Solar viewing, required filters.....

[stan26]

Hi there, just a quick question regarding viewing the sun....

I have a startravel 102 frac, do I just need to get some baader film and make a front apeture filter, OR will I also need a filter at the eyepiece end...? Thinking more from a safety point of view.....

Thanks

Stan.

[johninderby]

Yes you can just get some Baader solar film and make your own filter or you can buy the ready made filter.

http://firstlightoptics.com/proddetail.php?prod=AZsolarfilter

Cost for the ready made filter is about twice the cost of just the film on it's own. So is it worth paying a bit over £20.00 more for the ready made one???

BTW never ever, ever use one of those eyepiece filters, they're far too dangerous.

Now a Hershel wedge is another thing altogether. It replaces your diagonal and gives the best white light solar views and is very safe but a lot more expensive.

John

[Merlin66]

Baader solar film up front is all you need. Fitted securely! (and cover the finder!) 100% safe.

You can add a light green or continuum filter at the eyepiece to improve the contrast

[runoffshed]

Hi Stan, I don't use anything apart from the Baader film front aperture filter ( very good instructions on this site for making one with card etc..) but obviously the fit of the baader film filter should be tight. I always tape mine on with electrical insulation tape which keeps it secure but peels off for removal. The finderscope caps should be left on.

You should also check the baader filter each time by holding it up to the sun off the scope to look for any tell-tale pinpricks of light that would indicate damage etc...I would keep it in a tupperware type box with tissue rather than leave it in your equipment box.

You mentioned an eyepiece filter. If you have one of those "solar filters" supplied with telescopes in the 80s and 90s which screw into the eyepiece barrel it should be thrown in the bin!

Projecting the image is probably safer, although there are still dangers with this as there is always the possibility of someone 'having a peak' through the eyepiece whilst your back is turned.

Good luck

[DarkSky71]

Sorry to but in this thread , but if were to get a Baader Filter , what solar detail would i see. Do you see any surface details other than sunspots, anyone post an image that represents what your eye could see through one

[brianb]

Do you see any surface details other than sunspots

Yes, faculae. And, on very rare occasions, flares may be bright enough to be visible in white light, one was observed as far back as 1859.

I think this image is pretty representative of what can be seen by eye with a solar film filter on a 4" scope at around x100 in good conditions.

2011 Mar 26, 1055 UT. Faculae visible as networks of lighter areas around the active areas near the limb.

[michael.h.f.wilkinson]

I totally agree solar film is an excellent option, but for those considering a Herschel wedge: DO NOT USE THIS IN SCTS or MAKS. The primary is so fast that the secondary may shatter due to thermal stress. In a fast scope like the ST 102, thermal stresses can also build up, so beware of Herschel wedges in fast scopes as well.

[johninderby]

In a refractor the speed of the scope doesn't matter. In fact the 102 f/5 refractor makes an ideal white light solar scope for use with a Hershel wedge. I used to use that very combination for my white light viewing.

As mentioned never use a Hershel wedge with any mirror scope as that can be very dangerous.

John

[DarkSky71]

Thanks for the image BrianB, would still be wary of putting eye to eyepice . Was thinking of viewing through either a webcam or Meade LPI (would have to purchase either as well as Baader Filter) Trouble is if they have effective 6mm EP magnification will be 200x in an ETX90. This would probably be too high i assume. Looking at your image up to 100x is about best for FOV

[michael.h.f.wilkinson]

In a refractor the speed of the scope doesn't matter. In fact the 102 f/5 refractor makes an ideal white light solar scope for use with a Hershel wedge. I used to use that very combination for my white light viewing.

As mentioned never use a Hershel wedge with any mirror scope as that can be very dangerous.

John

At F/5 the light gets more concentrated, so thermal differences can be great. A well-built scope should not have any trouble, but I have seen plastic parts melt or deform. The effect is not nearly as much as at F/2. A reflecting scope is fine with a Herschel wedge provided it has a slow primary. My old F/8 Newtonian worked happily with EP projection on a white screen.

[John]

Thanks for the image BrianB, would still be wary of putting eye to eyepice . Was thinking of viewing through either a webcam or Meade LPI (would have to purchase either as well as Baader Filter) Trouble is if they have effective 6mm EP magnification will be 200x in an ETX90. This would probably be too high i assume. Looking at your image up to 100x is about best for FOV

I've recently got a Kendrick white light solar filter which uses the Baader filter film I believe. I was a little nervous about putting my eye to the eyepiece but I'm glad I did - Brian's photo is a pretty accurate representation of what I could see with my 4" refractor

I find an 8mm eyepiece (83x in my scope) gives a nice view of the solar disk.

And it's nice to observe with the warm sun on you as well

[brianb]

A reflecting scope is fine with a Herschel wedge provided it has a slow primary

Definitely NOT SAFE there is a good chance of cracking the secondary or melting glue used to attach it. Same applies to projection.

