Re: TN89 and hints on optical vignetting and stray light

[Tom Droege <tdroege2@earthlink.net>]

OK, I am taking box flats, so I just took down the TOM1 light shield and 
measured it.

Details of the Mark IV, TOM1 dew shield:

Length of shield   300 mm from lens face

Diameter at exit  200 mm

Baffle at 130 mm from lens face, diameter 140 mm

Baffle at 250 mm from lens face, diameter 160 mm

The shield is a 200 mm diameter cardboard tube with baffles made from black 
foam board.  The design is such that several rings center the whole 
structure to the lens.  I would estimate that it is centered to 5 mm.   It 
is light weight and quite rigid because of the four ring and tube 
construction.  It is a snug fit to the lens tube and does not flop around 
as the telescope is moved.

I know that "dave garnett" <dave.garnett@metapurple.co.uk>  has the optical 
design files.  Possibly Michael has squirreled them away on the home 
page.   I just looked, and could not find them.   Dave, possibly you can 
send a copy of what you have to Michael so that it can be archived.

There are no internal baffles in the lens itself.

Note that the focus scheme results in some tilt of the camera.  I have 
designed stiffeners which I hope will greatly reduce this and am waiting 
for their arrival.

Hope this provides enough information so that someone can search for the 
source of the problem.

Tom Droege



At 10:52 PM 1/1/03 +0000, you wrote:

>  Have corresponded with Michael Richmond, who has asked me to post some
>information on the TASS Mailing List which may be of general interest.
>Michael - I have expanded here somewhat on my original mailing.
>
>Have also expressed interest in seeing a similar analysis of the 7x7 data by
>Michael using only the dark-subtracted, non-flat-fielded image data since I
>am interested in the nature of the focal plane illumination, vignetting
>included.  This was also raised by Arne in his note to the List of 31st Dec.
>
>         -------------------------
>
>On the subject of vignetting, these are my comments, which were prompted
>after seeing the response characteristics across the MkIV field as detailed
>in Technical Note 89.  I know that this analysis deals with flat-fielded
>image data but care must be taken when generating flat-fields if the degree
>of vignetting of the sky background is different to that of point sources -
>this can be a serious limitation of the 'light-box' technique.  The general
>rule is to design optics, baffles and dew-shield so that vignetting is
>virtually absent - easier said than done.
>
>First what does vignetting look like.  If you analyse response across the
>frame (as was done by the methodology used in TN89) in terms of a contour
>plot, you see that sensitivity is a maximum at some point and that expressed
>as polar coordinates centered on that point, the response varies approx. as
>the second power of the radial distance, r, across the frame when the entire
>frame is suffering from vignetting.  When partial vignetting is present (the
>usual case), response is constant for the central region, radius, a, and
>then decreases by approx. the second power of (r - a).
>
>Now to the practice - vignetting can arise from one (or indeed all) of three
>sources.  These listed in order of likelihood are:
>
>1.  Undersized / off-centered dew-shield.
>For the MkIV optical configuration (100mm aperture, 400mm focal length, 40mm
>diagonal across CCD frame), the dew-shield must be oversized relative to the
>front element by at least [(40/400) x (distance from tip of dew-shield to
>front element)].
>For example if you have a dew-shield which is 200mm in length, then you need
>it to be oversized by 20mm, i.e. a diameter of 120mm minimum.
>In practice the dew-shield should be fairly rigid and larger than this (say
>125-130mm in diameter) to allow for lack of concentricity of the shield with
>the optical axis.
>
>If indeed the dew-shield (light shield as described by Tom) is about the
>same length as the focal length of the scope (400mm), then the front opening
>of the shield will need to be about 150-160mm in diameter and should be
>fairly rigid so that it does not flop when at different zenith distances -
>clearly this would otherwise be disastrous for all-sky photometry.
>
>2.  Undersized internal baffle stop.
>Sometimes blackened circular apertures are interposed at one or more
>positions along the optical axis.  It is common to design systems for visual
>observers working on-axis.  Since you need an unvignetted field some 40mm in
>diameter then care has to be taken, if baffles are used, that they are
>adequate in size.
>t located near to, or on the window of the CCD chip.
>
>And finally - diffuse reflections originating from stray light will
>generally only alter the sky background in a gradual fashion across the
>frame and this is easily compensated for by taking sky background measures
>in close proximity to stellar images.
>
>I know that TASS people are well aware of most, if not all of the above but
>I wouldn't like to think that any of the above points may have been
>inadvertently overlooked and so I do hope that this note is helpful.
>
>Happy New Year to all,
>Richard Miles