Old 30-01-2013, 12:18 AM
originaltrilogy (Petr)
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Filters for f1.9 use??

I want to get set of very narrow filters (3nm) for use with hyperstar at f1.9

I see some filters not rated for use at less than f3, why is this? How does it affect?
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Old 30-01-2013, 06:05 AM
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bert (Brett)
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The faster optics can cause the passband to shift off the emission line. Essentially the passband of the filter shifts from the emission line of the object. Also the narrower the filter the more likely it is to shift off the emission line.

Last edited by bert; 30-01-2013 at 08:51 PM.
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Old 30-01-2013, 11:14 AM
Poita (Peter)
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Take a look here:

Not 3nm, but still an interesting test with almost exactly your hyperstar setup.

I don't know who has 3nm filters rated for f1.9 use.
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Old 30-01-2013, 11:27 AM
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Interference filters are designed on the assumption the light passes through perpendicular to the filter. The filter consists of two thin semi-transparent, semi-reflective layers that are separated by an exact half wavelength, or multiples of this.


The reason they don't work well with fast lenses is that the light entering the filter at an oblique angle travels a longer path inside the filter and hence it will transmit light of a longer wavelength than it was designed for. You can see the same thing if you take say an OIII filter, hold it up and watch what happens as you tilt it.

f/3 is about the practical limit.
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Old 30-01-2013, 12:55 PM
Poita (Peter)
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Wasn't some new super fast f1.4 or something ridiculous telescope announced recently along with filters to cope with it? I'll have to find it.

Meanwhile, there is a bit of a chat re hyperstar on Astrodon's site:
Gary Gonnella, Posted on February 19, 2010 12:03 PM

Hi Don, I am using your 3nm filters with a celestron 14" and Hyperstar adapter for F/1.9. I am imaging in Southern California and the filters have worked great. My questions is since I am shooting so fast would your 5nm filters give me better results.
*****Hi, Gary. please the the Narrowband FAQ page at http://www.astrodon.com/Orphan/astrodonfaqnarrowband/ where this is discussed.
Could you provide a link to some images taken at f/2? There will be significant signal loss from the 3 nm filter at f/2 but, you're so efficient at grabbing photons at f/2 that it perhaps somewhat makes up for it. The issue is transmission loss vs. gain of perhaps 1/3 less sky noise due to light pollution at your location. I don't have much experience at f/2 as I do at f/3 with an E-180. My guess would be that you'd be better off with the 5 nm filter at f/2. Don
Gary Gonnella, Posted on February 27, 2010 12:15 AM

Thanks Don, I got the filters before I realized the difference made by fast F's. I have read your FAQ and wanted your opinion. I may have to get the 5nm filters from you but have pretty good results so far. I am using the QSI 583. I have put a 20 Minute unprocessed Ha at this site: http://www.linear-dims.com/out/Ha_20MIN.zip ****Hey, that's pretty good for f/2 wit ha 3 nm filter. Kevin Nelson at QSI indicated that there would be about 10% vignetting with our 31 mm inserts an that's about what I see in your image. I see the curtains in back of the HH, a clean Alnitak with almost no halo and a bright Flame nebula. If you stacked 5-7 of these, I think you'd get a good, high-contrast image. If you went with the 5 nm, you would add signal with increased transmission, but your bkg signal would also go up do the the wider filter, so the gain solely due to increased transmission would be partially cancelled, and the HH region and curtain are not a very strong emitting region. I think you're probably fine with what you have IMO. Don
Gary, may I use this 3nm H-a image of the HH on the site? What is your email?
Gary Gonnella, Posted on March 02, 2010 5:09 PM

Thanks Don, You are more than welcome to use the image. I post most of my images at: http://picasaweb.google.com/gary.gonnella I would be happy to send you high rez copies of any of these or just the raws if you want. The last 2 images in my ...LOOK HERE FIRST... folder were built with these images all at 10 Minutes with total exposure of about 2 hours for each filter. My confusion may be from the fact that short F's were said t cause filter shift. Being an old Ham I think of shift as moving the center frequency off of the desired point. From what I now think i understand it is not a frequency shift but a decrease on transmission. If this is the case I am thrilled with a little loss to get past my high light pollution. My email: gary.gonnella@gmail.com
********Please take a look at our Narrowband FAQ from the narrowband page. It is indeed a spectral shift of the bandpass with incident angle of light moving the peak transmission off the emission wavelength. So, instead of being on top of the curve, you're part way down the side. Don
Thanks again and I love the QSI with your filters.
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