The ONLY SAFE way of viewing or imaging the Sun with anything other than a refractor is to use an objective end solar filter made from Baader solar film (or one of the alternatives which are much harder to obtain). These really are very safe PROVIDING they're attached firmly to the objective end of the scope and are always checked for tears, splits and holes before use.

A solar wedge is marginally (but not hugely) better than solar film if you're using a refractor. As always, long focus 'fracs work better than short focus ones, unless the short focus one is of expensive triplet apochromatic construction. These days I tend to use a solar wedge except when the observing in public, when I use a solar film filter so that it's OBVIOUS that I'm using special equipment.

Projection is less good (though unconditionally safe for your eyes) and there is a serious risk of meting the cementing in modern eyepieces even when a small refractor is used.

[michael.h.f.wilkinson]

Actually, Kit Peak uses a very slow reflector without solar film. If the scope is slow enough and the secondary low enough coefficient of thermal expansion, you will not create large thermal stresses.

I would still recommend solar film as better any time. A main problem with a Herschel wedge is the sheer aperture of even small reflectors.

[brianb]

Actually, Kit Peak uses a very slow reflector without solar film.

And without a reflective film on the primary ... making it equivalent to having a huge solar wedge mounted in front of the objective.

A conventional reflecting catadioptric scope (with reflective coatings i.e. usable for night time observation) of any aperture is not safe for use with a solar wedge or using the projection method. The heat sink on my Lunt solar wedge gets only slightly warm when used with my 110 mm refractor, I don't know the maximum safe aperture but I'd suspect somewhere around 12". Since few amateurs have access to refractors of that sort of size, I rest my case.

[michael.h.f.wilkinson]

I had forgotten about the non-coated primary of Kit Peak. I do however note Polarex sold projection kits for their slow Newtonians when I was 17 year old. When I built a 6" F/8 with aluminium coated primary and secondary (40mm diameter, rated at 95% reflectivity), I made the following calculation.

The diameter of the unfocused solar image on the secondary is some 37 mm (25mm cone width convolved with the 12 mm focused solar image). This means the light is concentrated 16.88 times. It also means the light is distributed over the bulk of the mirror surface. Note however, that the secondary is at a 4 deg angle, reducing the intensity by a factor 0.71, yielding a concentration of 11.93. Also the central obstruction takes away about 5%, so a factor of 0.95 needs to be factored in, together with the same factor for the reflectivity of the primary, reducing the figure further to 10.77. More important, the bulk of both the visible AND infrared light is reflected, so only 5% is absorbed, yielding a figure of 0.538x the power absorbed by a fully blackened object lying in direct sunlight. This gets warm, not hot.

To be on the safe side, I redid the calculation with a 90% reflective mirror, and obtained a figure of 1.02, i.e. 2% more than a black object lying in direct sunlight. This can get uncomfortably hot (which means hotter than 43 deg C, which is the pain limit), IF it is not cooled convectively or otherwise, but should still not do damage. As the secondary is at the front of an open tube, and it is connected to a large thermal sink in the form of the holder, it is cooled to a certain degree (note that conductive and convective heat transfer way outstrip radiative in this case).

Having done these calculations, I set the scope up, and monitored the temperature of the secondary (by touching its side). It became warm, but not uncomfortable to the touch (after 20 min).

The fact that the coating is aluminium has a few important consequences: first, it reflects well in infrared (the infrared spectrograph I tested in Gornergrat for my BSc thesis used aluminium gratings and mirrors) and UV. Second, because it is metallic reflection, it relies on electrical conductance of the surface to form an equipotential surface, which is reflective. If the temperature rises a lot, the conductance drops, and increasing absorption, increasing temperature again, etc., leading to a catastrophic runaway effect. However, the observed heating of the mirror was insufficient to have that effect (unless reflectors routinely fail to work above 40 C ).

Dielectric coatings designed for visual use may well absorb a lot more IR and UV, leading to catastrophic failure much more easily.

After confirming the secondary did not overheat, I turned my attention to the eyepiece: a 25mm circle T ortho. This absorbs about 2% of the light, and can thus get quite hot. I checked the EP for plastic parts (none), and checked that the linear coefficient of expansion of glass is about half that of brass and steel, so the glass could not readily be pinched in its mounting, and shatter because of it. If anything, the fit might become a bit loose.

With some trepidation I inserted it, and projected the image of the sun. The scope did not track, which meant nudging it, which means the heat is distributed a bit more than in a tracking scope. I noted that the EP became quite hot to the touch, but did not hurt (meaning less than 43 C temperature), probably because the heavy metal focuser acted as a heat sink. You could light a piece of paper in the exit pupil!

Thus, I did the calculations, and I did the experiment, which confirmed the calculations.

Note that I never used a Herschel wedge in my newt: it was way too expensive! Besides, a very simple problem with Herschel wedges in Newtonians is simply that the latter cannot reach focus using such a device (quite apart from the risks).

Again, with modern dielectric coatings, and certainly in closed tubes like SCTs your are asking for serious trouble. Solar film is far safer (and way better than EP projection anyway). In that we certainly agree.