Solar projection

[Andrew*]

I'm planning to view the Venus transit on 6th June with a group of friends, so want to make it easily viewable for everyone using solar projection.

What kind of eyepieces are good to use? I know to stay away from eyepieces with cemented elements, but don't most modern designs have cemented doublets etc.? It would be most handy to use my Baader zoom but something tells me this is not a good choice...

The telescope used will be 100-120mm.

Also, what is the best magnification to use? Logic tells me the highest that will still show the whole disc, so about 100x??

Cheers

Andrew

[callump]

Hi Andrew,

the focal length of the scope is more important than the aperture in working this out (actually the aperture has no impact at all...).

The formula is basically

d = Sp x fe / (fo / 100)

where

d is the distance from the eyepiece to the screen

Sp is the diameter of the projected Sun

fe is the focal length of your eyepiece

fo is the focal length of your objective

So, to get a 100mm image of the Sun on your screen, with a 15mm eyepiece on a 500mm focal length scope, the screen would need to be 300mm away from the eyepiece.

This is an empirical formula, but should work out well enough (where the distance from the eyepiece to the screen is much bigger than the focal length of the eyepiece).

Regarding what sort of eyepiece to use, I have used plossls (Televue) in the past without any apparent problems, but I would probably suggest you use a 'cheap' one that you would not mind being destroyed should anything bad happen.

Refractors and plain newtonian reflectors are fine for projecting - SCT's and other cat's are not.

Callum

[garethmob]

i vagely remember reading that the TAL scopes come with a hygines (spelling) eyepeice for solar projection, im not 100% but i saw it somewhere

[iwatkins]

I would question using a newtonian for projection though. That'll be a lot of heat on the secondary ?

[callump]

Most of the heat ought to be reflected, though I expect the seconday may get warm. You'd have to be careful if the secondary is only glued to its mount.

You might also want to use an aperture mask over the front, to stop it down to 3 or 4 inches, maybe.

I've not had a problem when I was using a newtonian reflector (10") - but YMMV.

No warranty expressed or assumed.

Callum

[Bizibilder]

You could get a cheap Ramsden type eyepiece - it would do the job. Around 20mm focal length. I would also be careful about using any reflecting surface without a solar filter - it will still get very hot!

You may find it easier with a solar filter and webcam to show the image on a laptop screen is actually easier and safer?

[callump]

I wouldn't really recommend using a newtonian reflector, but it can work and was used by many for years. A refractor is better if you have one.

I'm not sure there is anything easier or simpler than just adding a screen to view the projected image onto. KISS in action.

Callum

[barkis]

In 2004, I videod the transit of Venus.

I used my 150mm f8 refractor and x3 Barlow projection to project the sun onto a 1 foot square white card, which was attached to a

frame fixed to the refractor. Another card slid over the front of the tube of the frac. served to cast a dark shadow, which allowed the projected image to to be highly visible. The scope was on a driven Astro 5 mount, which together with the hand paddle helped to keep the image central on the white card.

I viewed the transit from first contact of Venus, almost to the end of the event.

Although it was a historical astronomical happening, I'm afraid I found watching the tape later, a pretty boring thing, so I ended by speeding it up:D.

The upcoming transit on the 6th June, will be the last this century, so ignore my comments here, and get a record of it if you can.

Ron.

[BinocularSky]

When I have projected, I have used a 20mm Huyghenian. You need to make sure that there is no dust at all on the field lens.

[AndyH]

If you go down the 15mm ep route and want/need a decent but cheap eyepiece, then these guys still have the old Tal 15mm kellners(they used to be part of the kit in a few of the scope sets)

http://www.wexphotographic.com/buy-tal-15mm-tal-kellner-eyepiece-1-25-inch-31-7mm-/p10645

Andy.

[estwing]

i shall be using my zeiss to view this..but not with my cz ep's....just in case!!