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View Full Version : Solar Observing - so I learned my lesson!



hewhocaves
2010-Mar-19, 10:18 PM
(note - this story does not end in blindness. sorry to disappoint lol)

Its been pretty clear out the last few days and I've been taking the telescope out for some rudimentary astrophotography (see here http://picasaweb.google.com/hewhocaves/PhotosOfSaturn# and here http://picasaweb.google.com/hewhocaves/MoonShotsOnStPattySDayNotTheAlcoholicKind# ) Tonight looks like it might be another good night...

...but I got a little impatient and wanted to give a whack at the sun. So I pulled out the 8" dob, put in the lowest mag eyepiece (a 40mm), got a hand-sized whiteboard and pointed the scope at the sun. I DID NOT EVER look into the eyepiece. Rather, I put the whiteboard about a foot away from the eyepiece and observed the sun that way.

Well, we're at a solar minima, so there wasn't much to see. I turned the tube away from the sun and checked spaceweather.com. Apparantly there were two possible sunspots that were fading fast. I figured I'd try and catch one. To improve contrast, I figured I'd put in a color filter. So I attached a Red #25 filter in and reset everything including the whiteboard.

About thirty seconds after I started to watch the whiteboard I heard a "pop" and smelled something burning. Turning the scope away I took out the eyepiece and found the filter had cleanly and completely cracked down the center.

They say the sun can burn out your retinas. I believe it, of course, but there's nothing like seeing a filter crack under the intense heat of an 8" primary mirror. Lord knows what it would do to the human eye.

The downside is that I'm a little wary of the reflection on a whiteboard method and it's effect on my regular eyepieces. I could shell out the $80 for a filter to gover the front end of my mak-cas, but I think I'm a little wary of even that. I mean, the sun.. it just made it go "pop".

RickJ
2010-Mar-20, 12:36 AM
Baader Film is a good safe filter for your scope. All that dangerous light and heat never enters the scope I prefer it to glass or other film filters as it seems to give the best image. Though if you are serious about solar viewing there are a bunch of H alpha options you might want to consider. The view is spectacular.

John Dobson invented a solar scope in which a glass solar filter is used at the aperture but at a 45 degree angle. Not only is it a filter but it is also the scope's diagonal! Probably fun to align but has the safety that if the filter shatters or falls off there's no diagonal so no light ever hits the eye of the viewer. I've been surprised no one has made a solar scope using this system, at least I've not seen any.

As you found a solar filter at the eyepiece just absorbs the heat of the sun and soon is history. This is why such solar filters are so dangerous. As a beginner with a 2.4" refractor with one such filter I was looking at the sun through a window (I know but I was a beginner and the window was high quality float glass so it worked rather well) when the doorbell rang. It was the postman with a package I had to sign for. I came back to the scope to find a bright sliver of light on the ceiling. What was that? Looking at the scope I saw the filter (likely welding glass) had cracked in half, the middle part was missing. There was a small chip out of the plaster ceiling (this was a long time ago when ceilings were plaster). I never found any of the missing glass, the chip might have been there all along. But I do know for sure there was a chip taken out of the center of the eyepiece lens! So if the light didn't get me the glass likely would have. I've attached a shot of the chip in the center of the eye lens.

I've used simple two element type eyepieces for solar projection with a 10" scope without a problem but be sure the sun hits glass not the edge of the eyepiece! Cemented eyepieces or those with plastic parts likely won't survive except in small scopes or scopes stopped down to small apertures.

Only problem I had with eyepiece projection was when I walked right in front of a long focal length eyepiece. I knew it was well above eye level so didn't think about it. I smelled hair burning as I did this. I had a very weird part in my hairline for a few weeks! Burned the skin enough the hair fell out in a 6mm path and didn't grow back for what seemed like months. Oddly, I didn't feel it at the time, only smelled it.

Rick

mahesh
2010-Mar-20, 10:57 AM
Hey hew..., stunning, awesome shots of the Moon...how many hours new is it?
Great captures of Saturn too. Thanks for sharing.


Baader Film is a good safe filter for your scope...
I, too, use Baader film for my home-made filters ('scope + binoculars)


MrJ, I can't open your linked picture...keeps taking me back into BAUT log-in!?! ...I can't wait to see TheEvent[/I]