RE: [MaxImDL] Re: Reflected Light?


May 14, 2007

 


----------------------------

#28378 May 14, 2007

I have been struggling with what I thought were dust donuts in my

images and flats. After doing some testing, I have deduced it must be

reflected light inside the tube. I removed the filter wheel, so only

the telescope and camera are in the train. The scope is a Orion ED80

and the camera an Atik 16 HR. The donuts are in the same location

regardless of camera position angle (rotation). Can anyone point me to

a resolution.



I have uploaded a sample flat in the photos section, misc. album:



tech.ph.groups.yahoo.com/group/MaxImDL/photos/view/9edb?b=2



Thanks,

Mark



----------------------------

#28379 May 14, 2007

If they rotate with the camera, then they are probably part of the

camera system. Probably dust on a window in the camera?



Peter

--- In MaxImDL@yahoogroups.com, "Mark" mgjr@...> wrote:

>

> I have been struggling with what I thought were dust donuts in my

> images and flats. After doing some testing, I have deduced it must be

> reflected light inside the tube. I removed the filter wheel, so only

> the telescope and camera are in the train. The scope is a Orion ED80

> and the camera an Atik 16 HR. The donuts are in the same location

> regardless of camera position angle (rotation). Can anyone point me to

> a resolution.

>

> I have uploaded a sample flat in the photos section, misc. album:

>

> tech.ph.groups.yahoo.com/group/MaxImDL/photos/view/9edb?b=2

>

> Thanks,

> Mark

>



----------------------------

#28380 May 14, 2007

Mark wrote: >

> I have been struggling with what I thought were dust donuts in my

> images and flats. After doing some testing, I have deduced it must be

> reflected light inside the tube. I removed the filter wheel, so only

> the telescope and camera are in the train. The scope is a Orion ED80

> and the camera an Atik 16 HR. The donuts are in the same location

> regardless of camera position angle (rotation). Can anyone point me to

> a resolution.

>

> I have uploaded a sample flat in the photos section, misc. album:

>

> tech.ph.groups.yahoo.com/group/MaxImDL/photos/view/9edb?b=2



If you rotate the camera and the dust goes with it, the specs MUST be on the

camera window.



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



25 Conover Street

Ottawa, Ontario,

Canada, K2G 4C3



Phone: (613) 225-2732

Fax: (613) 225-9688



---------------



----------------------------

#28384 May 15, 2007

If you rotate the camera and the dust goes with it, the specs MUST be

on the > camera window.



I thought the same, however images taken with the camera off the scope,

and the window covered with white cloth have no donuts! I have heard

of calculating the distance to the dust using the diameter. Does

anyone know the math?



Mark



----------------------------

#28385 May 15, 2007

Mark wrote:

> I thought the same, however images taken with the camera off the scope,

> and the window covered with white cloth have no donuts! I have heard

> of calculating the distance to the dust using the diameter. Does

> anyone know the math?



That's because the light isn't collimated when you take it off the telescope.



The visible dust spot size is simply geometric, based on the f/ratio (ratio of

diameter of objective to focal length).



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



25 Conover Street

Ottawa, Ontario,

Canada, K2G 4C3



Phone: (613) 225-2732

Fax: (613) 225-9688



---------------



----------------------------

#28386 May 15, 2007

Mark, try CCDware's "Dust Donut Calculator" at www.ccdware.com/resources/ ...joe :)







------- Original Message -------

From : Douglas B. George[mailto:dgeorge@...]

Sent : 5/15/2007 9:22:13 AM

To : MaxImDL@yahoogroups.com

Cc :

Subject : RE: Re: [MaxImDL] Re: Reflected Light?



Mark wrote:

> I thought the same, however images taken with the camera off the scope,

> and the window covered with white cloth have no donuts! I have heard

> of calculating the distance to the dust using the diameter. Does

> anyone know the math?



That's because the light isn't collimated when you take it off the telescope.



The visible dust spot size is simply geometric, based on the f/ratio (ratio of

diameter of objective to focal length).



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



25 Conover Street

Ottawa, Ontario,

Canada, K2G 4C3



Phone: (613) 225-2732

Fax: (613) 225-9688



---------------







Yahoo! Groups Links







----------------------------

#28387 May 15, 2007

Mark-



Covering the camera with a white cloth will produce very diffuse,

undirected light (something like a scope with an extremely small focal

ratio), and therefore no dust shadows will be produced (actually, they

will, but only very large and very low contrast). Put the camera back

on the scope and make a flat. Now measure the diameter of your dust

shadows. Their distance from the sensor is just w * F, where w is the

width (number of pixels * pixel size) and F is the focal ratio. It is

common to see several diameters; the three main locations where you

get dust are the front and back surfaces of the camera window, and the

front of the sensor window. Except at very large focal ratios, most

other surfaces in the system are generally too far from the sensor to

produce noticeable shadows.



Chris



*****************************************

Chris L Peterson

Cloudbait Observatory

www.cloudbait.com



--- In MaxImDL@yahoogroups.com, "Mark" mgjr@...> wrote:



> I thought the same, however images taken with the camera off the scope,

> and the window covered with white cloth have no donuts! I have heard

> of calculating the distance to the dust using the diameter. Does

> anyone know the math?

>

> Mark



----------------------------

#28390 May 15, 2007

You can prove the collimation issue yourself. Just put a several foot

long cardboard tube in front of the camera to simulate a long focal

length or collimation, and put your white cloth on the outre end.

