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Phillips6549

Solar projection - on a budget?

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We have a fledgling Astronomical Society at my place of work and we're intending to have some telescopes available for colleagues to view the transit of Mercury in November.  While the majority will use appropriate solar filters it would be good to have at least one set up for projection.  However, we only have reflectors and Schmitt or Maksutov Cassegrain scopes among the members.  I was considering buying a small refractor second hand to use for this but thought I'd ask for advice here first.

Which is the best type of scope to use for projection?

What do we need to look out for (overheating & fire risks for example)?

What is the best umbrella to protect kit from the inevitable deluge on the 11th November? :clouds2:☔

 

Thanks

Mark

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Any reasonably long focal length 50mm/60mm achromatic refractor should be adequate. A Huygenian eyepiece, having no cemented optical components, would be preferred to avoid heat damage. There should be no significant fire risk, the heat will be concentrated at the focus of the eyepiece rather than the focused projected image. An umbrella might well be the best "umbrella" to protect kit against rain.   😀 

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Wouldn’t a cheap department store magnifying glass and a low power metal cased loupe work fairly well? I bet 1/4” foam core poster board, in black, could be flex glued using silicone (hot glue might melt) together to make a passing projector. A project I’ve thought about a good deal. Simple square boxing with a detachable huge front shield. Finding some nice projection material (finely frosted/etched glass plate, or maybe a film adhered to clear plexi) has been my hurtle. Yes, I’ve seen the sun cone design, but I want a larger image!

Color aberrations aren’t as noticeable, to me, using projection as in normal viewing, like looking at the moon. It might be fun to see what us CN’ers could come up with that is cheap, safe and simple.  Very briefly I have used my 32mm Plossl EP behind an old 4” scratched up simple magnifying glass to project the rising sun onto the side of my house. The sun was 4’ across!

I was in the Willamette valley of western Oregon during the 2017 eclipse, and I used half of an old pair of very cheap tiny binoculars to project the sun onto some thin paper. Sun spots and rough edge of the moon shadow were clear and crisp, and my iOptron SkyTracker Pro kept the sun on target. Travel restriction forced me leave my tripod at home so I had to zip tie one of those octopus micro tripods to a steel T post on the edge of a field, blind align by phone coords and use the settings on the little mount. Anyway, I gently touched the bino half from time to time, and it was never hot hours either side of totality.

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VIEWING THE SUN IS POTENTIALLY VERY DANGEROUS!

I'd be a little wary of any method of viewing the Sun which uses an unfiltered solar image. If any children, pets or careless adults just happened to get their eye to the eyepiece.......If you go along with the projection method, in terms of morality you ought to stay with the scope 100% of the time it is projecting which may be too much of a compromise. Peronally, I'd prefer just to stick with the Solar film method.

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Is the “sun funnel” design unsafe? How so?

I’m talking about something without an eyepiece, just two simple lenses, that projects an image on to a screen/film/substrate, but much larger than done in the following link. Unless someone/something tore at the “funnel” I cannot see how it could be dangerous, and doing so would surely knock a scope off target.

https://www.nightwise.org/sun-funnel

I’m thinking of something 18” to 24” for screen size inside a soft hood. A big front shield, a hood one would have to use to see the screen and lack of direct viewing access makes the idea seem pretty safe. What am I missing? I wouldn’t leave any scope unattended around kids too small to be careful, and even older kids only if they haven’t demonstrated responsibility. I’m only exposed to my grandkids, and their cohorts, and my expectations from them have been met with flying colors. YMMV.

I just think this is a simple solution that can be done safely and cheaply.

 

 

Edited by theropod
Typo

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39 minutes ago, theropod said:

Is the “sun funnel” design unsafe? How so?

I imagine they're great fun to put together and use. To avoid unnecessary mishaps one would be advised to use only refractors, eyepieces without any cemented/glued elements (are they still commonly available?) and to be aware that in fast scopes the heat concentration will be significant. If the scope, for example, is allowed to drift, focused and concentrated sunlight might damage the interior of the scope, might cause a fire, or what have you. Personally speaking, I'm of the opinion that solar projection stems from an era where there was no real alternative. These days there are many much better and safer options and which aren't necessarily anymore expensive.

Edited by Rob Sellent
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 Thanks everyone for all the replies.  I'm looking at a projection so that many people can see what's going on at the same time without the risk that a clumsy observer might knock the scope off target.  My thoughts regarding the sun-funnel (as opposed to a remote and separate screen) are that it would be safer because the light path is entirely enclosed with no opportunity for anyone to get direct access to the eyepiece.  

