I use the Lumicon Giant Easy Guider (GEG) to allow off axis guiding, my tube assembly is a C-11. I CCD image these days, but stray light is still my enemy, especially for photometry where I'm trying to take scientifically valid and useful data.

My GEG interior is flocked about as much as possible.  Same for the rear mounting area of the C-11 that comprises the 'sky end' of the GEG.

I've flocked the inside of my baffle tube.

My latest dew/light shield extends 28 inches beyond the front of the OTA.

I have added a flocked ring baffle to expand the central obstruction of my secondary mirror...this, along with the long light shield...means you can't look from the sky end of the OTA and see down into the baffle tube.  (This is very much as described in Rutten and Van V's book Telescope Optics, page 222, fig 19.5, 19.7, and 19.8.)  That means light has to scatter off at least one flocked surface before it can enter the baffle tube...and it enters the baffle tube at a less-glancing angle than before...which tends to make the flocking paper more effective, since just about anything is more reflective at a glancing angle.

The next time I work on the scope I will probably flock the inner and outer corrector rings and see what further reductions in scattered light I get. These are shiny surfaces, so this could be an area for improvement.

In it's current state, when I look up the tube (remove the camera/eyepiece) toward blue sky...the scattered light is pretty low, but one problem remains when I move my eye to the extreme edge of the focal plane...I can see skylight reflected off the back side of the corrector plate.  (This effect is not visible in the center of the focal plane.)  This is very much as described in Rutten and Van V's book Telescope Optics, page 232, fig 19.13. I don't know of an easy/inexpensive way to eliminate this problem in SCT's.

For my small chip CCD I have also put a baffle about three inches in front of the chip that just barely permits the light cone to make it to the chip unvignetted by this baffle.  This location is still behind the GEG's prism location, and I may add another baffle just behind the pickoff prism.  35mm film needs a larger baffle, but I bet adding one or two in a similar manner would help.

Some useful links:


PS.  Baffling the SCT is tough to do well.

25 Mar 01 - krajcit@3lefties.com