VintageBigBlue.org

 

Re: Periodic Error and Drift Measurements.


Dec 1, 2003

 


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

#18094 Dec 1, 2003

Last night I tried to measure periodic error and drift in my

relatively new G11 with Gemini level 3. I used a phillips ToUCan Pro

and K3ccd tools. I used a University Optics 2.8X klee barlow

directly in back of the flip mirror then the ToUCan Pro. Nothing

else except for a JMI-NGFS in the imaging train. I assume this gives

you exactly 2.8x the 2500 mm focal length of my Meade 10"SCT OTA. I

have read somewhere that magnification on barlow is kind of dependant

on where you place it. The readings I get from the K3ccd software

indicate that my Field of View is 1.76 ' x 1.32'. Pixel width is 5.6

microns, at a setting of 640 x 480.



First Question. Is the placement of the 2.8X Klee giving me exactly

2.8X.

Second Question. Do the values listed above sound correct? Is the

software giving me the correct FOV?



I used K3ccd tools to calculate the field of view by inputting the

parameters I described above, then quickly estimated the drift error

based on measuring the movements on the screen from the first frame

to the last frame. I took a 10 minute exposure. I tracked a star in

Orion's belt that was nearly at the equator. Orion was high but not

at the meridian. Polar alignment was ok but not great. Here are the

results.



RA drift was at approximately 2 arc sec per minute.

At the end of the run I had to press the RA + to recenter the

object. A little less than 20 arc-seconds of travel in a downward

direction on the screen over the 10 minute period. I think it runs

slow? I have to press the RA+ to keep up with the star. The camera

was positioned right side up and squared with the telescope. The

star moves down slowly on the screen from top to bottom.



Third Question. Any input? Am I thinking correctly. I wish the larger

drift could be Dec Drift but I don't think I have the buttons

reversed.



Dec drift was approximately 6.7 seconds in the 10 minutes.



Fourth Question - Is this average, above average or what. Could this

be producting the RA drift. I don't think so.



Fifth Question. Is there a way to speed up the sidereal speed if I

confirm that there is a problem?



Sixth Question. Anyone else experience a drift in RA in either

direction and corrected the issue?



I am plotting the movement verses time to get a better look at the

detail for the periodic error measurement. There are about 2400

frames so it may take a while unless someone has a program that will

calculate it for me. K3ccd may be the program but I just downloaded

it yesterday and don't completely know how to use it.



Any input?



Thanks in advance.



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

#18096 Dec 1, 2003

Emilio, some answers:



1. To compute the exact focal length, you should shoot a field with

two stars with a known separation. You can then compute the fl if you

know the pixel size in microns of your camera. As is, there are some

variations in the exact focal length between SCTs, and it also varies

based on where the barlow is placed in the optical path.



2. By my computations, your fov (assuming exactly 2.8x increase in

2500mm fl) is 1.8 x 1.3 arcminutes.



3. Most likely your mount runs at the right speed and the drift is

due to a slight polar misalignment.



4. You need to measure the periodic error subtracting out the drift

for a valid PE measurement. Periodic error is just that, periodic. It

doesn't keep drifting: after the full 4 minute worm cycle it should

come back to zero.



5. Yes, Gemini lets you set the exact speed by using the comet

tracking mode. It's unlikely that you'll need to do this, but read my

earlier post about removing drift using this mode.



6. Yes, and it's most likely due to polar misalignment. The first

step is to improve polar alignment. The second would be to try out my

Comet Tracking suggestion (and let me know how it works out G>).



Hope this helps,



-Paul

--- In Losmandy_users@yahoogroups.com, "ejrobau" ejr@c...> wrote:

> Last night I tried to measure periodic error and drift in my

> relatively new G11 with Gemini level 3. I used a phillips ToUCan

Pro

> and K3ccd tools. I used a University Optics 2.8X klee barlow

> directly in back of the flip mirror then the ToUCan Pro. Nothing

> else except for a JMI-NGFS in the imaging train. I assume this

gives

> you exactly 2.8x the 2500 mm focal length of my Meade 10"SCT OTA.

