PDA

View Full Version : H-alpha filters



BigJim
2004-May-17, 01:45 AM
Finally got to see one today. =D> They're quite impressive, although the white-light filter wasn't bad. I wish I could afford one.... sigh.

mutant
2004-May-17, 02:29 PM
I have seen them but cannot afford one. Does anyone know why they are so darned expensive. Is it the materials needed or productions costs?

Kevinito
2004-May-17, 04:16 PM
Check it out . . .

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=28181&item=3815496 493&rd=1&ssPageName=WDVW

-Kevin

Hale_Bopp
2004-May-18, 12:20 AM
Well, when they start shipping, you can get a Coronado PST for $499. It is a 40mm dedicated solar telescope with an h-alpha filter.

The filter on Ebay is for observing nebula and galaxies, not the Sun. An h-alpha filter for the Sun must be combined with some sort of blocking filter to keep you from frying your eyes.

So, here is a random thought that probably has a very good, obvious answer I am missing. Why not use a traditional full aperture white light filter on your telescope and plop an h-alpha filter on your eyepiece. Would be a lot cheaper than a full fledged h-alpha system. I assume if it were this easy, someone would already be doing it and promoting it, so what am I missing?

Rob

Kaptain K
2004-May-18, 10:13 AM
The filter on Ebay is for observing nebula and galaxies, not the Sun. An h-alpha filter for the Sun must be combined with some sort of blocking filter to keep you from frying your eyes.
Not true. As the posting on E-bay said, it blocks all EM, except for 20% of the light in a 0.15nm window centered on H-alpha. Such a filter would be useless for anything except solar viewing.

Hale_Bopp
2004-May-18, 05:23 PM
Sorry, Kaptain K. I still beg to differ. I would not look at the Sun with that filter. I would only use a full aperture solar filter for my telescope and judging from the size, this is not a full aperature filter. I see nothing that says it is safe to look at the Sun with that filter (at least by itself).

H-alpha filters are used for CCD imaging. Nebula tend to have lots of hydrogen and isolating the h-alpha line is great for photographing these objects where there is substantial light pollution.

Lumicon sells h-alpha filters for this purpose...you can read about them at

http://www.lumicon.com/haf.htm

Note that they specifically tell you NOT to use these filters for the Sun. I also see the transmission profiles are very different (over 90% claimed for the Lumicon versus over 20% for the eBay filter).

The ad also states


Other light from the x-ray spectral region to the far ir spectral range (10,000nm) is prevented from passing and so it is important to prevent focus of the beam at the filter.

This implies that it transmits light from the h-alpha region up to 10,000nm in the near infrared, similar to the Lumicon filter.

So, now I know why you can't use this with a full aperture filter on the Sun...it transmits too much other light to give h-alpha views of the Sun.

It would be really useful if the seller posted the filter transmission profile on the ad.

Let's also remember that sometimes sellers on eBay are not always well versed in the applications of the eqipment they are selling. It is possible this person got an h-alpha filter from somewhere, heard h-alpha filters were used to view the Sun, and assumed this one could also be used to view the Sun.

So, that's my two cents based on our admittedly incomplete data about the filter in question.

Rob

Russ
2004-May-18, 05:48 PM
I recalled S&T magazine having some articles on this topic, so I did and archive search and got these:


3 articles found for "hydrogen alpha filters", department: S&T Test Report, date range Nov.1941 to May2004

Pricing Info Archive Help New Search
1. S&T Test Report July 2003, p. 46-50 (PDF: 594 kilobytes)
Coronado 90-mm Better White Light Filter
BinoMite Solar Binoculars
CEMAX Eyepieces
By Dennis di Cicco
Purchase Article Purchase Archive Subscription Order Original Issue

2. S&T Test Report Video of the Stars
By David M. Moore | August 1999, p. 61-64 (PDF: 416 kilobytes)
Purchase Article Purchase Archive Subscription Order Original Issue

3. S&T Test Report August 1999, p. 64-66 (PDF: 295 kilobytes)
A First Look: SBIG's Enhanced ST-7E CCD Camera
By Dennis di Cicco
First Look, A: SBIG's Enhanced ST-7E CCD Camera

They charge for copies of the article or issue so I didn' t go any further than this. Hope it helps. 8)