Solar imaging with large glass.

[Ewan]

Just thought I'd post this snippet of info straight from the horses mouth as it were, Jen Dudley, Daystar owner / founder.

A guy on FB, Andrea Van on posted a fantastic solar prom capture with a 152mm scope. Jen was asked about ERF & safety etc using a 2" IR/UV in front of the diagonal & NO full frontal D-ERF & she confirmed it is safe.

FB link https://m.facebook.com/story.php?story_fbid=1966619613348202&id=100000005477295

That' a hell of a lot cheaper than a front D-ERF & the only downside is thermal currents could be an issue & you will have to watch your equipment like a hawk.

My D-ERF is 135mm, actual clear glass of 125mm, I know I am loosing roughly 1" all the way round which can equate to a fair bit in capture speeds ie ms & detail seen.

I may have to try this but may also grab a 2" Ha filter as well.

Just thought some might find it useful.

Atb

[michael.h.f.wilkinson]

You ERF does indeed not have to be front mounted in a refractor. I plan on getting a 152mm frac and placing my 85mm tri-band ERF (80mm clear aperture) halfway down the tube. Regarding tube currents, there is a potential issue. When imaging the sun with a reflector or catadioptric system, the ERF needs to be front mounted, especially with a fast primary, which will concentrate a huge amount of heat onto a small portion of the secondary, which might well shatter. I hadn;t heard that a 2" UV/IR block filter in the diagonal would suffice for a 6" scope (I read in Daystar documentation it is OK in an 80mm). When using an H-alpha filter I guess you should use a CCD version, which blocks IR too, or add a separate IR/UV block filter.

[Macavity]

I was of two minds... I hadn't shared that I HAVE run my 80mm Franken scope
"sans filtre" (without the ERF!), but I sense there is a certain risk factor as one
goes to greater aperture? My "back of envelope" calculation suggested that the
typical Etalon / Quark (ERF) would be dissipating about 10W(?) with an 80mm.
 

Not sure I'd be quite so happy with 6" & 40W (Thinking light bulb / resistor) 
As Michael infers, it's differential heating that might crack non-annealed glass?

I console myself that my (expensive enough) 90mm ERF will "always be useful"
located some way down tube of some future "if I live long enough" scope... 

[Merlin66]

Reducing the incoming energy as far as possible is always a good solution.

Thermal effects are minimal - think of a large aperture scope used with a Herschel wedge......

If the light beam is not impinging on anything - baffles etc there’s nothing to generate hot spot thermals.

The recommendations for the Quark are unique to the Quark design and may not be the best or safest for other solar filters.

 

[Peter Drew]

I have been using a 6" aperture objective along with an internal 80mm triband ERF visually for a few years now without issue and have built several others on the same layout basis. A Quark has its own built in ERF so maybe a special case. In any event I will also prefer to use a reflective ERF such as the Baader D-ERF series as the downstream heating is minimal. As Merlin66 says, there should be no more internal thermal problems to affect the final image in Ha than there is for white light using a Herschel wedge. 

[Macavity]

That's the thing, innit? The Herschel Wedge is transmitting 94%
of heat & light. A "clear" UV/IR filter is absorbing ALL the HEAT.

Not trying to "outsmart"! I was trying out my new ED80 for White
Light solar, the first time. I hadn't developed my "new system"...
I forgot I leave most open (optical) holes closed / PLUGGED. And: 

The hole does go *right through* albeit in a very few seconds!
Keen Eyes will doubtless note the *really* BAD colimation...

For me: £300 ERF versus (safety) re. £1200 Etalon? (Eyeball).

[michael.h.f.wilkinson]

A dichroic (or dielectric) UV/IR block filter will reflect rather absorb UV and IR. Visible light is transmitted, so isn't absorbed either, apart from a very small fraction. Apparently the filters can handle the residual heat

[ngwillym]

Ewna,

Looks like Daystar are updating their advice as people try out more things with the Quark.

It was a year ago that they confirmed to me that a UV/IR cut in front of the quark was sufficient for my 127mm Apo. I've now 'upgraded that filter to a Baader 35nm Ha filter instead - Daystar also thought this a good idea, with the caveat that it should be as far away from the quark as possible - in my case at the other end of a Revelation 80mm 2" extension tube. Has all been fine so far.

Neil