Stacking 2 Hutec LPr Filters

leon · #1 13-10-2008, 10:10 PM

Hi Guys, well this is probably the dumbest question I have asked for some time, but I will ask it anyway.

On the rear element of the Tak I use a 72mm Hutech LPR Filter, and this obviously gives me a much longer imaging range before the fogging limit is reached.

The CA-35 that holds the Camera to the Tak also accepts a 50 mm Filter.

I just happen to have a 50mm Hutech LPR filter as well, and was wondering, If I used both these filters in the imaging train, would I get twice the imaging time before fogging, or am I just dreaming.

Any response eould be appreciated, or just tell me i'm crazy.

Leon

dugnsuz (Doug) · #2 15-10-2008, 01:01 PM

Quote:

Originally Posted by leon

...If I used both these filters in the imaging train, would I get twice the imaging time before fogging, or am I just dreaming.

If the bandpass characteristics are identical on both filters, I don't reckon it would make any difference leon.

Doug

leon · #3 15-10-2008, 01:06 PM

Hmmm Doug, now I have to admit that didn't cross my mind,

see it must be a dumb question,

thanks for your response though.

Leon

Ian Robinson · #4 15-10-2008, 02:19 PM

If transmission at wavelength X = 95 % , the stacking two "identical" fllters will result in overall transmission of : 0.95x0.95=0.9050 = 90.5%

And so some desireable light will be absorbed and / reflected.

So longer exposures will be required, that will likely counter any benefits in cutting Hg and Na vapour lighting and W filament lighting pollution of stacking.

leon · #5 15-10-2008, 03:15 PM

Thanks Ian, ah well, it was just a quick thought that crossed my mind, anything to help the pollution, many thanks for your response.

Leon

bojan · #6 18-11-2008, 09:12 AM

Actually, it will help a lot.
Transmission will be lower (as Ian calculated) , but rejection of (some) undesired wavelengths will be vastly improved.
For example, if transmission of undesired wavelength is 1% (0.01x), if you stack two filters you will have 0.0001x (0.1%).
The problem may be additional reflections (from filter surfaces) and of course, price/performance ratio... because, undesired wavelengths are already sufficiently reduced....