I

> have read somewhere that magnification on barlow is kind of

dependant

> on where you place it. The readings I get from the K3ccd software

> indicate that my Field of View is 1.76 ' x 1.32'. Pixel width is

5.6

> microns, at a setting of 640 x 480.

>

> First Question. Is the placement of the 2.8X Klee giving me

exactly

> 2.8X.

> Second Question. Do the values listed above sound correct? Is the

> software giving me the correct FOV?

>

> I used K3ccd tools to calculate the field of view by inputting the

> parameters I described above, then quickly estimated the drift

error

> based on measuring the movements on the screen from the first frame

> to the last frame. I took a 10 minute exposure. I tracked a star

in

> Orion's belt that was nearly at the equator. Orion was high but

not

> at the meridian. Polar alignment was ok but not great. Here are

the

> results.

>

> RA drift was at approximately 2 arc sec per minute.

> At the end of the run I had to press the RA + to recenter the

> object. A little less than 20 arc-seconds of travel in a downward

> direction on the screen over the 10 minute period. I think it runs

> slow? I have to press the RA+ to keep up with the star. The camera

> was positioned right side up and squared with the telescope. The

> star moves down slowly on the screen from top to bottom.

>

> Third Question. Any input? Am I thinking correctly. I wish the

larger

> drift could be Dec Drift but I don't think I have the buttons

> reversed.

>

> Dec drift was approximately 6.7 seconds in the 10 minutes.

>

> Fourth Question - Is this average, above average or what. Could

this

> be producting the RA drift. I don't think so.

>

> Fifth Question. Is there a way to speed up the sidereal speed if I

> confirm that there is a problem?

>

> Sixth Question. Anyone else experience a drift in RA in either

> direction and corrected the issue?

>

> I am plotting the movement verses time to get a better look at the

> detail for the periodic error measurement. There are about 2400

> frames so it may take a while unless someone has a program that

will

> calculate it for me. K3ccd may be the program but I just

downloaded

> it yesterday and don't completely know how to use it.

>

> Any input?

>

> Thanks in advance.







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

#18098 Dec 1, 2003

--- In Losmandy_users@yahoogroups.com, "Paul K" pkane2001@h...>

wrote: > Emilio, some answers:

>

> 1. To compute the exact focal length, you should shoot a field with

> two stars with a known separation. You can then compute the fl if

you > know the pixel size in microns of your camera. As is, there are

some > variations in the exact focal length between SCTs, and it also

varies > based on where the barlow is placed in the optical path.

>

> 2. By my computations, your fov (assuming exactly 2.8x increase in

> 2500mm fl) is 1.8 x 1.3 arcminutes.

>

> 3. Most likely your mount runs at the right speed and the drift is

> due to a slight polar misalignment.

>

> 4. You need to measure the periodic error subtracting out the drift

> for a valid PE measurement. Periodic error is just that, periodic.

It > doesn't keep drifting: after the full 4 minute worm cycle it should

> come back to zero.

>

> 5. Yes, Gemini lets you set the exact speed by using the comet

> tracking mode. It's unlikely that you'll need to do this, but read

my > earlier post about removing drift using this mode.

>

> 6. Yes, and it's most likely due to polar misalignment. The first

> step is to improve polar alignment. The second would be to try out

my > Comet Tracking suggestion (and let me know how it works out G>).

