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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 988
Loc: Oman
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Quote:
I see there are two kinds of H-alpha filters: narrowband (~7nm) and broadbande (~13nm). I'm planning on buying one to use with my DSLR. Which one should I buy or do I really need both?
Which brand do you guys like?
The filter to get is the Astronomik 6nm Clip Filter (with certified 90+% transmission at Ha, available only by direct purchase from Astronomik in Germany). The uncertified filters available from US retailers are probably fine, but I did not want to take the risk. The full answer can get very, very long. But these comments may lead you in the right direction:
Doppler line broadening is very minute, fast focal ratio broadening is also minute. Both these are generally under a nm. They do matter if you are talking of filter bandwidths of a few Angstroms (NB 1 nm = 10 Angstroms). Unsharp stars are due to misfocus, not really the Bayer Array, since the Ha leaks onto the G pixels in Canon DSLRs so loss of resolution is trivial. I have a few pages addressing filter issues on my website. 13nm filters are, frankly, too wide if you have a lot of light pollution. Been there, done that. Pay up and get the best up front, as indicated above. If you are willing to use 2" filters then the Bayer 7nm Ha CCD is also great. I do own the Bayer 7nm, Astronomik 13nm and Astronomik 6nm, but I have not shot the same target with a couple of these for comparison. Some things just take too long for our short lifespans.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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Samir, If you are willing to make a comparison of the 13 and 6nm filters I just picked up a Canon EF 200mm f/2.8L II USM Lens. I believe you also have this lens. Lets pick a target, iso number and length of subs ect. Perhaps not a perfect test but it might be fun.
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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s58y
Postmaster
Reged: 12/12/04
Posts: 5509
Loc: Eastern NY
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Quote:
Interesting. What filter is on your Hutech 30d? Clear or astronomic ir cut? I ask because I noticed much more in the way of star halo effect with my self modded 350d "no filter replacement" than my new 1000dh with the mild ir cut, when using the ha filter. My 80ed also seemed to cause more bloat and halo than my meade sn6 reflector or a quality camera lens would.
The 30D has the Hutech UV/IR blocking filter. For some reason, I never can find a sharp focus point when using either of the H-alpha filters, compared with an Astronomic UHC or IDAS LPS filter (or no filter). The problem was worst with the 13nm H-alpha filter. Maybe some other setup would get the worst bloating with the Baader 7nm filter?
Unfortunately, I can't use the Baader 7nm inside my ancient Nikon 800mm lens, so I may end up getting one of those Astronomik 6nm clip filters, an Astronomik 48mm threaded 6nm filter (rim is narrower than the Baader), or even a 50mm round Astrodon 5nm filter to fit inside the lens.
Murphy's law says if I get the 6nm clip filter, I'll soon move to either a full-frame DSLR or a real astrocam. If I get the 5nm Astrodon, I'll never move to a real astrocam. If I get the 6nm threaded filter, I'll end up with a camera requiring square 50mm filters.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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cweinhei
sage
Reged: 01/14/06
Posts: 258
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It looks ro me like the halo is only a big problem for really bright stars. Is that the conventional thought? I didn't notice the halos in your shots above.
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Orion Atlas EQ-G w/ EQMOD
8" F/4 LXD75 Schmit-Newt (Flocked)
JMI NGF-55D Focusor
Hutech Canon Rebel 350 XT (Clear filter)
DSI Pro (for guiding)
Shoestring USB Shutter Control
Shoestring Focusor Control
HNSky,PHD,DSLRFocus,WCS,Iris
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 988
Loc: Oman
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Quote:
Samir, If you are willing to make a comparison of the 13 and 6nm filters I just picked up a Canon EF 200mm f/2.8L II USM Lens. I believe you also have this lens. Lets pick a target, iso number and length of subs ect. Perhaps not a perfect test but it might be fun.
