Re: Question about dithering


Dec 20, 2004

 


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

#15340 Dec 20, 2004

While I'm driving you all crazy, here's another beginner question(s).



Dithering. I know what it is but I'm unclear about is using the SBIG

internal guider. As we all know, when the shutter is closed, the

guider can't image. So, during downloads, the guider is off-line.

This is explained in the Maxim manual. There is a delay parameter

that allows the guider to re-sync with the star following any

inadverent drift or dithering move. This is also mentioned in the

manual.



What I'm not clear on is this. When the shutter opens and the guider

is resynchronizing, the main CCD is being exposed. Naturally the

guider will move the mount a bit to get resynchronized so that

portion of the exposure is trash.



How is this eliminated? Does Maxim close the shutter, reset the main

CCD, reopen the shutter and start again? I suppose this would work

fine, it probably doesn't take much time.



I'm just curious about this.



Also, is anyone using the Offset Guiding? I plan to try this on

Maccholz. My question is how do you determine the parameters to

enter for the x/y offsets?



This would vary according to the camera orientation as well as the

motion of the comet. Getting an accurate set of values could be

tough. But maybe there's an easy way that I'm overlooking....



Thanks!



Steve...



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

#15342 Dec 20, 2004

k4ikm wrote:

> What I'm not clear on is this. When the shutter opens and the guider

> is resynchronizing, the main CCD is being exposed. Naturally the

> guider will move the mount a bit to get resynchronized so that

> portion of the exposure is trash.



No, it's not exposing. The main CCD chip is in flush mode; the

photonelectrons are all being shuffled out to the readout register and

dumped.

> Also, is anyone using the Offset Guiding? I plan to try this on

> Maccholz. My question is how do you determine the parameters to

> enter for the x/y offsets?



At www.cyanogen.com/products/maxim_extras.htm you will find a

useful utility for generating an offset file (MaxTrack.zip), contributed

by John Farrell.



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



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



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

#15345 Dec 20, 2004

Doug,



Two followup questions on the Offset Guiding.



What happens when Maxim hits the end of the file? Will it continue

using the last offset values? Or does it stop offsetting?



Does Maxim restart at the top of the file each time a new exposure

begins? (by this I mean a "Slot", not the guider's exposure).



Thanks!



Steve...





--- In MaxImDL@yahoogroups.com, "Douglas B. George" dgeorge@c...>

wrote: > k4ikm wrote:

>

> > What I'm not clear on is this. When the shutter opens and the

guider > > is resynchronizing, the main CCD is being exposed. Naturally the

> > guider will move the mount a bit to get resynchronized so that

> > portion of the exposure is trash.

>

> No, it's not exposing. The main CCD chip is in flush mode; the

> photonelectrons are all being shuffled out to the readout register

and > dumped.

>

> > Also, is anyone using the Offset Guiding? I plan to try this on

> > Maccholz. My question is how do you determine the parameters to

> > enter for the x/y offsets?

>

> At www.cyanogen.com/products/maxim_extras.htm you will find

a > useful utility for generating an offset file (MaxTrack.zip),

contributed > by John Farrell.

>

> Doug

>

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

>

> Doug George

> dgeorge@c...

>

> 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

>

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



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

#15346 Dec 20, 2004

k4ikm wrote:

> What happens when Maxim hits the end of the file? Will it continue

> using the last offset values? Or does it stop offsetting?



It stays at the last offset values.

> Does Maxim restart at the top of the file each time a new exposure

> begins? (by this I mean a "Slot", not the guider's exposure).



No. It starts at the top whenever you start guiding.



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



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







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

#15352 Dec 20, 2004

Doug,



Thanks, this is very good news I think.



Sky6 gives the motion of Machholz in arcseconds per second for RA

and DEC. This is easy to convert to fractions of a pixel per guiding

frame based on the calibration Maxim does to compute arcseconds per

pixel and camera angle. A very simple Excel spreadsheet does this

calculation. Then just pop the seconds per guiding frame, the x and

y offset in pixels per frame into a file and off you go. This single

value will be applied by Maxim until you change it. That should be

adequate for tracking over a moderate period.



This seems very simple, it also includes camera angle which seemed

to be missing from the application you pointed me to.



Thanks again! Both for the answer and your patience.



Steve...



--- In MaxImDL@yahoogroups.com, "Douglas B. George" dgeorge@c...>

wrote: > k4ikm wrote:

>

> > What happens when Maxim hits the end of the file? Will it

continue > > using the last offset values? Or does it stop offsetting?

>

> It stays at the last offset values.

>

> > Does Maxim restart at the top of the file each time a new

exposure > > begins? (by this I mean a "Slot", not the guider's exposure).

>

> No. It starts at the top whenever you start guiding.

>

> Doug

>

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

>

> Doug George

> dgeorge@c...

>

> 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

>

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



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

#53207 May 14, 2013

It's somewhat likely that your mount cannot move accurately at that scale,

depending on the image scale you're working at. It's much better to use a

larger dither amount. I use 10 pixels, and guider dithering rather than

mount. That said, unless you're going to capture an adequate number of

frames (at least 10) dithering may not remove the dead pixel artifacts

anyway. "More frames + dithering + sigma clip + ignore black pixels" will

give you good results.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tim Stone



On Tue, May 14, 2013 at 6:41 PM, Pete Su pete.su@...> wrote:



> **

>

>

> If I am dithering my light frames using an offset or 1 or 2 pixels how much

> shift should I see from frame to frame as I capture frames?