>

> Hope this helps,

>

> -Paul

>

> --- In Losmandy_users@yahoogroups.com, "ejrobau" ejr@c...> wrote:

> > Last night I tried to measure periodic error and drift in my

> > relatively new G11 with Gemini level 3. I used a phillips ToUCan

> Pro

> > and K3ccd tools. I used a University Optics 2.8X klee barlow

> > directly in back of the flip mirror then the ToUCan Pro. Nothing

> > else except for a JMI-NGFS in the imaging train. I assume this

> gives

> > you exactly 2.8x the 2500 mm focal length of my Meade 10"SCT

OTA. > I

> > have read somewhere that magnification on barlow is kind of

> dependant

> > on where you place it. The readings I get from the K3ccd

software > > indicate that my Field of View is 1.76 ' x 1.32'. Pixel width is

> 5.6

> > microns, at a setting of 640 x 480.

> >

> > First Question. Is the placement of the 2.8X Klee giving me

> exactly

> > 2.8X.

> > Second Question. Do the values listed above sound correct? Is

the > > software giving me the correct FOV?

> >

> > I used K3ccd tools to calculate the field of view by inputting

the > > parameters I described above, then quickly estimated the drift

> error

> > based on measuring the movements on the screen from the first

frame > > to the last frame. I took a 10 minute exposure. I tracked a star

> in

> > Orion's belt that was nearly at the equator. Orion was high but

> not

> > at the meridian. Polar alignment was ok but not great. Here are

> the

> > results.

> >

> > RA drift was at approximately 2 arc sec per minute.

> > At the end of the run I had to press the RA + to recenter the

> > object. A little less than 20 arc-seconds of travel in a

downward > > direction on the screen over the 10 minute period. I think it

runs > > slow? I have to press the RA+ to keep up with the star. The

camera > > was positioned right side up and squared with the telescope. The

> > star moves down slowly on the screen from top to bottom.

> >

> > Third Question. Any input? Am I thinking correctly. I wish the

> larger

> > drift could be Dec Drift but I don't think I have the buttons



Thanks for your kind reply. I am surprized and pleased that what I

perceived was a small amount of polar misalignment could be the

issue. This makes me happy if true. I will continue to work on the

polar alignment. May even try to drift after polar access correction

to get it right on. Then I will remeasure everything. > > reversed.

> >

> > Dec drift was approximately 6.7 seconds in the 10 minutes.

> >

> > Fourth Question - Is this average, above average or what. Could

> this

> > be producting the RA drift. I don't think so.

> >

> > Fifth Question. Is there a way to speed up the sidereal speed if

I > > confirm that there is a problem?

> >

> > Sixth Question. Anyone else experience a drift in RA in either

> > direction and corrected the issue?

> >

> > I am plotting the movement verses time to get a better look at

the > > detail for the periodic error measurement. There are about 2400

> > frames so it may take a while unless someone has a program that

> will

> > calculate it for me. K3ccd may be the program but I just

> downloaded

> > it yesterday and don't completely know how to use it.

> >

> > Any input?

> >

> > Thanks in advance.



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

#18099 Dec 1, 2003

Emilio, one more place to look for a source of drift is the focuser

assembly and the mounting of your camera. If there's any slop in the

focuser, or anywhere between the focuser and the camera, this will

often manifest itself as a constant drift as the mount is moving in

RA. Double-check all the screws and tighten the focuser well.



Regards,



-paul

--- In Losmandy_users@yahoogroups.com, "ejrobau" ejr@c...> wrote:

> --- In Losmandy_users@yahoogroups.com, "Paul K" pkane2001@h...>

> wrote:

> > Emilio, some answers:

> >

> > 1. To compute the exact focal length, you should shoot a field

with

> > two stars with a known separation. You can then compute the fl if

> you

> > know the pixel size in microns of your camera. As is, there are

> some

> > variations in the exact focal length between SCTs, and it also

> varies

> > based on where the barlow is placed in the optical path.

> >

> > 2. By my computations, your fov (assuming exactly 2.8x increase

in

> > 2500mm fl) is 1.8 x 1.3 arcminutes.

> >

> > 3. Most likely your mount runs at the right speed and the drift

is

> > due to a slight polar misalignment.

> >

> > 4. You need to measure the periodic error subtracting out the

drift

> > for a valid PE measurement. Periodic error is just that,

periodic.