Hi, it's actually that specific lens (and others) that have steered me towards a pop-in, clip filter. As I said, it's a very looong story. Used to be that the only pop-in Ha filter available was a Hutech, but that had to be used with a UV/IR blocker (because it leaked some IR). My DSLR at the time had clear glass. When I acquired another modded DSLR (with UV/IR blocker) Hutech had stopped selling the filters. So I bought the Baader 7nm 2" Ha. An excellent filter, narrower than my 13nm Astronomik. At the time Astronomik had problems with manufacturing their 6nm Ha, even though they were still being advertised. I was in correspondence with the owner of Astronomik. When they finally sorted out the fabrication issues, they also offered a pop-in version. This was a godsend! Usable with the 200mm lens wide open (also with all my lenses from 14mm to 600mm). The difference between the 6nm and the 13nm is actually immediately obvious. The image through the viewfinder is much dimmer, and autofocus is rarely possible with the 6nm. Last time I autofocused on a street light about a mile away. I do have an image with the Astronomik 13nm and the 200mm lens here but not much use for comparison. Like most stuff in astrophotography, without a controlled, side-by-side comparison one can easily end up with the wrong conclusions. Stuff like light pollution and post processing are very critical. Of course, just being able to use your 200mm lens at f2.8 instead of f4.5 (2" filter attached at the front of the 200mm lens with step-down rings) is worth quite a bit. So far the only nagging issue I have experienced was that in a C14 Hyperstar the Astronomik clip filter showed more flare from bright stars than my 2" Baader, but I had not used both filtyers on the same star field; so was it the star field that had much brighter stars? or the filters? I doubt that the flare had anything to do with the filters, but if somebody reports something similar then it would be worth examining.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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Ok Samir, and we are using different cameras as well. Too many variables indeed. I have yet to try the ha filter at my dark site so I am curious as to the improvement over my home suburban site. But I am pumped about this lens, seems like an ideal solution at 200mm.
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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s58y
Postmaster
Reged: 12/12/04
Posts: 5509
Loc: Eastern NY
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Quote:
It looks ro me like the halo is only a big problem for really bright stars. Is that the conventional thought?
You can see off-center reflection haloes around the two bright stars in Orion's belt in this narrowband shot:
haloes
It's not clear that they're caused by the filter, of course.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, TV102iis
Old camera lenses: 800mm f/5.6, 180mm f/3.4
AP900, Barndoor tracker
http://www.pbase.com/s58y
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 988
Loc: Oman
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Quote:
Ok Samir, and we are using different cameras as well. Too many variables indeed. I have yet to try the ha filter at my dark site so I am curious as to the improvement over my home suburban site. But I am pumped about this lens, seems like an ideal solution at 200mm.
The narrowband filters do make a HUGE difference to the visibility of nebulae even at a very dark site. Here is a comparison (sort of, since integration times were not equal) using a not particularly narrowband filter (40nm passband at Ha). Worth a click, IMHO. Generally I found that you need subs about 3x as long as for white light, minimal length for white light at a dark site is one minute at f2.8, narrowband 3-minutes. Adjust subexposure length for focal ratio. Since the subs can get very long for slow OTAs one might wish to start with a very short lens. A Canon 50mm/1.8 or 50/1.4 are excellent at f2.8 in Ha, and almost any mount should be able to track that for 3 minutes unguided. Note: because there is NIL chromatic aberration when shooting narrowband, f2.8 works well with these 50mm lenses, but in white light one needs to close down to f4. I have also come to the conclusion that the bulk of the imaging time (say, 75+%) should be with the narrowband filter, the white light is really just to provide star colors.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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justabob
Carpal Tunnel
Reged: 05/05/07
Posts: 1685
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Very nice Samir. I have a two week holiday this new moon and adding ha data to my 50mm Cygnus is on the list of targets. I will try it at f2.8 as you suggest. But I think I will go with 5 min subs, because I am building a collection of five min darks at various temps.
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http://www.pbase.com/rkn/astro&page=all
Vixen Sphinx SXW
Meade sn6
Canon EF 200mm f/2.8L II USM Lens
Hutech 1000d
Self modded 350d
ST8300c on order
DSI PRO II
Bob
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KevinUK
Vendor (DSLR-AstroMod)
Reged: 08/22/07
Posts: 720
Loc: N 51'53 W 00'25
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Quote:
Quote:
It looks ro me like the halo is only a big problem for really bright stars. Is that the conventional thought?
You can see off-center reflection haloes around the two bright stars in Orion's belt in this narrowband shot:
haloes
It's not clear that they're caused by the filter, of course.
Hi I get exactly the same off-centre affect shooting with my modded 450D and Astronomik UHC clip in filter through my Meade 8" SN . I am still unsure what causes this.
The focus looks fairly sharp and only the bright stars with haloes have their centres skewed. If it was rotation or coma all objects would be similiarly effected and focus would be compromised and soft in final image which it isn't.
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DSLR AstroMod
DSLR filter removal and replacement packages
http://www.dslrastromod.co.uk
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