Right..... I think I'll stick to the herschel wedge from now on....

[Andrew*]

I was hoping to view the recent Venus transit with a few friends and decided to give eyepiece projection a go, as it would allow everyone to get a peek. To save frying one of my precious (sss) Zeiss orthos, I purchased a second hand 12.5mm Circle V H.M. Only eyepieces designs without cemented elements are said to be suitable for projection, so I thought this would be fine.

Well, it was a pretty dismal morning during the Venus transit, so it never got used.... until a few days ago. I just decided to give it a go and project the sun onto a card with my ED120. I got quite a nice view. The card surface was not really fine enough to show granulation, but I got a great view of the spots. However, when I checked out the eyepiece afterwards, this is what I saw!

Amazingly, the views on the card were still reasonably good.

I think I'll stick to the herschel wedge from now on! Extremely sharp and no danger of damage to eye, eyepiece or clothing...

[Moonshane]

ouch!

[Mark at Beaufort]

Andrew I can appreciated your statement. Herefordshire Astro Society is organising a star party in November and during the daytime we will undertake some solar viewing. I was worried about showing people the Sun via my Herschel Wedge just in case they got the wrong idea. I therefore decided it will be solar projection through my Astro Tech 4" frac.

I thought I would undertake a trial run at home because I had an old 12mm Kellner EP which I thought was all metal construction. Set up the scope and had a good image on a projection screen. Then smoke started coming from the EP - yes it melted!! So its back to the drawing board.

[Luke]

Thanks for the warning. Scary stuff!

I was wondering whether to give projection a go having read a book that seemed to favour it. I'll give it a skip as I'm probably more accident prone than you folks...

[JamesF]

Eyepiece projection isn't that uncommon for solar viewing though, is it? How have people got it to work in the past? Restricting the aperture at the objective?

James

[Merlin66]

Using an old all metal Ramsden was the favourite method.....

[Andrew*]

Andrew I can appreciated your statement. Herefordshire Astro Society is organising a star party in November and during the daytime we will undertake some solar viewing. I was worried about showing people the Sun via my Herschel Wedge just in case they got the wrong idea. I therefore decided it will be solar projection through my Astro Tech 4" frac.

I thought I would undertake a trial run at home because I had an old 12mm Kellner EP which I thought was all metal construction. Set up the scope and had a good image on a projection screen. Then smoke started coming from the EP - yes it melted!! So its back to the drawing board.

I've been thinking about this and have a few ideas. One of the most effective would be to get a live view on a monitor with a camera. Do you have any imaging experience? A gamepad could be connected with EQMOD and viewers could pan across the sun's face taking in the features. I was even thinking of a dual-monitor/scope setup showing white light and Ha side-by-side.

I don't think people will get the wrong idea of solar observation if you put up a notice to the effect of "Don't try unless you know what you're doing or instant blindness will result".

I think solar projection still has its dangers even if you keep the eyepiece intact. The focused beam coming from the eyepieice is extremely intense, and even near the card it will be hot. Aside from using a camera, herschel wedge is probably the safest as unless it actually falls out, it's impossible to access the focused beam. The back gets hot to the touch but is no more hazardous than a cup of tea.

Andrew

[David Smith]

I used to do all my solar viewing using projection using my Tasco 60mm refractor. Never had any issues with melted or smoking eyepiece's so I guess it must be the aperture.

[RikM]

I used a webcam through a scope with Baader film a couple of years ago at Cheltenham Science festival. It worked well but the difficulty was getting the lappy screened off dark enough to see clearly. I put it in a folding crate with a blanket thrown over.

[michael.h.f.wilkinson]

The issue is not (just) aperture, it is focal ratio. A slow refractor is fine for projection. We used a 70mm F/10 at school with a 20mm Huygens with a completely brass and glass construction. I have seen F/13 and F/15 4" scopes being used without problems. I even used a 6" F/8 Newtonian with an all metal and proper glass EP (after working out the thermal loading on the secondary, which turned out to be minimal, it never got warmer than 35-40 Celsius (i.e. safe to touch)) without ever shattering an EP. I did tend to use longer F/L EPs (20-25mm), which might have helped.

One reason the O.P.'s EP may have shattered is differential thermal expansion of the glass compared to the metal. It might be good to ensure that the retaining rings holding the lenses in place aren't too tight.

[Andrew*]

The issue is not (just) aperture, it is focal ratio. A slow refractor is fine for projection. We used a 70mm F/10 at school with a 20mm Huygens with a completely brass and glass construction. I have seen F/13 and F/15 4" scopes being used without problems. I even used a 6" F/8 Newtonian with an all metal and proper glass EP (after working out the thermal loading on the secondary, which turned out to be minimal, it never got warmer than 35-40 Celsius (i.e. safe to touch)) without ever shattering an EP. I did tend to use longer F/L EPs (20-25mm), which might have helped.

One reason the O.P.'s EP may have shattered is differential thermal expansion of the glass compared to the metal. It might be good to ensure that the retaining rings holding the lenses in place aren't too tight.

I don't understand why this would be the case. With a steeper light cone in a faster scope, the concentration of light will decrease faster away from the focal point, so I would assume there would be less heating.

I think the main issue in my case is the tiny (3mm) eye lens upon which the entire solar disk was directly focused by the field lens (11mm). A slightly less "harsh" design may well have fared better.

Interesting about the secondary mirror. Optics generally absorb very little energy. Mirrors reflect over 88% and lenses transmit at least the same. It's only when optics are very close to the focal point that there is significant energy being absorbed. Reflectors are mostly not recommended for solar, but what is the general experience I wonder?

Andrew

[RikM]

The first white light experience I had was with a 75mm off axis Baader film mask for my 150P. Effectively operating as a uninstructed f/10 reflector. When in good collimation it was great. The only reason I don't use it now is the physical size and set up time compared to a small refractor on a photo tripod.

[stephenramsden]

hey, Im an "all metal Ramsden"…I like it!

[Andrew*]

I still wouldn't put you at the focal point of a 120mm scope though.... Sorry.