Re: [ap-gto] 1200RA drift?


Jul 7, 2002

 


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

#5282 Jul 7, 2002

I have a new 1200GTO that seems to have stars drifting in RA to the west,

suggesting to me that the mount is tracking a little slow. I can see this

in a few minutes with a CCD in that if there is no guiding, the star will

slowly drift to the west. I have a pretty good drift polar alignment with

this thing and Tpoint is suggesting that I am a couple of arc minutes

off. I have noticed that if I run a long guider exposure with the ST8 and

Maxim, the stars will be ever so slightly oblong in RA so I have to run an

exposure of 4 seconds. Ten seconds will show stars slightly oblong in RA

and the guider information window shows RA making bigger corrections with

larger errors. RA will have errors like .5-.9 while DEC will be happy with

.1-.2 errors (somewhat seeing dependant, of course) but results are

consistent. This is with PEM on and trained.



Is there a way to check RA drive rate or crystal rate? Reason I'm curious

is that at one time we were able to put three Losmandy G-11 drive boxes on

an oscilloscope and one ran fast, one was slow, and one was about

right. This was verified visually when the different boxes were put on the

same mount. Just curious if anyone else has noticed this problem with a

1200's RA drive rate.



Might it be clever to send the electronics back to AP and let them check it

out now that we are in monsoon pattern here in SE Arizona?





Mike J. Shade: mshade@...

Sonoita Hills Observatory, Sonoita Arizona



See work done at the observatory: c3po.cochise.cc.az.us/astro

under the "photographs and images" panel on the left of the screen.



"I like the dark, it's cheap." E. B. Scrooge www.darksky.org "I'm a

member, are you?"



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

#5292 Jul 8, 2002

Mike,



There was a long discussion on this list three weeks ago about mount drift

rates and errors. This issue is quite complex and there are many factors

that influence the resulting drift rate. Roland says that the AP mounts run

at the King rate which is about 0.25% slower than the sidereal rate. The

crystal controlled time base has a tolerance +-0.01%. Also, it turns out

that the biggest drift error is often caused by variable

tube/mount flexure. I have an AP1200 mount and often guide using 6-8 sec

exposures without problems. Typical guiding errors are about 0.4 arc sec

RMS for both axes, with 3 arc sec seeing, but the errors do change with

declination, camera angle and seeing.



Richard







At 04:27 PM 7/7/2002 -0700, you wrote: >I have a new 1200GTO that seems to have stars drifting in RA to the west,

>suggesting to me that the mount is tracking a little slow. I can see this

>in a few minutes with a CCD in that if there is no guiding, the star will

>slowly drift to the west. I have a pretty good drift polar alignment with

>this thing and Tpoint is suggesting that I am a couple of arc minutes

>off. I have noticed that if I run a long guider exposure with the ST8 and

>Maxim, the stars will be ever so slightly oblong in RA so I have to run an

>exposure of 4 seconds. Ten seconds will show stars slightly oblong in RA

>and the guider information window shows RA making bigger corrections with

>larger errors. RA will have errors like .5-.9 while DEC will be happy with

>.1-.2 errors (somewhat seeing dependant, of course) but results are

>consistent. This is with PEM on and trained.

>

>Is there a way to check RA drive rate or crystal rate? Reason I'm curious

>is that at one time we were able to put three Losmandy G-11 drive boxes on

>an oscilloscope and one ran fast, one was slow, and one was about

>right. This was verified visually when the different boxes were put on the

>same mount. Just curious if anyone else has noticed this problem with a

>1200's RA drive rate.

>

>Might it be clever to send the electronics back to AP and let them check it

>out now that we are in monsoon pattern here in SE Arizona?

>

>

>Mike J. Shade: mshade@...

>Sonoita Hills Observatory, Sonoita Arizona



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

#5302 Jul 10, 2002

In a message dated 7/7/2002 6:29:07 PM Central Daylight Time,

mshade@... writes:



> I have a new 1200GTO that seems to have stars drifting in RA to the west,

> suggesting to me that the mount is tracking a little slow



Again, I doubt that the frequency is wrong. I'm betting that your polar

altitude is off. Another thing to check is whether the drift occurs toward

the zenith, or if you get drift at lower altitudes. At the lower altitudes I

would expect drift due to atmospheric refraction. Also, do you know if your

telescope optics are flexing or not?



