ASTRO: Question for John Gretchen

I have a friend using the CCD Labs Q453 camera from near Houston. I
think that is about identical to your camera except yours uses dry
nitrogen to fill the CCD chamber while his uses dry Xenon. Each
supplier thinks his gas better of course.

From what I read neither camera has temperature control of the cooling.
The dark current of the chip is such matching temperature of darks to
lights isn't all that important removing the need for controlled
temperature cooling. But with his humidity turning down cooling would
be a good feature it appears.

He's having a real problem with dew on the window on all but the
"driest" nights. He says the humidity on a "dry" night is pretty severe
so dry is a very relative word. I'd think you, being almost on the
ocean, would have a similar problem but you don't seem to. How do you
avoid this?

Rick

The cameras are not air tight. They may be purged with one gas or
another but that escapes at sometime. The way the camera deals with
moisture is with "cold fingers", a strip of metal on both sides of the
chip which get cold before the chip. All the moisture is a collected on
the metal strips. To avoid there being more moisture than the fingers
can handle I store the camera in a sealed case with silica beads, any
desiccant will work but silica beads can be regenerated. I have heard of
some people incorporating a heater in the storage box but I have not
found that necessary. There are also rubber plugs on the side of the
camera that some people remove during storage, I have also found that
unnecessary. Now if he is dewing on the outside of the optical glass the
is part of the nose piece, that can be caused if the camera is facing
down as the cold air in the camber will sink towards the glass. This
cooling of the glass will make it dew up on the outside. I use an
eyepiece heater around the drawtube if the focuser near the camera that
I turn on it I think it will be needed.
I don't use darks, just bias, flats, and defect maps.

Rick Johnson wrote:
I have a friend using the CCD Labs Q453 camera from near Houston. I
think that is about identical to your camera except yours uses dry
nitrogen to fill the CCD chamber while his uses dry Xenon. Each
supplier thinks his gas better of course.

From what I read neither camera has temperature control of the cooling.
The dark current of the chip is such matching temperature of darks to
lights isn't all that important removing the need for controlled
temperature cooling. But with his humidity turning down cooling would
be a good feature it appears.

He's having a real problem with dew on the window on all but the
"driest" nights. He says the humidity on a "dry" night is pretty severe
so dry is a very relative word. I'd think you, being almost on the
ocean, would have a similar problem but you don't seem to. How do you
avoid this?

Rick


--
John N. Gretchen III
http://www.tisd.net/~jng3

Thanks for the info.

I didn't realize you had to go through all that. I passed the info on.
Not sure if his version of the camera is the same as to those
"fingers" or not. Though he indicated it was the window that was
frosting over rather than the chip. So the warmer may be all he needs.

In winter I have heat tape around my camera to prevent the front of the
chamber from frosting over. Normally waste heat from the camera's
electronics warms it just enough that this isn't a problem even on the
most humid of nights in summer. But when the outside temp is below
about -30C and I try to cool the CCD below -40C then I can get frost
without the tape. Or if it is -25C and I cool to -50C I get the problem
as well. At -60C CCD temp I get frost no matter what the outside temp
without heating. The waste heat just is swamped by such cold temps. I
need 75 watts of heat tape to get the warming needed as well as lots of
insulation around the camera to hold it in. All this with the fan off
of course.

Rick

John N. Gretchen III wrote:

The cameras are not air tight. They may be purged with one gas or
another but that escapes at sometime. The way the camera deals with
moisture is with "cold fingers", a strip of metal on both sides of the
chip which get cold before the chip. All the moisture is a collected on
the metal strips. To avoid there being more moisture than the fingers
can handle I store the camera in a sealed case with silica beads, any
desiccant will work but silica beads can be regenerated. I have heard of
some people incorporating a heater in the storage box but I have not
found that necessary. There are also rubber plugs on the side of the
camera that some people remove during storage, I have also found that
unnecessary. Now if he is dewing on the outside of the optical glass the
is part of the nose piece, that can be caused if the camera is facing
down as the cold air in the camber will sink towards the glass. This
cooling of the glass will make it dew up on the outside. I use an
eyepiece heater around the drawtube if the focuser near the camera that
I turn on it I think it will be needed.
I don't use darks, just bias, flats, and defect maps.

Rick Johnson wrote:

I have a friend using the CCD Labs Q453 camera from near Houston. I
think that is about identical to your camera except yours uses dry
nitrogen to fill the CCD chamber while his uses dry Xenon. Each
supplier thinks his gas better of course.

From what I read neither camera has temperature control of the
cooling. The dark current of the chip is such matching temperature of
darks to lights isn't all that important removing the need for
controlled temperature cooling. But with his humidity turning down
cooling would be a good feature it appears.

He's having a real problem with dew on the window on all but the
"driest" nights. He says the humidity on a "dry" night is pretty
severe so dry is a very relative word. I'd think you, being almost on
the ocean, would have a similar problem but you don't seem to. How do
you avoid this?

Rick



--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Not much trouble..., take the camera off the scope and up it in the
case, regenerate the silica every couple of months. Not a bad idea for
any electronic equipment that is outside from time to time.

Rick Johnson wrote:
Thanks for the info.

I didn't realize you had to go through all that. I passed the info on.
Not sure if his version of the camera is the same as to those "fingers"
or not. Though he indicated it was the window that was frosting over
rather than the chip. So the warmer may be all he needs.

In winter I have heat tape around my camera to prevent the front of the
chamber from frosting over. Normally waste heat from the camera's
electronics warms it just enough that this isn't a problem even on the
most humid of nights in summer. But when the outside temp is below
about -30C and I try to cool the CCD below -40C then I can get frost
without the tape. Or if it is -25C and I cool to -50C I get the problem
as well. At -60C CCD temp I get frost no matter what the outside temp
without heating. The waste heat just is swamped by such cold temps. I
need 75 watts of heat tape to get the warming needed as well as lots of
insulation around the camera to hold it in. All this with the fan off
of course.

Rick


John N. Gretchen III
http://www.tisd.net/~jng3