H alpha filters

[acropolite]

A question on filters and solar observing. We know that the Coronado PST gives superb views of the sun, particularly it's structure and prominences. Given that the PST allows only h alpha wavelengths through, has anyone tried an h aplha EP filter in combination with solar film to see if the same effect occurs.:shrug:

Note: Before anyone jumps in, I am not suggesting using the h aplha EP filter without a suitable (baader or 1000oaks film etc) solar filter on the front of the OTA.

[Dennis]

Hi Phil

I tried and with an H alpha and the Baader Astrosolar ND 3.8 and it didn't really work. The image on the notebook was quite poor; it looked very tonally compressed and not as good as without the filter.

I then clicked colour mode and the image turned red as expected, but again, not as good as the B&W without the H alpha. I'll see if I kept some of the experimental avi's, although a nagging thought tells me I deleted them. Oh, and there were no filaments or prominences - not even a hint of them.

Cheers

Dennis

[[1ponders]]

You want to see some nice Ha images Phil, check this out (http://www.coronadofilters.com/). I'm finding it very hard to resist.

[seeker372011]

are you familiar with Larry Alvarez's work

http://mysite.verizon.net/armicheal/
stunning stuff.. the guy is exceptional in what he is able to produce consistently

[[1ponders]]

He definately does great work. Oh to be able to afford a Solarmax 90 :love:

[iceman]

Phil I don't see how that would work - the solar film cuts out all wavelengths except visible light, right? So adding a H-A filter at the EP isn't going to produce anything I would've thought?

The H-a solar filters need to be on the front of the scope, so that only the h-a wavelengths are passed through.

I could be wrong?

[Dennis]

Hi Mike

I read your post as Registax was chugging through an H Alpha avi. Here is an image in H Alpha using a Schuler H Alpha filter. The image was taken through the Vixen 102mm f9 refractor with x2 Barlow. 73 frames from 1800. Not very pretty!

The ToUcam was in colour mode, so I assume that only the x1 Red pixels were being utilised in the Bayer matrix, not the x2 Green and x1 Blue?

Cheers

Dennis

[iceman]

Dennis did you have a white-light filter on the front as well?

[Dennis]

Hi Mike

A BIG YES! I had the Baader Astrosolar photographic filter (ND3.8) which is for photographic use only. Here is an image from an avi where I captured in B&W. I was expecting better results but it looks like this is not the way to go with H Alpha?

Cheers

Dennis

[iceman]

What i'm thinking Dennis, is that the white light filter blocks the h-a wavelengths coming through, so putting a h-a EP filter in isn't going to help at all except to make the image red, as your avi shows.

I've never heard of the H-a EP filters (used mainly for deep-space imaging) used for solar imaging.

[Dennis]

That's why I tried the Schuler, to see if I could find a cheap way into H Alpha solar imaging. :D :D

I cannot remember the tech details from the filter, but a search revealed the following details: Schuler - H-Alpha 1.25" 10nm Bandpass, 656nm Narrowband CCD Imaging Filter.

Anyhow, it didn't work; either in colour or B&W mode. :(

Cheers

Dennis

[Don Pensack]

This doesn't work because the bandwidth of the H-Alpha filter is not narrow enough to capture ONLY the H-Alpha line. The bandwidth of a typical H-Alpha filter is easily 200nm or more. You need 0.1nm at most.
There are available (see lumicon.com) H-Alpha filters that cover the front of the scope and provide the correct bandwidth, but they are expensive (~$900US for a filter appropriate for an 8" scope).

[Suzy_A]

I've got a 7 nm 2" Baader Ha filter and have tried it with various combinations of solar filters and ND filters to see what I could see. Visually or with a Canon 30D camera set to colour, you get a red circle.... The number of colours available on a dig camera is not enough to show up much in photos. Maybe you can use photoshop to streach out the colours? Set to monochrome, you get to see a little bit more, but not a lot.

Basically, the bandwidth of these types of Ha filters is not enough to cut out all the 'noise' light around the Ha line and all you see is what you would see in white light, but it's red. You need at least 1 nm to see something decent, and really less than 0.7 nm for it to be really good. When you get down to 0.2 nm, well, you can see everything. But a 0.2 nm filter is probably worth (or rather costs) more than every bit of camera gear and astro gear I have, plus my car thrown in as well.

The best way to see something useful is to get a Coronado PST for $800. These have a ~1 nm filter and show up a reasonable amount. The problem with these is that they are fitted onto a 40 mm f10 telescope, so the maximum maginification is about 100 (and that's pushing it) and you really need more than that.