>

> I have noted that even with dithering on the frames that I get seem like

> they are perfectly registered, which is confusing since my understanding is

> that the whole idea is to introduce some random offsets so that the various

> stacking algorithms will filter out pixels that stay in place.

>

> FWIW, I'm using a Mach-1 mount and have set things to both guide and dither

> using the mount, not the guider cables.

>

> Thanks

> Pete

>

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

>

>

>





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



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

#53208 May 14, 2013

OK. I hardly ever actually have enough time to capture 10 or more frames

except in L, so I will re-evaluate my schemes.



Thanks

Pete





On Tue, May 14, 2013 at 8:07 PM, Tim Stone tim.stone.piano@...>wrote:



> It's somewhat likely that your mount cannot move accurately at that scale,

> depending on the image scale you're working at. It's much better to use a

> larger dither amount. I use 10 pixels, and guider dithering rather than

> mount. That said, unless you're going to capture an adequate number of

> frames (at least 10) dithering may not remove the dead pixel artifacts

> anyway. "More frames + dithering + sigma clip + ignore black pixels" will

> give you good results.

>

> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> Tim Stone

>

>

> On Tue, May 14, 2013 at 6:41 PM, Pete Su pete.su@...> wrote:

>

> > **

> >

> >

> > If I am dithering my light frames using an offset or 1 or 2 pixels how

> much

> > shift should I see from frame to frame as I capture frames?

> >

> > I have noted that even with dithering on the frames that I get seem like

> > they are perfectly registered, which is confusing since my understanding

> is

> > that the whole idea is to introduce some random offsets so that the

> various

> > stacking algorithms will filter out pixels that stay in place.

> >

> > FWIW, I'm using a Mach-1 mount and have set things to both guide and

> dither

> > using the mount, not the guider cables.

> >

> > Thanks

> > Pete

> >

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

> >

> >

> >

>

>

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

>

>

>

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

>

> Yahoo! Groups Links

>

>

>

>





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







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

#53209 May 14, 2013

In that case, you're probably better off using software filters like

Photoshop's dust and scratch filter, to reduce the defects. Good flats and

darks will also help by reducing the level of noise around the defect.

Curving in Photoshop can also help reduce the levels around the defects.

All together, you can render small defects relatively unnoticeable. Column

defects are a different story. They can be very nasty to get rid of without

sufficient dithered frames.



Dark skies and good imaging!



~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tim Stone



On Tue, May 14, 2013 at 7:09 PM, Pete Su pete.su@...> wrote:



> **

>

>

> OK. I hardly ever actually have enough time to capture 10 or more frames

> except in L, so I will re-evaluate my schemes.

>

> Thanks

> Pete

>

> On Tue, May 14, 2013 at 8:07 PM, Tim Stone tim.stone.piano@...

> >wrote:

>

>

> > It's somewhat likely that your mount cannot move accurately at that

> scale,

> > depending on the image scale you're working at. It's much better to use a

> > larger dither amount. I use 10 pixels, and guider dithering rather than

> > mount. That said, unless you're going to capture an adequate number of

> > frames (at least 10) dithering may not remove the dead pixel artifacts

> > anyway. "More frames + dithering + sigma clip + ignore black pixels" will

> > give you good results.

> >

> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> > Tim Stone

> >

> >

> > On Tue, May 14, 2013 at 6:41 PM, Pete Su pete.su@...> wrote:

> >

> > > **

>

> > >

> > >

> > > If I am dithering my light frames using an offset or 1 or 2 pixels how

> > much

> > > shift should I see from frame to frame as I capture frames?

> > >

> > > I have noted that even with dithering on the frames that I get seem

> like

> > > they are perfectly registered, which is confusing since my

> understanding

> > is

> > > that the whole idea is to introduce some random offsets so that the

> > various

> > > stacking algorithms will filter out pixels that stay in place.

> > >

> > > FWIW, I'm using a Mach-1 mount and have set things to both guide and

> > dither

> > > using the mount, not the guider cables.

> > >

> > > Thanks

> > > Pete

> > >

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

> > >

> > >

> > >

> >

> >

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

> >

> >

> >

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

> >

> > Yahoo! Groups Links

>

> >

> >

> >

> >

>

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

>

>

>





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



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

#53211 May 14, 2013

I disagree. With a minimum of 5 exposures per color channel SD masking does a good job of hiding most hot pixels.

Ruben

Www.stardoctor.org

--- In MaxImDL@yahoogroups.com, Tim Stone tim.stone.piano@...> wrote:

>

> It's somewhat likely that your mount cannot move accurately at that scale,

> depending on the image scale you're working at. It's much better to use a

> larger dither amount. I use 10 pixels, and guider dithering rather than

> mount. That said, unless you're going to capture an adequate number of

> frames (at least 10) dithering may not remove the dead pixel artifacts

> anyway. "More frames + dithering + sigma clip + ignore black pixels" will

> give you good results.

>

> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

> Tim Stone

>

>

> On Tue, May 14, 2013 at 6:41 PM, Pete Su pete.su@...> wrote:

>

> > **

> >

> >

> > If I am dithering my light frames using an offset or 1 or 2 pixels how much

> > shift should I see from frame to frame as I capture frames?

> >

> > I have noted that even with dithering on the frames that I get seem like

> > they are perfectly registered, which is confusing since my understanding is

> > that the whole idea is to introduce some random offsets so that the various

> > stacking algorithms will filter out pixels that stay in place.

> >

> > FWIW, I'm using a Mach-1 mount and have set things to both guide and dither

> > using the mount, not the guider cables.

> >

> > Thanks

> > Pete

> >

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

> >

> >

> >

>

>

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

>



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