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Is this typical guiding performance for an older g8? RMS around.25 px , 1.0 to 1.6 arc seconds

 

Stuck at home, I decided to learn more about phd2, and try to tweak my mount.? ?All my efforts did not seem to have much of an effect.? ?I generally achieve RMS of about? .25px ,? and about 1.0" to 1.6 ".? ?This doesn't sound great to me, I was hoping for about half of that - but my images seem to be ok.? ?(Guide camera is QHY 5L-II with 3.75 um, with 130mm QHY mini guide scope.? ?Camera for imaging is a K5 Pentax dslr.? Imaging scope is an 80mm f6 APO with a Televue 0.8x flattener.)? ?

The most irritating part has been trying to reduce the Dec backlash measured by PHd2 - which has stayed at 6 - 9 seconds despite numerous lubes, adjustments, and even Belleview washers.? ? Motor is attached very tight.? I have tried changing the balance, tight axis, loose axis.? Everything I know to try other than replacing the motors or the little plastic motor connectors - but they seem tight.? ?I tried switching the motors, but no change in Dec backlash.? ?Any thoughts?

The mount has the separate worm blocks and brass worms.? ?Using the 492 controller and the ST4 camera controlled interface and USB 3.0.

Thanks, Gil
Ventura, CA
 

I¡¯ve been using PHD2 for the last 3 or 4 months. The best guiding I¡¯ve had is 0.8¡± RMS (about 0.7¡± RA and 0.5¡± DEC) but I¡¯ve also seen it go as high as 1.5¡±. It is very dependent on seeing, and how windy it is, or whether any cables are dragging or snagging. It also depends on where you aim the telescope. I usually see slightly worse figures for RA than for DEC (maybe 30% or so higher). The worst guiding for me is objects at around zero DEC (such as Orion). The guiding gets better for me as you get higher up in DEC, most likely because periodic error is less there. Even though I am imaging at a scale of about 1.4¡± per pixel, the guiding i have been getting seems good enough. I think I am in a place where the seeing is not great and there is also light pollution.
 

I should add I use a 50 mm guide scope (focal length 207 mm) and I usually get PHD2 DEC backlash estimates of between 0.8 and 2 seconds. Mine is also a 492 system guided by ST4.
 

Hi Robert,

My GM8 had a loose DEC saddle dovetail.? I discovered the 2 top mounting bolts were slightly too long...the dovetail was slightly loose.??

My solution was to file off the 1/4-20 bolt ends so the plate was finally snug.??

Please check your dovetail plate for any wiggle.

After that, mount your scope.? Grab the front and back of the scope and try to wiggle it in DEC.? It should ideally not wiggle at all. If it dies wiggle, figure out where:. Is the worm shifting left to right?? Is the worm lightly snug into the ring gear?

All the best,
Michael

On Thu, Mar 4, 2021, 2:55 AM McFarlane, Gilbert via <gilboastro=[email protected]> wrote:
Stuck at home, I decided to learn more about phd2, and try to tweak my mount.? ?All my efforts did not seem to have much of an effect.? ?I generally achieve RMS of about? .25px ,? and about 1.0" to 1.6 ".? ?This doesn't sound great to me, I was hoping for about half of that - but my images seem to be ok.? ?(Guide camera is QHY 5L-II with 3.75 um, with 130mm QHY mini guide scope.? ?Camera for imaging is a K5 Pentax dslr.? Imaging scope is an 80mm f6 APO with a Televue 0.8x flattener.)? ?

The most irritating part has been trying to reduce the Dec backlash measured by PHd2 - which has stayed at 6 - 9 seconds despite numerous lubes, adjustments, and even Belleview washers.? ? Motor is attached very tight.? I have tried changing the balance, tight axis, loose axis.? Everything I know to try other than replacing the motors or the little plastic motor connectors - but they seem tight.? ?I tried switching the motors, but no change in Dec backlash.? ?Any thoughts?

The mount has the separate worm blocks and brass worms.? ?Using the 492 controller and the ST4 camera controlled interface and USB 3.0.

Thanks, Gil
Ventura, CA

 

Err... Gilbert (where did I imagine it was Robert?)...


On Thu, Mar 4, 2021, 8:23 AM Michael Herman via <mherman346=[email protected]> wrote:
Hi Robert,

My GM8 had a loose DEC saddle dovetail.? I discovered the 2 top mounting bolts were slightly too long...the dovetail was slightly loose.??

My solution was to file off the 1/4-20 bolt ends so the plate was finally snug.??

Please check your dovetail plate for any wiggle.

After that, mount your scope.? Grab the front and back of the scope and try to wiggle it in DEC.? It should ideally not wiggle at all. If it dies wiggle, figure out where:. Is the worm shifting left to right?? Is the worm lightly snug into the ring gear?

All the best,
Michael

On Thu, Mar 4, 2021, 2:55 AM McFarlane, Gilbert via <gilboastro=[email protected]> wrote:
Stuck at home, I decided to learn more about phd2, and try to tweak my mount.? ?All my efforts did not seem to have much of an effect.? ?I generally achieve RMS of about? .25px ,? and about 1.0" to 1.6 ".? ?This doesn't sound great to me, I was hoping for about half of that - but my images seem to be ok.? ?(Guide camera is QHY 5L-II with 3.75 um, with 130mm QHY mini guide scope.? ?Camera for imaging is a K5 Pentax dslr.? Imaging scope is an 80mm f6 APO with a Televue 0.8x flattener.)? ?

