[Aavso-photometry] Screen for "dome" flat?

[Wolfgang Renz]

Hi Yenal

The idea is to let in just light that is as parallel as possible and that
actually can illuminate the FOV at all and thus minimize issues due
to scattered light. But one must take care to not introduce an addi-
tional vignetting by this.

One point is to use a long light shield (> ~ 80 cm with my 8" SCT).
If you screw off everything from the back of your Meade 14" LX200R
and look off-axis through the back of your scope, you will very pro-
bably be able to see directly through the primary baffle tube and the
Schmidt corrector plate to the outside (just wave with something in
front of the corrector plate). Just try it yourself, I would be interested
how it looks like with one of the large Meade scopes.
Every light that enters the scope this way is prone to cause a scat-
tered light issue in the flats if it falls into a flat pimary mirror baffle
tube (like with the (old) Celestron SCTs) or a flat adapter tube if they
are blackened less than perfect. The Meade pimary mirror baffle
tubes are baffled (at least all I've seen up to now), but their baffels
are calculated for on-axis light and they don't get rid of light that falls
directly through the Schmidt corrector plate behind the pimary baffle
tube.

To decrease scattered and reflected light issues, some astrono-
mers (pros and amateure) also add additional baffles to flat exten-
sion tubes and/or a square/rectangular mask in front of the CCD
chip or the camera chaimber window (the surrounding of the active
area on the CCD chip is often amazingly reflective).

If the diagonal corner-to-corner FOV of your CCD would be e.g. 2°
than all parallel light within an opening angle of a 2° cone might fall
on the CCD. 1° off-axis to all sides. Parallel light that comes from
outside this range cann't fall on the CCD and might just cause scat-
tered light issues.
In general the diameter C of the cone of an opening diameter O of
a scope at the distance d will be:
        C = O + 2 * tan(FOV/2) * d
which is for small angles FOV equivalent to:
        C ~~ O + tan(FOV) * d
Without any baffels and tubes, the diameter O would have to be
choosen to be the diameter of the primary mirror of the reflectors
and the free lens diameter of refractors. But many scopes are
constructed in a way that they already vignet the FOV. E.g.
Newtons for visual usage have usually an undersized secondary
that full illuminates just the very center of the FOV. On the other
side, some scopes have oversized promary mirrors. This is very
obvious with Schmidt cameras that have a much larger primary
mirror than Schmidt plate diameter to fully illuminate the FOV of
several degrees. Meade also claims to use oversized mirrors, but
that is probably just good for not much more than 1°(I've actually
never seen an user comparison of the free openings of the primary
mirrors and the Schmidt plates yet).
As the baffels are usually the most limiting diameter, one can and
probably should choose the diameter of the first front baffle of the
scope and measure the distance from there. With refractors its
located at the lens cell, with SCTs its at the Schmidt corrector plate,
with Newtons its usually a few mm to cm inside the tube. But this
baffle is not necessarily the most limiting one. E.g. the most limiting
baffle of my 8" LX90 is the baffle deep inside the primary baffle tube
with the smallest diameter of of baffles in the tube (I'm pretty sure that
this one negates any advantage of the oversized primary).

The width of the black boarder around the white disk of the dome flat
screen should be IMO choosen as large as possible and practical.
If the lightshield isn't long enough to obstruct the direct light throught
the Schmidt corrector plate into the primary baffle tube close to the
focal plane, the black boarder should be wide enough to do accomp-
lish this.

For illustrative examples for lens hoods of camera lenses and how to
design them to minimize internal refelction/flaring issues , see the
Photographic optics pages of  Paul van Walree:
http://www.vanwalree.com/optics.html
Especially take a look at his lens hood and vignetting pages:
http://www.vanwalree.com/optics/lenshood.html
http://www.vanwalree.com/optics/vignetting.html

One could of course make the dome screen not round, but rectangular
with the aspect size of the CCD chip, but then one would not have just
to position the optical axis of the scope right, but would also have to
rotate the camera with the scope always accordingly or make the
dome screen rotatable.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Yenal Ogmen"
To: "Wolfgang Renz"
Sent: Monday, May 12, 2008 3:51 PM
Subject: Re: [Aavso-photometry] Screen for "dome" flat?

>> Its also a good idea to not to use a square or rectangular dome
>> screen but a round one with a wide black boarder. The white
>> center must be just as wide as the opening of the scope plus
>> the effective FOV angle of the CCD diagonal implies (depending
>> on the distance of the dome flat screen to the scope).
> 
> Mr. Renz, can you please explain in detail how to
> calculate the screen size in relation with the size
> and the distance of the scope?
> 
> Thank you in advance.
> 
> Yenal Öðmen
>  Green Island Observatory Homepage
> www.geocities.com/yenalogmen 
> Geçitkale 
> North Cyprus