Re: [ap-gto] Re: 1200GTO PE and Dec Curves


Jun 9, 2004

 


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#10108 Jun 9, 2004

I have placed an Excel file in the Files folder entitled "PE PEM Off

06 09 04". It comprises data and PE and Dec drift curves taken on

my newly delivered 1200GTO.



Drift aligning on the southern star was done with an MX716 camera, C-

11 scope and MaximDL reading out the differential motion. I had the

up/down drift adjusted to within epsilon of zero arc seconds per

minute. Unfortunately this increased to 0.15 arc seconds per minute

when I tightened the knobs. Strangely, I observed some oscillatory

motion in Dec that was larger than seeing jitter.



I was disappointed that the new "washers" did nothing for me to

alleviate the misalignment introduced by tightening the azimuth

knobs. I had initially ordered the bearing base plate, but was

advised that I didn't need it for my permanent mount and that the

washers would solve my problem. I also employed Roland's technique

of tightening the rear knob so it would act as a pivot. No joy.



The PE and Dec drift curves were run without PEM . I still haven't

gotten it to work. I was aiming at a star near Duhbe . the only

cloudless sector of the sky.



I'd appreciate feedback on the interpretation of the PE and Dec

drift curves. In particular, to back out the high rate of Dec drift

from the PE curve, do I simply subtract the Dec drift in quadrature?



Also, why is the Dec curve non-monotonic? I find this behavior most

strange. I do not think it is associated with mirror sag because it

appears to be oscillilatory.



Clearly I have lots to learn about my new mount and the

characterization of its properties and I welcome tutoring from the

group . thanks.



Clear skies,



Dennis Persyk

Igloo Observatory Home Page dpersyk.home.att.net

Hampshire, IL



New Images home.att.net/~dpersyk/new.htm



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#10110 Jun 9, 2004

In a message dated 6/9/2004 4:15:04 PM Central Daylight Time, dpersyk@...

writes:



> Also, why is the Dec curve non-monotonic? I find this behavior most

> strange. I do not think it is associated with mirror sag because it

> appears to be oscillilatory.

>



If your camera is not perfectly square with the two axes, it will appear that

the Dec drift has slight periodic error. This is because part of the dec

drift is really due to the RA motion. I am assuming that you have the camera

fairly close to square and you are interpreting a Y motion as Dec and an X motion

as RA (or vice versa).



Roland Christen





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#10113 Jun 9, 2004

--- In ap-gto@yahoogroups.com, chris1011@a... wrote: > In a message dated 6/9/2004 4:15:04 PM Central Daylight Time,

dpersyk@a... > writes:

>

>

> > Also, why is the Dec curve non-monotonic? I find this behavior

most strange. I do not think it is associated with mirror sag

because it appears to be oscillilatory. > >

>

> If your camera is not perfectly square with the two axes, it will

appear that the Dec drift has slight periodic error. This is

because part of the dec drift is really due to the RA motion. I am

assuming that you have the camera fairly close to square and you

are interpreting a Y motion as Dec and an X motion as RA (or vice

versa). >

> Roland Christen

>

>



According to MaximDL CCD the camera was aligned to within 3.8

degrees of horizontal as determined in the Calibrate Autoguider

routine. Pressing the E button while exposing produced a horizontal

star in the image. X is RA and Y is Dec as is the common convention

in imaging software.



The oscillatory behavior was observed during the south star drift

procedure and in the PE curve acquisition to the northwest. I will

acquire more data next relatively cloudless night. With my present

self-guide camera and 530 mm focal length scope (piggybacked on the

C-11), I prefer to take unguided exposures and hence my interest in

precise drift aligning. At the moment, my Dec drift seems to be far

greater in magnitude than the periodic error.



Thanks again for the speedy reply and help.



Dennis



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#10115 Jun 9, 2004

In a message dated 6/9/2004 5:34:41 PM Central Daylight Time, dpersyk@...

writes:



>

> According to MaximDL CCD the camera was aligned to within 3.8

> degrees of horizontal as determined in the Calibrate Autoguider

> routine. Pressing the E button while exposing produced a horizontal

> star in the image. X is RA and Y is Dec as is the common convention

> in imaging software.





I don't know what is common convention, but I have my camera normally aligned

with Dec=Y.

>

> The oscillatory behavior was observed during the south star drift

> procedure and in the PE curve acquisition to the northwest.



Dec is not driven. The motor is absolutely still unless it is told to move.

If your camera is even slighly out of square with respect to the mount axes,

then you will see a small RA motion superimposed on the apparent Dec motion. If

you want to be absolutely sure that the Dec motor is not turning the axis, you

can temporarily disconnect the cable from the motor. Just be sure that you

put it back before pressing any N-S buttons.





With my present > > self-guide camera and 530 mm focal length scope (piggybacked on the

> C-11), I prefer to take unguided exposures and hence my interest in

> precise drift aligning. At the moment, my Dec drift seems to be far

> greater in magnitude than the periodic error.

>



Since the sky is distorted due to atmospheric dispersion, you will always

have some drift in one or both axes depending on where you are trying to image.

One other source of apparent drift manifests itself in the C14 as well as the

smaller SCTs. The star image will drift primarily in RA because the weight of

the camera equipment on the back of the SCT causes a differential flexure

between the optical axis and the mechanical axis. It disappears only at the zenith

and re-appears as you cross the meridian. I have seen similar effects in other

Cassegrain telescopes. No matter how well you are polar aligned, there will

be some drift somewhere in the sky when you use these types of scopes. With

smaller scopes and shorter focal lengths you should be able to eliminate most of

the drift, but you may still have some near the horizon.



Roland Christen





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