The most irritating part has been trying to reduce the Dec backlash measured by PHd2 - which has stayed at 6 - 9 seconds despite numerous lubes, adjustments, and even Belleview washers.? ? Motor is attached very tight.? I have tried changing the balance, tight axis, loose axis.? Everything I know to try other than replacing the motors or the little plastic motor connectors - but they seem tight.? ?I tried switching the motors, but no change in Dec backlash.? ?Any thoughts?

The mount has the separate worm blocks and brass worms.? ?Using the 492 controller and the ST4 camera controlled interface and USB 3.0.

Thanks, Gil
Ventura, CA

 

¿ªÔÆÌåÓý

*IF* you could Barlow that guide scope, or use a guide camera with smaller pixels, OR BOTH) and still guide at a 1/4 pixel.. ?

?

But ¡°1.0 ¨C 1.6¡± ¡° with a focal reduced 80mm scope and a DSLR would qualify as pretty much perfect tracking already, wouldn¡¯t it??

?

Derek

?

From: [email protected] [mailto:[email protected]] On Behalf Of McFarlane, Gilbert via groups.io
Sent: Wednesday, March 03, 2021 9:58 PM
To: [email protected]
Subject: [Losmandy_users_io] Is this typical guiding performance for an older g8? RMS around.25 px , 1.0 to 1.6 arc seconds

?

Stuck at home, I decided to learn more about phd2, and try to tweak my mount.? ?All my efforts did not seem to have much of an effect.? ?I generally achieve RMS of about? .25px ,? and about 1.0" to 1.6 ".? ?This doesn't sound great to me, I was hoping for about half of that - but my images seem to be ok.? ?(Guide camera is QHY 5L-II with 3.75 um, with 130mm QHY mini guide scope.? ?Camera for imaging is a K5 Pentax dslr.? Imaging scope is an 80mm f6 APO with a Televue 0.8x flattener.)? ?

The most irritating part has been trying to reduce the Dec backlash measured by PHd2 - which has stayed at 6 - 9 seconds despite numerous lubes, adjustments, and even Belleview washers.? ? Motor is attached very tight.? I have tried changing the balance, tight axis, loose axis.? Everything I know to try other than replacing the motors or the little plastic motor connectors - but they seem tight.? ?I tried switching the motors, but no change in Dec backlash.? ?Any thoughts?

The mount has the separate worm blocks and brass worms.? ?Using the 492 controller and the ST4 camera controlled interface and USB 3.0.

Thanks, Gil
Ventura, CA

?

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Thanks for the ideas and comments.? ?Yes, the tracking on the images seems to be OK, so I guess the backlash is more academic than troublesome at this point.? ?(I checked the saddle and all is tight and "flat").? ? ?It just bugs me knowing it is there!

Question:? ?When I hold the motor free of the mount, there is a tiny bit of play hack and forth as I twist the motor axle.? ?Is this normal?? ?(It is similar for both motors.)??
Gil
BTW, Robert would be a fine name!? ?:? )? ? ??
 

Can't answer the autoguiding pixel movement question...too much is involved in the auto guiding system and pixel sizes vary widely like 2 to 3x.? ?1 arcsec peak to peak?? Seeing is often said to make stars fluctuate about that much on average seeing.??

On your other question....motors...

The DC motors have permanent magnets and electric "poles" that are switched on and off as the rotor turns, to get the highest torque.? If you have power to the motors, even if they are "stopped" you will feel the motor pulsate as you try to turn the pinion gear by hand.? That's the Gemini sending power to keep the optical encoder in?place.??

If you have no electric power on the motor you might feel the pinion have small rotation positions...but probably it just spins.? ?

-----


Stay well and safe,

Michael




On Thu, Mar 4, 2021, 8:20 PM McFarlane, Gilbert via <gilboastro=[email protected]> wrote:
Thanks for the ideas and comments.? ?Yes, the tracking on the images seems to be OK, so I guess the backlash is more academic than troublesome at this point.? ?(I checked the saddle and all is tight and "flat").? ? ?It just bugs me knowing it is there!

Question:? ?When I hold the motor free of the mount, there is a tiny bit of play hack and forth as I twist the motor axle.? ?Is this normal?? ?(It is similar for both motors.)??
Gil
BTW, Robert would be a fine name!? ?:? )? ? ??

 

I have a late 90s vintage GM8 with digital drive. ? It has a stainless steel worm. ? I get about the same guiding performance. ?With my 80mm and focal reducer, that is more than good enough. ? Have not tried it out since I remedied gears, and cleaned everything out. ? the big adjustment I had is that you control your guide speed ON the 493 controller. ? If you had that speed cranked up for using the hand controller, you will not guide very well at all:-). ?All was fine once I set the speed lower. ? It is different than guiding the modern mounts but doable. ??

Cheers!

JMD

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