Voila: dust doughnuts. See my paper last year at SAS for more (than you

probably ever wanted to know) on dust doughnuts and similar problems.



John Menke



Mark wrote:

>

>

> > If you rotate the camera and the dust goes with it, the specs MUST be

> on the

> > camera window.

>

> I thought the same, however images taken with the camera off the scope,

> and the window covered with white cloth have no donuts! I have heard

> of calculating the distance to the dust using the diameter. Does

> anyone know the math?

>

> Mark

>

>



----------------------------

#28395 May 15, 2007

Guys,

Does this mean 'T-Shirt' flats won't work? I was hoping to start

some work with such flats....



Frank



--- In MaxImDL@yahoogroups.com, john@... wrote:

>

> You can prove the collimation issue yourself. Just put a several

foot

> long cardboard tube in front of the camera to simulate a long

focal

> length or collimation, and put your white cloth on the outre end.

> Voila: dust doughnuts. See my paper last year at SAS for more

(than you

> probably ever wanted to know) on dust doughnuts and similar

problems.

>

> John Menke

>

> Mark wrote:

>

> >

> >

> > > If you rotate the camera and the dust goes with it, the specs

MUST be

> > on the

> > > camera window.

> >

> > I thought the same, however images taken with the camera off the

scope,

> > and the window covered with white cloth have no donuts! I have

heard

> > of calculating the distance to the dust using the diameter. Does

> > anyone know the math?

> >

> > Mark

> >

> >

>



----------------------------

#28396 May 15, 2007

fccolosimo wrote: > Guys,

> Does this mean 'T-Shirt' flats won't work? I was hoping to start

> some work with such flats....



It will work, but the T-Shirt has to be in front of the telescope!



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



25 Conover Street

Ottawa, Ontario,

Canada, K2G 4C3



Phone: (613) 225-2732

Fax: (613) 225-9688



---------------



----------------------------

#28397 May 15, 2007

If the t-shirt is at the scope's entrance pupil, the light coming

through it looks the same to the imager as the sky. T-shirt flats

often aren't the best, but that's because the material can be a bit

non-uniform. But such flats will often be fine for all but the most

critical requirements.



Chris



*****************************************

Chris L Peterson

Cloudbait Observatory

www.cloudbait.com





--- In MaxImDL@yahoogroups.com, "fccolosimo" fcc0@...> wrote:

>

> Guys,

> Does this mean 'T-Shirt' flats won't work? I was hoping to start

> some work with such flats....

>

> Frank



----------------------------

#28399 May 15, 2007

Hi, Chris,



Interesting you should say that. I've found I do better taking flats

(SXV-M25C and BRC-250) with no covering at all. I simply point the scope at

the uniformly lit daytime sky and tell Maxim to do its thing. Like you, I've

found I can induce patterns on my flats using a tee shirt. Definitely a

no-no!



Regards,



Greg Hartke

Sykesville, MD USA



-----Original Message-----

From: MaxImDL@yahoogroups.com [mailto:MaxImDL@yahoogroups.com]On Behalf Of

cloudbait

Sent: Tuesday, May 15, 2007 5:54 PM

To: MaxImDL@yahoogroups.com

Subject: [MaxImDL] Re: Reflected Light?





If the t-shirt is at the scope's entrance pupil, the light coming

through it looks the same to the imager as the sky. T-shirt flats

often aren't the best, but that's because the material can be a bit

non-uniform. But such flats will often be fine for all but the most

critical requirements.



Chris

.





[Non-text portions of this message have been removed]







----------------------------

#28404 May 15, 2007

Nothing beats twilight flats if properly taken (too short exposure will

give shutter vignetting, too long, too many stars). Well, the former if

the camera HAS a shutter :-)



----------------------------

#28412 May 16, 2007

Thanks everyone,



100 pixel diameter dust spots 6.45 pixels binned 2, f-7.5 ~10mm to the

dust. Cleaned the window dust spots are gone:)



Bob- Although the dust was invisible to the eye, the effect on my

lights was so severe that flat fielding (from ACPs dawn flats) was not

correcting the images. But now I have a clean sensor window and my

flats look good.



Thanks again for everyones help.



Mark



----------------------------

#28434 May 20, 2007

Hello Greg



I cannot find any page in the online manual that describes this process. Exactly

what do you mean with the "tell Maxim to do its thing"?



TIA



Lawrence Harris



> I simply point the scope at

>the uniformly lit daytime sky and tell Maxim to do its thing.

>

>Regards,

>

>Greg Hartke



----------------------------

#28435 May 20, 2007

Hi, Lawrence,



Oops. Sorry. This was a silly reference to having set up Maxim to take the

sequence automatically. I do this (as I'm sure most of us do) for all of my

flat, bias, and light frames.



Regards,



Greg



-----Original Message-----

From: MaxImDL@yahoogroups.com [mailto:MaxImDL@yahoogroups.com]On Behalf Of

Lawrence Harris

Sent: Sunday, May 20, 2007 4:24 AM

To: MaxImDL@yahoogroups.com

Subject: [MaxImDL] Re: Reflected Light?





Hello Greg



I cannot find any page in the online manual that describes this process.

Exactly

what do you mean with the "tell Maxim to do its thing"?



TIA



Lawrence Harris

> I simply point the scope at

>the uniformly lit daytime sky and tell Maxim to do its thing.

>

>Regards,

>

>Greg Hartke

.





[Non-text portions of this message have been removed]



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