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

[Wolfgang Renz]

Hi Tim

> I will confess that for awhile I "foolishly" (IMO) used an opaque
> screen over the corrector plate until a friend asked me why in
> the world I would introduce something in to the optical path that
> is not there when I am imaging.... Never used it again.

A diffusor has the task to even out the illumination as one does
effectivly image the diffusor and not what illuminates the diffusor.
This works well if there are gradients and stars in the twilight sky.
The diffusors in lightboxes also cause the light to get scattered
multiple times which also evens out the illumination. To accomp-
lish this the diffusor must IMO have a opacity of well above 50%.
 
If one doesn't use a diffusor, the imaged area must be evenly illu-
minated to get correct flats. This is also valid for the screens of
dome flats and IMO not reachable without a few precautions.
As most light sources (except maybe frosted bulbs) and most
reflectors don't give an even illumination over a wide angle range
of the beam, and as the scattering at the illuminated screen is
slightly directed and the illuminating source must be placed off-
axis,  the dome flat screen and its scattered light is not neces-
sarily even.
To account for the first its a good idea to indirectly illuminate the
dome flat screen. If the second is a significant issue, one can e.
g. rotate the illuminating source in 4 steps or use 4 light sources.
For scopes with a small field of view, it might not cause a large
effect, but as wider the FOV gets, as difficulter it gets to get an
evenly illuminated dome screen.

Its also a good idea to blacken and baffle the scope very well
and to place the diffusor not at the optics directly but in front of
a long light shield (that just not obstrucs the FOV of the CCD
and is used for imaging too) to minimize scattered light issues.
E.g. the common obstructed scopes (like SCTs) let fall light
coming from ~ 6-11° off-axis directly through the front opening/
Schmidt corrector plate into the primary baffle tube close to the
focal plane. To avoid this, one must either place an additional
baffle around the secondary (which will reduce the effective
opening of the scope) or use a light shield that is longer than ~
80 cm (measured from the Schmidt plate).

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).

Clear skies
 Wolfgang

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