So.... one possibility is to rip it apart (gently!) and use the good bits and attach them to a better telescope - eg a ED 100 f9, preferably better. Now you can get about 200 or 250x or more with a better scope. As it's now putting through more light, you will need to either stop the scope down a bit (or you will fry the filter and/or your eyeball) but that brings you back to square 1, so the other way is to use a ND filter and/or a dark red filter over the objective so that you still have the aperture but only a similar amount of light to the original 40 mm coming through. I also used a IR/UV cut filter to make sure the nasty IR and UV are blocked.

All the main bits on a PST unscrew - if you are careful and use a bit of muscle. There is heaps of red glue on the threads to try and stop you from doing that...

I had someone machine up an adapter so that the thread on the PST body fits onto my telescope. You will need to experiment with tube lengths and focus etc.

If you want to use the PST bits for photography (and for best visual results as well), you need to make sure the focus is outside the tube and eyepiece holder thingy - with the standard PST you can't take photos as the focus is too far inside the eyepiece thingy. You can get around this by unscrewing the eyepiece thingy and removing the bit closest to the main body. BUT be very careful as the eyepiece thing (with the plastic knob on it) will now touch the prism inside the main body. You need to put a spacer in (about 2 mm) to stop this from happening and scratching the prism.

Have a look at http://www.astrosurf.com/re/pst_dismantle_20060617-01.jpg and http://www.astrosurf.com/re/sun.html - almost down to the end of the page.

I'll put up some photos of what I have done with my PST - one day!

[Don Pensack]

Note that 1nm = 10 Angstroms.
An H-alpha filter that's usable for solar viewing has a maximum bandwidth of 1 Angstrom (like the PST), or 0.1nm.
Larger telescopes need 0.7 Angstroms, or .07 nm in order to be dark enough.
If doing H-Alpha at larger aperture, bandwidths get down to ~0.2 Angstroms, or 0.02 nm.
When 0.7 Angstrom H-Alpha filters are used on larger telescopes, they typically stop the scope down to 90mm or so.
A 7nm H-alpha filter is passing 70 Angstroms, and this is simply not safe for viewing on any size scope.
I've seen some H-Alpha filters that are simply a long-wavelength pass filter, and this style will not work well photographically with modern CCD cameras unless an IR filter is added.

[Suzy_A]

[QUOTE=Don Pensack]Note that 1nm = 10 Angstroms.
An H-alpha filter that's usable for solar viewing has a maximum bandwidth of 1 Angstrom (like the PST), or 0.1nm.
QUOTE]

Whoops! Yes, sorry. It was late at night. (That's my excuse - it wasn't the night-cap...) Yes in some places it should have been Ångstrom where 1 Å = 10E-10 metres, although this is now no longer a 'real' unit and has been officially dropped and we are now all suppose to the SI unit, the nm = 10E-9 m.

So to correct what I wrote - the Baader 7 nm (70 Ångstrom) filter and other similar filters are useless for solar prominence and flare etc observation, even when correctly filtered with other blocking filters. For prominences and flares etc, you need at least 10 Ångstrom - preferably much less - and for granulation you need a 0.1 nm (1 Ångstrom ) or less filter, although the good stuff isn't really visible until you are down to at least 0.7 Ångstrom, as Don points out.

The easiest and cheapest way that I know to get reasonable performance is to strip a PST and put the black box on a better scope.

A problem with most of the sub-Ångstrom filters are that they only work at large f numbers - typically f10 or preferably more. The angle at which the light comes through the filter is too great on slower telescopes and due to the thickness of the film and the optics of the filter, the bandwidth of the filter increases and you lose the sub-Ångstrom filtering capability. What you tend to get on a telescope that is too fast (ie the light coming down is to convergent) is a donut area of the image where it is OK and the rest of the image is outside the H-alpha wavelength.

'Tuning' of the filter is usually done by tilting the filter - which changes the effective thickness of the film.

The other way is to build yourself a solar spectroheliograph. You can do this for about $1000 - $1500 and a month of Sundaze. Basically it uses a diffraction grating, other optics and a slit and a rotating element to tune to the H-alpha - or any other frequency - you want.

There are a few websites devoted to this topic. A guy by the name of Veio has written a lot about them and you can download his book as a PDF. Also Westland and there are others. Have a look halfway down http://www.digilife.be/club/Franky.Dubois/sit.htm and there are links to a number of sites.