Roland Christen





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



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

#5305 Jul 10, 2002

In a message dated 7/8/2002 8:16:05 PM Central Daylight Time,

reseavey@... writes:



> Roland says that the AP mounts run

> at the King rate which is about 0.25% slower than the sidereal rate.



Actually, that is not correct. The rate is closer to the true sidereal rate.



Roland Christen





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







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

#5307 Jul 10, 2002

At 02:33 PM 7/10/2002 -0400, you wrote: >In a message dated 7/7/2002 6:29:07 PM Central Daylight Time,

>mshade@... writes:

>

>

> > I have a new 1200GTO that seems to have stars drifting in RA to the west,

> > suggesting to me that the mount is tracking a little slow

>

>Again, I doubt that the frequency is wrong. I'm betting that your polar

>altitude is off. Another thing to check is whether the drift occurs toward

>the zenith, or if you get drift at lower altitudes. At the lower altitudes I

>would expect drift due to atmospheric refraction. Also, do you know if your

>telescope optics are flexing or not?

>

>Roland Christen

>



It could be possible that the optics are flexing but I do have the primary

locked in the C-14 so I think that this accounting for what I'm seeing is

not a real high probability. When I get the rings attached to the plate

and this attached to the mount I will again do another drift alignment but

will use the CCD camera to check it and make corrections if need be.





Mike J. Shade: mshade@...

Sonoita Hills Observatory, Sonoita Arizona



See work done at the observatory: c3po.cochise.cc.az.us/astro

under the "photographs and images" panel on the left of the screen.



"I like the dark, it's cheap." E. B. Scrooge www.darksky.org "I'm a

member, are you?"



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

#5309 Jul 10, 2002

In a message dated 7/10/2002 3:30:58 PM Central Daylight Time,

mshade@... writes:



> It could be possible that the optics are flexing but I do have the primary

> locked in the C-14 so I think that this accounting for what I'm seeing is

> not a real high probability



Both optics and the tube can flex. Even with the optics tied down, there can

be flex.



Roland Christen





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



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

#5310 Jul 10, 2002

At 02:33 PM 7/10/2002 -0400, you wrote:

>>In a message dated 7/7/2002 6:29:07 PM Central Daylight Time,

>>mshade@... writes:

>>

>>

>> > I have a new 1200GTO that seems to have stars drifting in RA to the west,

>> > suggesting to me that the mount is tracking a little slow

>>

>>Again, I doubt that the frequency is wrong. I'm betting that your polar

>>altitude is off. Another thing to check is whether the drift occurs toward

>>the zenith, or if you get drift at lower altitudes. At the lower altitudes I

>>would expect drift due to atmospheric refraction. Also, do you know if your

>>telescope optics are flexing or not?

>>

>>Roland Christen

>>

>

>It could be possible that the optics are flexing but I do have the primary

>locked in the C-14 so I think that this accounting for what I'm seeing is

>not a real high probability. When I get the rings attached to the plate

>and this attached to the mount I will again do another drift alignment but

>will use the CCD camera to check it and make corrections if need be.



Hi Mike,



it is a bit off, but how you are locking your c14. I am looking for a

while for a goos solution.



Konstantin

--

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

Konstantin v. Poschinger

Hammerichstr. 5

22605 Hamburg

Germany

Tel. (040) 880 57 47

Fax. (040) 881 22 79

E-Mail: KPoschinger@...

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



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

#5311 Jul 10, 2002

Roland,

Thanks for correcting the King rate number I misquoted from one of your

previous posts.



Richard





> > Roland says that the AP mounts run

> > at the King rate which is about 0.25% slower than the sidereal rate.

>

>Actually, that is not correct. The rate is closer to the true sidereal rate.

>

>Roland Christen



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

#5313 Jul 10, 2002

A friend and I have taken apart his newer model C-14.

We found that the mirror is siliconed onto the slider

tube. Infact, the back of the mirror was free floating

of the slider tube backplate. The center lock ring was

set firm but not tight. As it was, we could put light

pressure (about 1/4 mirrors weight) on the mirror's

back side and watch the opposite end move. Our

solution was to slightly backoff the center ring,

insert some cork shims that took up the exact space on

three sides, and lightly retighten the center ring.

With the shims in place we can put 3-4 times the

pressure on the rear of the mirror to make the same

amount of deflection as before.



Hope this may help you out.



Tim



BTW: The mirror lock only locks down the slider tube

that the mirror is attached to.











