Re: [MaxImDL] Re: RA creep during autoguiding


Nov 23, 2005

 


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

#21714 Nov 23, 2005

I have noticed that whilst autoguiding a number of consecutive images,

the sequence of images shows a gradual movement in RA, towards the

east. I cannot see how this can happen because the guide star is being

kept 'central'. Any ideas?



Lawrence Harris



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

#21717 Nov 23, 2005

I have noticed the same effect - the creep you describe. I have a

piggyback refractor with my 10" reflector. Generally expose about 5

or 10 minutes, and when stacking many frames, I notice a few of the

hot pixels creeping across the image, tracing a diagonal path onthe

image. Up to now I have just assumed that it is some image

rotational effect due to imperfect polar alignment, but now that I

think about it, I don't think I can explain it so. The trace is a

line across the image. I will have to count pixels to quantify, but

it is clearly there, spaced about 1mm apart in each image. Would

like to know if it is simply due to alignment issues. Been thinking

I need to take some dark frames with my SXVH9C or work some hot

pixel magic or something!



randy

albuquerque

--- In MaxImDL@yahoogroups.com, Sam Barziza sbarziza@c...> wrote:

>

> Hi Lawrence:

> Can you describe your setup ? What type of mount do you have,

scope

> etc.? How are you guiding, thru a guidescope ?

> What type of camera do you have ? How long are your exposures, in

which

> you notice RA creeping, both AG and main camera ?

> How much movement do you see in pixels in your consecutive frames ?

> As much info that you can provide, will help in possibly

diagnosing your

> problem.

>

> Sam

>

> Lawrence wrote:

>

> >I have noticed that whilst autoguiding a number of consecutive

images,

> >the sequence of images shows a gradual movement in RA, towards

the

> >east. I cannot see how this can happen because the guide star is

being

> >kept 'central'. Any ideas?

> >

> >Lawrence Harris

> >

> >

> >

> >

> >Yahoo! Groups Links

> >

> >

> >

> >

> >

> >

> >

> >

> >

> >

> >

>



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

#21718 Nov 23, 2005

It happens because over time the guider and imager optical axes change

slightly with respect to each other. It is something that always happens

when using a secondary guidescope-it is just a matter of degree. You may

have some flexure of the guidescope on its mounting system, you may have

some shifting of one or both cameras, and if your scope is an SCT, you

almost certainly have some movement of the primary mirror.



The drift is harmless (or even beneficial) as long as it doesn't show in the

longest subframes you make. But if you are compromising your exposure time

just to control drift, you should look at your mechanical setup and see what

might be improved.



Chris



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

Chris L Peterson

Cloudbait Observatory

www.cloudbait.com



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

From: "rogauntt" rogauntt@...>

To: MaxImDL@yahoogroups.com>

Sent: Wednesday, November 23, 2005 7:20 AM

Subject: [MaxImDL] Re: RA creep during autoguiding





>I have noticed the same effect - the creep you describe. I have a

> piggyback refractor with my 10" reflector. Generally expose about 5

> or 10 minutes, and when stacking many frames, I notice a few of the

> hot pixels creeping across the image, tracing a diagonal path onthe

> image. Up to now I have just assumed that it is some image

> rotational effect due to imperfect polar alignment, but now that I

> think about it, I don't think I can explain it so. The trace is a

> line across the image. I will have to count pixels to quantify, but

> it is clearly there, spaced about 1mm apart in each image. Would

> like to know if it is simply due to alignment issues. Been thinking

> I need to take some dark frames with my SXVH9C or work some hot

> pixel magic or something!

>

> randy

> albuquerque



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