> It

> > doesn't keep drifting: after the full 4 minute worm cycle it

should

> > come back to zero.

> >

> > 5. Yes, Gemini lets you set the exact speed by using the comet

> > tracking mode. It's unlikely that you'll need to do this, but

read

> my

> > earlier post about removing drift using this mode.

> >

> > 6. Yes, and it's most likely due to polar misalignment. The first

> > step is to improve polar alignment. The second would be to try

out

> my

> > Comet Tracking suggestion (and let me know how it works out G>).

> >

> > Hope this helps,

> >

> > -Paul

> >

> > --- In Losmandy_users@yahoogroups.com, "ejrobau" ejr@c...> wrote:

> > > Last night I tried to measure periodic error and drift in my

> > > relatively new G11 with Gemini level 3. I used a phillips

ToUCan

> > Pro

> > > and K3ccd tools. I used a University Optics 2.8X klee barlow

> > > directly in back of the flip mirror then the ToUCan Pro.

Nothing

> > > else except for a JMI-NGFS in the imaging train. I assume this

> > gives

> > > you exactly 2.8x the 2500 mm focal length of my Meade 10"SCT

> OTA.

> > I

> > > have read somewhere that magnification on barlow is kind of

> > dependant

> > > on where you place it. The readings I get from the K3ccd

> software

> > > indicate that my Field of View is 1.76 ' x 1.32'. Pixel width

is

> > 5.6

> > > microns, at a setting of 640 x 480.

> > >

> > > First Question. Is the placement of the 2.8X Klee giving me

> > exactly

> > > 2.8X.

> > > Second Question. Do the values listed above sound correct? Is

> the

> > > software giving me the correct FOV?

> > >

> > > I used K3ccd tools to calculate the field of view by inputting

> the

> > > parameters I described above, then quickly estimated the drift

> > error

> > > based on measuring the movements on the screen from the first

> frame

> > > to the last frame. I took a 10 minute exposure. I tracked a

star

> > in

> > > Orion's belt that was nearly at the equator. Orion was high

but

> > not

> > > at the meridian. Polar alignment was ok but not great. Here

are

> > the

> > > results.

> > >

> > > RA drift was at approximately 2 arc sec per minute.

> > > At the end of the run I had to press the RA + to recenter the

> > > object. A little less than 20 arc-seconds of travel in a

> downward

> > > direction on the screen over the 10 minute period. I think it

> runs

> > > slow? I have to press the RA+ to keep up with the star. The

> camera

> > > was positioned right side up and squared with the telescope.

The

> > > star moves down slowly on the screen from top to bottom.

> > >

> > > Third Question. Any input? Am I thinking correctly. I wish the

> > larger

> > > drift could be Dec Drift but I don't think I have the buttons

>

> Thanks for your kind reply. I am surprized and pleased that what I

> perceived was a small amount of polar misalignment could be the

> issue. This makes me happy if true. I will continue to work on

the

> polar alignment. May even try to drift after polar access

correction

> to get it right on. Then I will remeasure everything.

> > > reversed.

> > >

> > > Dec drift was approximately 6.7 seconds in the 10 minutes.

> > >

> > > Fourth Question - Is this average, above average or what.

Could

> > this

> > > be producting the RA drift. I don't think so.

> > >

> > > Fifth Question. Is there a way to speed up the sidereal speed

if

> I

> > > confirm that there is a problem?

> > >

> > > Sixth Question. Anyone else experience a drift in RA in either

> > > direction and corrected the issue?

> > >

> > > I am plotting the movement verses time to get a better look at

> the

> > > detail for the periodic error measurement. There are about

2400

> > > frames so it may take a while unless someone has a program that

> > will

> > > calculate it for me. K3ccd may be the program but I just

> > downloaded

> > > it yesterday and don't completely know how to use it.

> > >

> > > Any input?

> > >

> > > Thanks in advance.







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