Some information about my instrumentation
As I am always working with my "CAR OBSERVATOY" the instruments are small
and easy and quick to install. This is necessary because we have no stable weather
conditions in our area.
At the picture you see the Genesis 100/500mm refractor with the AUDINE CCD camera
at the end. Most of my pictures are made with this setup. Beside the tripod with Super
Polaris DX mout and refractor you see a blue box, including the water cooling unit
for the camera. The camera is always connected to that unit via 2 tubes and transported
after the photosession in that blue box.
The pictures are collected with a Toshiba Tecra 720 Notebook, which is situated at
the back seat from the car.
The Celestron Radial Guider, which is between camera and telescope, includes a small moving
mirror system which makes it very easy to center the objects to the CCD-frame.
Beside the Genesis refractor and for brighter comets I use a 300mm f=1:1.8 Schmidt Camera
Meanwhile I also have a TAKAHASHI EPSILON 160 Reflector and a ZEISS APQ 100/640 Refractor
to take pictures from comets.
Remarks about the transmission characteristics of the
instrument and filters
Genesis refractor total transmission curve (including both lens groups)
The RGB filterset consists from 3 interference filters, because they have a higher transmission.
The NIR cutoff filter is build in the AUDINE CCD camera as a front window. It is also
an interference filter.
This is the curve from the KAF 401e chip. I transfered the spectral response curve
from Kodak into transmission values. Now I can very easy calculate the overall
efficiency of the complete setup.
Whatīs about the efficiency from different setups?
The normal setup is Genesis refractor + AUDINE CCD camera. If we make a convolution from
both spectra, we get
In my opinion there are two reasons to cut off the light above 700nm
1) The optical system is not corrected for wavelengts above 700nm
2) Photometric calculations will be better comparable to visible observations.
But is is clearly to see that the curve is not ideal compared to the Johnson-V filter,
Typical Johnson-V filter ( Transmission variates with filter thickness!)
The overlay from both curves shows were the differences are
The Telescope-Camera system still has too much transmittance in the blue and red region.
Now I can add the green interference filter to the setup. Than we get the following curve:
We play the same game and compare it with the Johnson-V characteristics.
Here is the overlay:
That looks much better. There is no blue sensitivity left and the high amount of red sensitivity
we had before droped down dramatically. I can imagine that the result is good enough to
compare visual observations with that setup.
There are some reasons why I donīt want to use an original Johnson-V filter:
a) The low energy throughput
Normaly those filter systems have only 50% maximum transmission. If I would use it, the
efficiency of the system decreases to
This is less than 50% from the energy I have with the green interference filter (setup 2)
b) Filter system has too much layers
I first try to work with setup 1 or 2 and compare my photometric results to those of the visual
observers. My feeling is, that the difference should not be that much.
By the way-- I am wondering why visual observations mostly are refered to V-type filters.
The human eye is also sensitive for blue and red. This is good to see in the curves from
the CIE 1931 for normal observers
Determination of the RGB ratios
To get the ratios for RGB images it is necessary to know the Transmittance of all used
componets, f.e. telescopes, filter and CCD-efficiency. So I measured all of the needed
spectra from the TAK, APQ, NIR-cutoff filter (front window of the Audine camera) and
As a first result I got the following efficiencies from 2 different setups:
1) Zeiss APQ 100/640 + NIR-cutoff filter + KAF 401e sensitivity
2) TAKAHASHI Epsilon 160 + NIR-cutoff filter + KAF 401e sensitivity
To get the RGB ratios I folded those curves with the 3 curves of my RGB filter set. The area under each curve
gave me the factors. Green is set to 1.0
1) Factors for Zeiss APQ 100/640 setup
1) Factors for TAKAHASHI Epsilon 160 setup