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johninderby

Baader 2" Cool-Ceramic Safety Herschel Wedge - First Light

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Cheers Gina. One of my luckier ones!! Everything in the right place at the right time to make for a pleasing composition. Technically more trial and error than design I think. I need more practice.

John, given the high price of a good Herschel wedge, do you think a cheaper continuum filter alternative would add anything to image definition?

Stu.

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Cheers Gina. One of my luckier ones!! Everything in the right place at the right time to make for a pleasing composition. Technically more trial and error than design I think. I need more practice.

John, given the high price of a good Herschel wedge, do you think a cheaper continuum filter alternative would add anything to image definition?

Stu.

The continuum filter will help with granulation and contrast but it's a matter of probably improving things overall by about 5% to 10%. Some people just use a plain green filter which helps a bit but it's not as good as the continuum filter. I use the continuum filter with the the Baader wedge.

It probably sounds like a broken record by now but getting closer to the best possible views is a real pain in the wallet. :D.

Joh

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Hi John.

I don't see why this is true :

The Hershel wedge can only be used with a refractor so unfortunately it can't be used with a reflecting mirror type scope such as your 4SE Mak. With your scope a front aperture solar filter is the only one you can use and I think the Baader is best of any readily available.

John

Have you got any rationale for this ?

I have happily used my Herschel with both refractor and reflector - there is no issue with either, just make sure you have the appropriate aperture stops yo match the filtration.

I also can't see that there is a specific reason that a Herschel wedge would be better than film - you will get increased contrast from a refractor - in general - but the wave front errors must be equivalent in size between fixed glass in reflection with additional filter surfaces and a single layer of 25 micron-thick plastic.

Just a few thoughts.

cheers

Mike

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I've tried to respond a few times, but for some reason the thread has not been getting updated.

Trying to say, thanks Gina. I appreciate your comments. I think this was a lucky shot with composition and exposure. I need to do this consistently to make any claims regarding skill.

John, I wanted to ask if you had any thoughts on the use of a continuum filter in combination with the Baader film. Does this improve things?

Thoughts welcome.

Cheers

Stu.

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Ahhh...now it's updated. I think I had a problem with my server or something.

Thanks for the comments John......

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The Baader wedge is better than any film, period. The Baader film is excellent, and not that far behind the wedge, but it's still second best. One look through the Baader wedge instantly ends any discussion as to whether it's better than film. :D

The reason why Herschel wedges are not recommended for use with reflectors is that the focused light from the primary to the secondary can cause all sorts of problems. There will be a heat build up at the secondary mirror which could either cause it to loosen if it's the glued on type or even crack the secondary. It's something that I'd never recommend due to the potential risks involved. You may be lucky enough to get away with it but Baader simply says to never use the wedge with any sort of reflector.

John

PS

There are also the Zeiss full aperture solar filters. They are as good as if not better than the wedges but sooooo expensive (££££££) and virtually impossible to get hold of nowadays anyway. In comparison the Baader wedge is dirt cheap.

Edited by johninderby

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So....as a newbie....I've got, refractor vs reflector vs Herschel vs Baader film with and without continuum filter........dare I ask, which is best to capture the photosphere? I guess there is always an opinion one way or the other.:D

Not wanting to open any cans of worms..........of course:D

Stu.

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Just go over to the dark side and start using CaK or Ha and maybe sell a car or something to pay for it!!!

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Thanks John - you've now got me engrossed in this as a thought experiment.

<experiment mode on>

Given that the solaric irradiance at top of atmosphere is 1350 Wm^2,

that the heat capacity of float glass is 500J/Kg Kelvin,

that the density of float glass is 2.5 g/cm^3 and that the reflectivity of aluminum coating on the secondary is 96%.

For my Visac 200mm cass, there is a total of

PI*0.10^2*1350 = 42W solar irradiance focused onto the secondary mirror which then reflects all but 4% or 0.004 of that energy towards the eyepiece, leading to a grand total of 0.17 W retained by the glass.

The secondary mirror is 76 mm in diameter and assumed 1cm thick, so has a heat mass of PI*3.8^2*1.0 = 150 grammes and a heat capacity of 75 J/K for this mass.

So the secondary mirror is changing in temperature by 0.17/75 Kelvin per second in the absence of conduction or convection, say roughly 0.025 degree K per sec.

This gives me 4000 seconds to reach 120 Centrigrade from 20 Centigrade ambient, or just over an hour, again in the absence of conduction or convection.

I think on the basis of this I would strongly recommend not using optics with smaller e.g. Petzval intermediaries, or like my Visac with tertiary focuser optics due to the even smaller thermal mass but the argument for heating of secondaries as being a cause of concern remains not clear to me since convection cooling will set in when the glass temperature difference is above 15 degrees or so (by analogy to house central heating).

<Experiment mode off.>

that was fun.

Mike

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Thanks for that info. I think it may well be a matter of certain reflector designs can cope with the heat, but to play it safe I stick with the "Try it at your own risk" recommendation.

SCTs are probably the most at risk as the secondary is thin and glued into the holder, so very little thermal mass. Newts would also be at risk depending on just how the secondary is affixed to it's holder, not too mention the effect bad collimation could also have.

John

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Given that the solaric irradiance at top of atmosphere is 1350 Wm^2,

that the heat capacity of float glass is 500J/Kg Kelvin,

that the density of float glass is 2.5 g/cm^3 and that the reflectivity of aluminum coating on the secondary is 96%.

The real issue here is that the beam will not always be falling only on the secondary mirror - some of the energy will get to the supports which have a lower thermal capacity and an absorbtion at least an order of magnitude higher.

Do not use a solar wedge or attempt eyepiece projection with any scope except a pure refractor (one without flattening optics near the bottom end of the tube). Do not use a barlow upstream of the wedge, the heat will uncement the elements. Do not use an eyepiece with cemented elements for projection for the same reason.

A reflector with an unsilvered primary mirror is equivalent to having a solar wedge (but of course cannot be used sensibly for anything other than observing the sun).

If you're at all unsure, play safe and use a solar film filter fitted over the objective.

This is quite apart from the fact that the use of a diagonal (of any type) with a Newtonian reflector will result in a very uncomfortable viewing position, in the rather unlikely event that there is enough focus adjustment to get a sharp image at all.

Edited by brianb

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I'll stick with Baader film for my 130mm Newt even if it was a relatively cheap scope. I have great respect for the power of the sun.

Edited by Gina

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