Their were these laser scanners that computer identics was making.
They had some filters made up.
They worked fine straight ahead but off axis they would change color
from red to green (I think).
They couldn't use them.


Richard Crisp wrote:
> Of course that is only an approximation; not all rays have the same angle on
the filter.
>
> the on-axis rays are perpendicular
>
> the chief ray angle for a extreme off axis target gets more and more sloped as
the FOV widens and that is the source of the problem
>
> if you ray trace those possibilities, you see a very complicated problem that
is nearly intractable. The best solution is a compromise and the faster the
optics the wider the passband needs to be to compensate
>
> redshifting the passband is not going to solve the problem. It may make part
of the image better but I can show you by raytracing that you will hurt other
parts of the image
>
> The best solution short of relay optics to collimate the light, will be a
filter with a wider passband.
>
>
>
> -----Original Message-----
> From: Wolfgang Renz
> Sent: Oct 20, 2009 12:49 PM
> To: Richard Crisp
> Cc: Astroimaging_filters
> Subject: Re: [Astroimaging_filters] Re: Anybody here using a HeII filter??
>
>
>
>
>
> Re: filters shifts to the red to compensate for fast optical systems
>
>
>> I don't know how that works: not all of the light rays are angled....
>>
>
> If a NB filter has a flat top, the transmission profile is placed in a way
> that the rest wavelength of the emission line is at the blue end of the
> flat top.
>
> If a narrower NB filter has a Gaussian or PSF like profile, the max
> transmission peak is shifted to the red to compensate for the blue
> shift of the mean incident angle. The mean incident angle is about
> half the the max incident angle of the used system. So pro observa-
> tories ray trace their systems to determine the max incident angle
> and then let manufacture their NB filter specifically for this value.
> This also means that they partially have different NB filters for differ-
> ent f# focii of the same scope and for the different f#s of different
> scopes.
>
> Clear skies
> Wolfgang
>
>

Hi, this filter will be returned for refund. I did an out-of-bandpass
transmission test, 190-950nm. The file can be seen if file folder Filter tests,
file Omega_HbLL.xls. Notice the light leakage that begins at ~850nm. Since a CCD
camera is sensitive from ~380-1000nm, this filter is unsuitable for CCD
application. A workaround would be to add a UV/IR cut filter.

Regards,
Richard


--- In Astroimaging_filters@yahoogroups.com, "ssb73q" <ssb73q@...> wrote:
>
> Hi, I just couldn't resist the purchase of a $119 Omega 2" Hb filter sold on
eBay. This filter is from a lot of 10, I purchased the first one, nine remain.
This is a CCD filter, not an eyeball nebula filter. I received that filter today
and did a transmission test. The results of that testing can be seen in file
folder Filter tests, file Omega_Hb.xls.
>
> The filter's eBay claim is to have a ~5nm bandpass centered at 486.1nm and,
80-90% transmission. You can see that the filter nails the Hb line with 89.8%
transmission with a ~5.5nm bandpass, nice.
>
> The filter looks visually good, but I don't know about its flatness or other
optical traits. Imaging testing will be required. I also need to do an
out-of-bandpass light leakage test on this filter.
>
> I must admit to being impressed with the transmission characteristics, compare
it to my 2" Baader Hb filter.
>
> Regards,
> Richard
>