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
Hi Deb, My CCD instruction manual doesn't say the frames have to be focused. One technique recommended is to do it when the sky is fairly evenly lighted (and I think for safety sake, this should be before the sun goes up or after it sets). You can stretch something like a piece of cloth evenly across the tube to diffuse the light further, then make the exposures. I have to admit that while I made several flat-field photos, I don't think I was able to actually use them. Best wishes, Joe
--- 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
--- Canopus56 <canopus56@yahoo.com> wrote: After working on my "trash can" light box last night, it turns out that I could not get a uniform flat field from an inline design - flashlight, diffuser plate and white field plate. So, I'm back to the basic Berry & Burnell reflective design - the lights point backwards away from the white field plate and into a trash can lined with white board. Canopus56 wrote before:
I'm new to this myself but happen to be in the middle of building a white light box, this evening in fact. . . . . Berry & Burnell's _Handbook of Astronomical Image Processing_ recommends building a white light box. . . . 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_ . . . . 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.
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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
I hope someone could explain more about flat frames. I know what they're supposed to do. My question is, why isn't it good enough to take one set of flat frames, and then have them to reuse forever? Or is it good enough? Thanks, Joe
Joe Bauman wrote:
I hope someone could explain more about flat frames. I know what they're supposed to do. My question is, why isn't it good enough to take one set of flat frames, and then have them to reuse forever? Or is it good enough? Thanks, Joe
Morning Joe, This comes up on the Software Bisque groups from time to time. My understanding is that you can resuse flats _provided_ the focus is at the exact same place and the position of the camera and any focal reducers are in the exact same position. But if any of that changes, even a little, a new flat must be made. Patrick
OK, Patrick, this is REAL dumb, but I thought focus didn't matter with a flat field. So why does it? And Deb: I guess I passed along some bad info to you. Sorry. -- Joe
Joe Bauman wrote:
OK, Patrick, this is REAL dumb, but I thought focus didn't matter with a flat field. So why does it? And Deb: I guess I passed along some bad info to you. Sorry. -- Joe
Flats contain data (it's just kind of hard to see) and all of those data need to be in the same place as the corresponding data from the white light image. Changing focus changes the location of the data. There's more detail in the on line Bisque manuals. I'm flying off to Wendover in a few minutes to check on a site for January's Stardust Watch but after I get home tonight I'll try and find the URL and forward it to you. Blue skies! Patrick
participants (4)
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astrodeb@charter.net -
Canopus56 -
Joe Bauman -
Patrick Wiggins