--- astrodeb@charter.net wrote:

For those of you that do CCD imaging I was wondering how you take the flat field frames. I've heard that that a twilight sky makes a nice flat frame . . . SBIG recommends a white T-shirt. Has anyone tried this before?

I'm new to this myself but happen to be in the middle of building a white light box, this evening in fact. Twilight is fine, but what happens when you arrive after dark? Berry & Burnell's _Handbook of Astronomical Image Processing_ recommends building a white light box. Their version involves a plywood box with a single piece of milk white plastic and four shielded lights placed in the corners to reflect shadowless light on the screen. Here's a link to another amateur's page who built a Berry & Burnell type screen (below the fold, in the middle of the page): http://www.angelfire.com/space2/tgtan/equipment.htm In this version, the lid of the box has a white field screen. The light is projected _backwards_ and away from the scope into a box lined with white artist board. That diffuse light then reflects back up onto the white plastic at the front of the box. Milk white plastic is at the heart of these light boxes. This is the plastic that you see in photography stores on boxes to to view your 35mm slides. Milk white plastic 3/16" thick can be purchased from any company under the yellow page category "Plastic - Rolled and Flat". In SLC, I bought my pieces (this morning) from Regional Supply Co. on about 3900 South and 300 West. It was $22 total for a 10" and a 12" round cut piece. I hadn't thought of the T-Shirt trick. A variant on the T-Shirt trick, if there are no other astrophotographers around to be bothered by white flashlights, was suggested by Berry and Burnell. For a large scope, you can set up one of those roll-up projector screens. Then you need to get a diffuse white field of light projected on the screen. That is done with a bright flashlight and a small piece of diffuser milk white plastic held in front of the flashlight. Of course, setting up a project screen and spewing white light all other the place in a group setting will probably tick off your co-amateur astronomers. That's one problem I see with the design at: http://www.angelfire.com/space2/tgtan/equipment.htm The bottom is made of semi-transparent white plastic. The bottom would light up like a light bulb. The key point for these type of in-line light boxes, is that you need a white field of view screen, and a second piece of white diffuser plastic to diffuse the light source into a shadowless uniform field. Probably, the T-shirt, and a flashlight a few feet away with a piece of diffuser plastic in front of it my work. But again, I haven't tried it and don't know for sure. Maybe others here with more experience can comment. I'm trying to build a simplier version of the Berry & Burnell that I call the "inline trash can light box." After I'm finished, I'll post again and let you know if it produces flat white field. In my proposed "inline trash can" model, under construction, a 2" hole is cut in the bottom of a black-opaque and round kitchen type plastic trash can. A small magnilight flashlight stuck up the bottom hole shines on a diffuser plate piece of plastic. (The flashlight can be held in place with 6" of waterpipe insulation.) This diffuses a uniform field (I hope) onto a the field-of-view plate at the front of the trash can. A fender of suitably placed closed-cell weather stipping makes the can fit snugly over the front of telescope tube. I'll let you know if it works out. For lurkers, any comments on the proposed trash design, or alternatives, would be appreciated. - Canopus56(Kurt) __________________________________ Start your day with Yahoo! - Make it your home page! http://www.yahoo.com/r/hs

Hi Debbie, For my flat fields I put a large white cardboard target on my observatory wall and illuminate it with an X-mas tree light placed down near the floor such that no shadows are cast on the target. At the end of an imaging session I shut the roof, turn off all the interior lights except the tiny one and point the scope at the target with the imager and focus positioned and set where they were during imaging. Then I shoot whatever length exposure is needed to come up to about half what is needed to saturate the chip. Patrick astrodeb@charter.net wrote:

For those of you that do CCD imaging I was wondering how you take the flat field frames. I've heard that that a twilight sky makes a nice flat frame but the camera has to be focused first. Without stars to focus, this is quite difficult.

I was going to try putting some printer paper over the dewshield but I filled up my memory card before I had a chance to try it. SBIG recommends a white T-shirt. Has anyone tried this before? I'm looking for ideas how to incorporate flat field frames into my images to remove dust and other artifacts that accumulate on the lens.

Clear Skies,

Debbie