s, and all metal, fore to aft.  It's the focusser, and the cell that holds the lenses, that need to be of metal, along with the optical-tube of course.  If you use a diagonal, use an Amici- or star-prism with a metal housing; also, all-metal eyepieces.  You can be assured that most if not all vintage .965" eyepieces are all-metal.

eBay has quite a few vintage, all-metal, Japanese refractors like my own, but from the U.S. however, although they do tend to ship worldwide; for example...

https://www.ebay.com/itm/234027361254?hash=item367d1e23e6:g:76gAAOSwLSVgpuKW#shpCntId

In so far as current models, certain Synta refractors are all or mostly metal, the Sky-Watcher "Evostar" 102mm f/10 achromat, for example.

Regrettably, the current 60mm, 70mm, 80mm and even the 90mm achromats for sale are going to contain plastic, quite a bit actually.

Incidentally, do you have a telescope, or telescopes, and a mount or mounts?

[Goldfinger]

On 09/06/2021 at 13:31, Gfamily said:

My understanding is that refractors are the only scopes that are suitable, as glass has more or less 100% transparency for the IR light that is going to cause problems. 

Reflectors and Catadioptric systems are more likely to be damaged because reflecting surfaces may only reflect about 95%, and if the light has been brought close to focus, the 5% that's absorbed can be pretty intense. 

However, this assumes glass throughout, if the light goes through any plastic optical elements close to the focal point, there is the likelihood of damage being caused. 

I've used 50mm binoculars for projection for tens of minutes at a time without any problem, so I would expect anything up to 70 or 80 mm should  be OK for reasonable length periods - though maybe make a cover for the objective to pop back on between observations  

I have a Celestron Nexstar 6SE. Would any type of filter prevent damage viewing the sun?

[Stu]

21 minutes ago, Goldfinger said:

I have a Celestron Nexstar 6SE. Would any type of filter prevent damage viewing the sun?

A Baader AstroSolar filter would work well on that, full aperture over the front.

There is a ready made one here for £82

https://www.firstlightoptics.com/solar-filters/astrozap-baader-solar-filter.html

[RT65CB-SWL]

4 hours ago, Goldfinger said:

I have a Celestron Nexstar 6SE. Would any type of filter prevent damage viewing the sun?

It would need to be a full aperture filter, either solar safety film or glass. I seem to recall one ‘specialist filter brand’ also makes solar filters in polymer that are threaded for use with camera lenses.

Any solar filter, irrespective of what material it is made from needs to be checked for pinholes, scratches, etc., before each and every use. If in doubt, do not use it...
and to finish off... DO NOT BUY THIS TYPE…
 

Edited June 11, 2021 by Philip R

[Arcturus2013]

9 hours ago, Alan64 said:

Incidentally, do you have a telescope, or telescopes, and a mount or mounts?

I have two telescopes. I don't believe either are particularly suitable, one is a Celestron PowerSeeker 70AZ telescope (70 mm f/10) and well the other I will never use for the Sun as it's an open tube dob.

Thanks for the advice about all metal scopes though.

I have to admit Baader film was particularly useful today for the eclipse especially for photography and possibly less impacted by clouds. That said I'll still think about solar projection some more.

[Goldfinger]

23 hours ago, Stu said:

A Baader AstroSolar filter would work well on that, full aperture over the front.

There is a ready made one here for £82

https://www.firstlightoptics.com/solar-filters/astrozap-baader-solar-filter.html

I found one on the Celestron website for the Nexstar 6 SE. I ordered one but it's on back order.

Thanks. I thought solar viewing was off limits.

[Gfamily]

1 hour ago, Goldfinger said:

I found one on the Celestron website for the Nexstar 6 SE. I ordered one but it's on back order.

Thanks. I thought solar viewing was off limits.

With a Baader full aperture filter, there's no need to be concerned regardless of the optics.

You can make a DIY filter up to 200mm aperture with an A4 sheet of Baader film 

[Les Ewan]

I don't think I would risk any modern refractor with projection  with so much plastics being used these days. I used solar projection with my Prinz 60mm refractor in the 70's with no problems and the cheap suppied  0.95" Huygen eyepieces which were pretty awful for night time use were a advantage for Solar projection. I have a Siberia 150 TAL 6" Newtonian which was issued with a projection screen in the kit. I did use it for projection often in the 90's without a problem but it must be said these scopes were all metal and had metal bodied Huygen and Ramsden eyepieces. 

I don't do projection these days my Skywatcher 90mm refractor and Baader filters do most of my solar work.

[Alan64]

12 hours ago, Goldfinger said:

I found one on the Celestron website for the Nexstar 6 SE. I ordered one but it's on back order.

Thanks. I thought solar viewing was off limits.

I found one in stock...

https://www.bhphotovideo.com/c/product/1340583-REG/celestron_94243_eclipsmart_white_light_solar_filter.html/overview

The image may not be as sharp as that through a Baader filter, but then the Baader is twice the price, although perhaps not twice as nice.