What about Solar Scopes make them so expensive?

[AlexB]

Hi learned peeps,

I just wanted to know, technically, why are specialist solar scopes so expensive and on a side not, how come there isn't a proper 'lens' that can be placed over the front of our existing scopes to give us the same kinds of images?

I'm guessing it has something to do with optics rather than the filtering out of the necessary wavelengths.

[Peter Drew]

I would think that the answer lies with the filtering rather than the optics. To be able to selectively isolate a narrowband such as Ha requires very difficult to manufacture filters involving specialist coatings and incredibly close tolerances in respect of thickness and eveness of coatings as well as parallelism of optical components. If you compare the cost of these filters with or without a telescope then the telescope is almost a give away!

[John]

It's to do with the filters actually. Optically they are pretty simple. I'm no expert on solar astronomy but I was pretty amazed that I could buy a scope for a few hundred quid that would actually show me the sun in hydrogen alpha light and let me see features that were hitherto only visible during a total eclipse.

I did a little reading about the Etalon filter and it's a pretty amazing piece of kit and even more amazing that Coronado can bring the PST to market at well under £500.

Edit: Peter and I typing at the same time !.

[PeterW]

High spec narrow band filters, high spec uniform reflective coatings, large extremely flat and parallel plates of glass, optical contacting.... The list goes on. Thenproblem is that coatings cost goes up with area.... You can only get so many substrates on the chamber. Making large bits of glass extremely flat is very hard. Sub aperture filters have dropped the entry price, but they are never going to be cheap, the kit to make them is costly and you need skilled people to get it right. The optics are dead simple by comparison as you are only interested in performance at one wavelength so any old achromatnwill work perfectly.

I'd start saving fast and look to pick up one of the starter scopes, they still show a good deal and will get you in the game as we build up to solar max again.

PEterW

[AlexB]

Thanks for the info. chaps I didn't think the filters were such a big deal given there are so many available in the market (of varying type not just solar.)

Are there any options at all for satisfactorily equipping an 8" Newtonian with solar capability (other than just solar film.) Could I use solar film and then use a darn good eyepiece filter?

[Merlin66]

Don't confuse the Ha imaging filters you see for DSO type objects with those used for Ha solar... they are COMPLETELY different concept and design.

The Ha solar requires a VERY small bandwidth around 1A whereas a "conventional" Ha is around 60 to 150A bandwidth and will not work for solar.

To get this precise bandwidth centred on the Ha wavelength requires some very specialised filter designs - these are the etalons..not just a piece of "coloured glass"

[AlexB]

Understood.

Still, the question remains, is it not feasible to obtain/make one that is either 2" or 1.25" and have it on the focus tube and twin it with solar film (for safety?)

Please forgive my lack of knowledge on the subject.

[Merlin66]

No. Not really, the type of filter required to get the 1A bandwidth is very a complicated optical design.

It automatically filters out all other visible wavelengths so a white light type filter is neither necessary or beneficial.

An energy reduction filter (ERF) which is basically a UV-IR + Red filter, can be used on the front of larger Ha scopes to reduce the heat/ energy on the sensitive etalon.

[AlexB]

I guess these are an ever-so slightly more cost-effective route, as I understand it, I would need a 2" filter system for F/1000mm and a double stack to go with it?!

[Merlin66]

Not sure I understand your reply.

A "typical" Ha scope has either a full sized etalon/ERF combo up front at the objective ie SolarMax 40 or 60. These are 40mm diameter or 60mm diameter. At the rear, you need an additional filter called a blocking filter..these are usually 5mm to 15mm diameter (BF5 or BF15) - There's no additional 2" or 1.25" filters involved.

In the PST there's no ERF up front... the etalon is positioned closer to the focus and a BF5 is built into the eyepiece holder.

Don't get confused with "normal" astro filters - when we talk about Ha solar, we're talking about something completely different.

[AlexB]

Sorry, I meant to drop a link in that last reply. On the end of the link they state that the filters in question can be used in conjunction with a normal telescope, unless I misunderstood it.

"Retrofit your Astronomical Telescope"

www.lunt-solarsystems.eu/index.php?id=22&L=1&tx_commerce_pi1[catUid]=4&cHash=c2909c1a63

[AlexB]

You'll need to copy and paste that link as it has square brackets in it :/

[Merlin66]

Yes for 1159 euro this filter would allow you to image in the wavelength of Calcium......not the same challenge as a Ha filter!

[AlexB]

Yes for 1159 euro this filter would allow you to image in the wavelength of Calcium......not the same challenge as a Ha filter!

Somehow managed to copy the wrong link lol.

This one:

lunt-solarsystems.eu/index.php?id=22&L=1&tx_commerce_pi1[catUid]=4&cHash=c2909c1a63

It says if you double stack you can get to <0.5 Angstroms.

[AlexB]

Hmm, looks like the link doesn't quite qork no matter what! Basically, on that site there are H-Alpha filters on menu on the left just above the Calcium link.

[Merlin66]

Calcium Ca-K Solar Telescopes and Modules : Products: Lunt Solar Systems[catUid]=13&cHash=2a87f85d6cb5f3285412f9f63444a29f

Yes I see the 50mm etalon with a BF1200 for 1969 euro.

[AlexB]

Means I can get a 200mm aperture (newtonian) solar scope...!! (I bet that's wrong somehow lol)

[Merlin66]

The 50mm filter is the maximum aperture!

If you were using it on a 200mm scope you'd need to manufacture an adaptor plate to cover the 200mm and leave a 50mm hole for the 50mm etalon...what were you thinking happened????

[AlexB]

This is where my knowledge on the matter is failing me. I assumed it would filter the image being seen at the focus tube.

*sigh*

[FraserClarke]

My knowledge of etalons is rusty, but I think they only work properly in a parallel beam (i.e. before the telescope optics). If you put them in a coverging beam (i.e. in the focus tube), they don't work as the angle of the light cone coming in changes the effective pathlength between the plates, and hence changes the wavelength at which the etalon works.

[brianb]

My knowledge of etalons is rusty, but I think they only work properly in a parallel beam (i.e. before the telescope optics). If you put them in a coverging beam (i.e. in the focus tube), they don't work as the angle of the light cone coming in changes the effective pathlength between the plates, and hence changes the wavelength at which the etalon works.

Yes. You can use a collimating system to parallelize the light & get away with a subaperture etalon (the PST and the Lunt scopes do this), the penalty is more of a tendency to "sweet spot" effects with both tuning and vignetting. There's no doubt that a full aperture etalon is best but it does get very expensive indeed in "large" ( > 70mm ) sizes!