On 18/10/2019 at 22:45, Rob Sellent said:

These days there are many much better and safer options and which aren't necessarily anymore expensive

I recognise that a full solar filter on the objective would be better for individual observations but I'm unaware of a better way to enable a group to view at the same time,  short of using a camera and TV screen.  But seeing something with your own eyes has you be better then watching the TV doesn't it?  Otherwise we might just as well watch it on Slooh.

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I used a sun funnel to good effect at the 2017 eclipse 🙂   It was great for group use, and for just keeping an eye on what was happening while still being able to be part of the event, see the surroundings etc.  With a cheap refractor and a small tracking mount you're sorted 🙂 

Although I have some lots of other kit, I'm certain to use this for the transit if the Welsh weather cooperates!

Helen

 

 

 

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Thanks Helen

I'll be using an 80mm / 900mm Celestron refractor on an Alt-Az goto mount.  My only query now is where to get just a small amount of rear projection fabric without ruining a perfectly good screen. 

 

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I actually tried projecting Mercury at the time of the last transit a few years ago.  I set up my 102mm f5 achro refractor, which did not quite manage to resolve Mercury's dark dot.  I switched to projection using my 127mm Maksutov, which resolved Mercury's dot okay - by now some way inside the solar limb.

Lesson one: inferior optics will not be good enough to show Mercury. 

Lesson two: you can use a 127mm Mak. (I would not advise using any larger aperture telescope).

Lesson three: don't use an eyepiece with plastic body elements - they tend to melt if you get the Sun off-axis.

Projection screen - you do not need anything special - a white fine grained card will work just fine.

If you use a diagonal it saves the bother of making a sunshade for the projection screen - just hold it so the sun does not shine on it.

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2 hours ago, Phillips6549 said:

Thanks Helen

I'll be using an 80mm / 900mm Celestron refractor on an Alt-Az goto mount.  My only query now is where to get just a small amount of rear projection fabric without ruining a perfectly good screen. 

 

If you want some screen I probably still have some you could have.... How big a funnel are you going to use?

Or I think I remember a suggestion of using doubled up shower curtain 🤣

Helen

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I tend to be very wary of the sun, I much prefer the idea of a full aperture solar filter securely fastened at the sun end of the telescope so that no concentrated suns rays can get anywhere close to my expensive telescope bits and pieces.   Then everyone can come and take a peek at a decently magnified image (and I get the impression that this will only be a tiny dot on the sun) as the automated system tracks.  I don't think I'd feel comfortable pointing the open end of any telescope at the sun without some form of covering.

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12 hours ago, Helen said:

If you want some screen I probably still have some you could have.... How big a funnel are you going to use?

Or I think I remember a suggestion of using doubled up shower curtain 🤣

Helen

The funnel is 123mm across at the widest point (~117mm inside diameter).  It's identical to the one referenced here https://eclipse2017.nasa.gov/make-sun-funnel as far as I can tell.

Not sure that Mrs P would appreciate me cutting bits out of the shower curtain either!  Also the patterns would probably not enhance the image 🛀 

 

Geoff

13 hours ago, Cosmic Geoff said:

Lesson two: you can use a 127mm Mak. (I would not advise using any larger aperture telescope).

Lesson three: don't use an eyepiece with plastic body elements - they tend to melt if you get the Sun off-axis.

Projection screen - you do not need anything special - a white fine grained card will work just fine.

Thanks for the pointers.  I think all my eyepieces (those I'm prepared to risk anyway) have some plastic components.  I may need another visit to a well known auction site. 

 

 

Edited by Phillips6549

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Hi  Geoff / @Phillips6549 

Good to see that you are projecting the Sun, rather than viewing it for a group.

And talking of which, if anyone comes along to your viewing with one of these...

738812906_Screenshot(37).png.f89f51468cec5fa9a157afa9643d8e0b.png

...hit it with a hammer or other tool ...and then tell them why you did it.

Just my 2p's worth.

Edited by Philip R
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That eyepiece solar filter should be illegal, and no I’m not kidding.

They might make good small arms targets.

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Mark

The attached pic is my ED80 on a EQ5 Pro mount, using solar filter

Taken at recent club viewing day

I now have glass solar filter, which find effective

To project, something like a ZWO ASI290 MC which you will find useful with your other club projects such as AP

John

Skywatcher ED80.jpg

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@Phillips6549 sorry, will see if I can find where I put the spare. As long as I can find it I will have enough to send you 😁. PM me your address (unless you are going to the star party 😉).

Helen

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