--- chris1011@... wrote: > In a message dated 7/10/2002 3:30:58 PM Central

> Daylight Time,

> mshade@... writes:

>

>

> > It could be possible that the optics are

flexing > but I do have the primary

> > locked in the C-14 so I think that this

accounting > for what I'm seeing is

> > not a real high probability

>

> Both optics and the tube can flex. Even with the

> optics tied down, there can

> be flex.

>

> Roland Christen

>

>

> [Non-text portions of this message have been

> removed]

>

>

> To UNSUBSCRIBE, or for general information on the

> ap-gto list

> see groups.yahoo.com/group/ap-gto

>

> Your use of Yahoo! Groups is subject to

> docs.yahoo.com/info/terms/

>

>





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

#5314 Jul 10, 2002

Mike,

I have made a number of measurements (using Tpoint) of my Meade 10" SCT

and AP1200 mount to try to improve pointing/tracking accuracy. The results

seem to clearly indicate that there is considerable flexing someplace. I

suspect (but can't yet prove) that the metal mirror support is flexing and

allows the mirror to tilt on the baffle tube. I lock the mirror down by

pulling the mirror mounting plate toward the OTA back plate using a

threaded rod inserted into the (single) mirror transport hold-down hole.

This is really a poor way to lock down the mirror, but other methods are

much more difficult and involved. In Tpoint, the "TF" and "DAF" terms do a

good job of modelling this flexure, but this only helps *pointing* accuracy

and not *tracking* accuracy. The flexure will increase the effective track

rate at lower OTA altitudes. As discussed in previous posts, a variable

tracking rate is needed.



Richard







At 01:28 PM 7/10/2002 -0700, you wrote:



>It could be possible that the optics are flexing but I do have the primary

>locked in the C-14 so I think that this accounting for what I'm seeing is

>not a real high probability. When I get the rings attached to the plate

>and this attached to the mount I will again do another drift alignment but

>will use the CCD camera to check it and make corrections if need be.

>

>

>Mike J. Shade: mshade@...

>Sonoita Hills Observatory, Sonoita Arizona



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

#5317 Jul 11, 2002

I had terrible pointing errors in my C11/1200 system which I tracked

down to mirro flexure: my mirror had the same lousy silicone caulk

mounting inside as yours. I also used cork in the annular space, and

tightened the collar. The pointing error vanished, though my "fix" did

introduce astignmatism (not a problem in photometry). I will be taking

it apart again soon, and after a series of measurements to determine and

correct interior reflections, I plan to remove the cork and replace it

with expanding foam which should introduce much less astigmatism. You

are on the right track!



John Menke

Menkescientific.com



Tim Khan wrote: >

> A friend and I have taken apart his newer model C-14.

> We found that the mirror is siliconed onto the slider

> tube. Infact, the back of the mirror was free floating

> of the slider tube backplate. The center lock ring was

> set firm but not tight. As it was, we could put light

> pressure (about 1/4 mirrors weight) on the mirror's

> back side and watch the opposite end move. Our

> solution was to slightly backoff the center ring,

> insert some cork shims that took up the exact space on

> three sides, and lightly retighten the center ring.

> With the shims in place we can put 3-4 times the

> pressure on the rear of the mirror to make the same

> amount of deflection as before.

>

> Hope this may help you out.

>

> Tim

>

> BTW: The mirror lock only locks down the slider tube

> that the mirror is attached to.

>

> --- chris1011@... wrote:

> > In a message dated 7/10/2002 3:30:58 PM Central

> > Daylight Time,

> > mshade@... writes:

> >

> >

> > > It could be possible that the optics are

> flexing

> > but I do have the primary

> > > locked in the C-14 so I think that this

> accounting

> > for what I'm seeing is

> > > not a real high probability

> >

> > Both optics and the tube can flex. Even with the

> > optics tied down, there can

> > be flex.

> >

> > Roland Christen

> >

> >

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

> > removed]

> >

> >

> > To UNSUBSCRIBE, or for general information on the

> > ap-gto list

> > see groups.yahoo.com/group/ap-gto

> >

> > Your use of Yahoo! Groups is subject to

> > docs.yahoo.com/info/terms/

> >

> >

>

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

> Do You Yahoo!?

> Sign up for SBC Yahoo! Dial - First Month Free

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>

> To UNSUBSCRIBE, or for general information on the ap-gto list

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>

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