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RE: [Losmandy_users] Re: Polar Alignment Frustrations


Feb 23 3:14 PM

 


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#58809 Feb 23 3:14 PM

I'mfrustrated by my inability to obtain a good polar alignment on mypier mounted G11. I'm experiencing image shift in images greaterthan 40 seconds. I've have better experienced better performance inthe past (120 second images weren't a problem). My setup is G11mounted and leveled on a concrete pier, a well balanced 10. F 8Ritchey-Chretien, and a Mallincam XT-418 camera. I typically useone or more focal reducers to get a larger field of view and betweenF 4 & 5, but last night I worked at prime focus since I wasplanning to drift align. I control the mount via the Gemini II handcontroller, no computer control.



Lastnight I installed the polar scope and carefully aligned on Polaris.I have a limited Northern view so I can't see the second star in thereticle and even Ursa Major was visible only through trees. Thatshould be close enough to then drift align. I pointed the scope atMirzam (front foot of Sirius) which was South near the meridian andequator. I centered the star on the vertical cross hair of theMallincam. After 5 minutes there was no perceptible drift to theleft or right, only a very slight downward drift (perhaps 1/3 thestar's diameter). I considered that good and then went to Mirachwhich was low (~ 25 degrees) in the West a little North of theequator. I saw drift to the right after 2 minutes, so I adjustedthe altitude knob clockwise (raised the mount). After a couple ofiterations there was no left or right drift after 5 minutes. I wentback to the Southern sky and centered on Sirius. After 5 minutes thestar had drifted only ~ . the star's diameter to the left. Iconsidered that should be adequate and went started viewing. Afterdoing a 4 star align, including Regulus and Denebola, I went togalaxies in Leo and had trouble with shift in 60 second images.Trying to stack 60 second or longer frames was futile due to imageshift.



Iclearly am doing something wrong, but what? �� How can Southern & Western stars sit on across hair for 5 minutes with little to no perceptible drift and thenwhen I try to observe objects have image shift in 1 minute ?Any suggestions would be greatly appreciated.



Tim



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

#58813 Feb 24 7:20 AM

Tim,

A drift an image could be caused by a number of things. Certainly polar alignment is one. Flexure and slop in the imaging train is another. Mount periodic (or non-periodic) error is yet another.��

If you want to take long exposure images at longer focal lengths, you'll want to use an autoguider. You can also try to program PEC to reduce the periodic error.

Regards,

�� �� -Paul



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

#58814 Feb 24 8:39 AM

Hi Tim,

F10 poses challenges in precision...I can't get around them without an OAG type of autoguider .for deep sky long exposures.. Autoguiding with a piggyback scope never proved 100% reliable, only off axis guiding was far better in my f10 work.. As Paul noted, any gradual sag in your image tubing or focusser could cause star streaks.. If gradual, that could be handled by the OAG, but could not be detected by a piggyback guider. .

I polar align using stars nearest I can get to the celestial equator.. These give the maximum shift per unit time spent drift aligning. With my guide camera on the imaging scope (f10 shall we say, coming automatically from the Off Axis Guider) .I use the PHD2 drift tool.. It does a nice job of showing the drift graphically, and separates out the critical DEC drift from the ignorable RA movement.. The polar scope is not good enough, of course, for deep sky long exposures.. Not changing my imaging camera also reduces the chance I will jiggle the mount and somehow affect the polar alignment. .

As Paul says, if there is PE in your drive causing RA to oscillate, you will see that at f10.. The Gemini systems have PEC built in, but you must first "train" the PEC with your Video camera and say PHD2, and then you must always tell the Gemini to turn on OEC since it defaults to off (I think Paul has got some Gemini.net feature to put it on at boot up...if you use Gemini.net).. Autoguiding will usually clean up any residual gradual polar or drive variations.. You can run one of several programs to measure the G11 PE...PecPrep is one of those. .

Stacking programs ought to still work for minor movement between sub frames, but perhaps you were saying that your stars are streaked in each sub.. Those would be unrecoverable.. If the stars are sharp, but there is sub frame movement, I'd hope the stacking software would still work (if video, try Registax...?) .

Very best of luck,

Michael





.Fri, Feb 24, 2017 at 7:49 AM yh@... [Losmandy_users] Losmandy_users@yahoogroups.com> wrote:

.Tim,

A drift an image could be caused by a number of things. Certainly polar alignment is one. Flexure and slop in the imaging train is another. Mount periodic (or non-periodic) error is yet another..

If you want to take long exposure images at longer focal lengths, you'll want to use an autoguider. You can also try to program PEC to reduce the periodic error.

Regards,

. . -Paul



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

#58815 Feb 24 10:24 AM

If it helps I can strongly recommend using Sharpcap with a guide camera on a (say) 200mm guide scope (or a focal reducer in a longer scope) to do the Polar alignment. It.s so much simpler and more accurate that I sold my polarscope!



Dave



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

#58816 Feb 24 2:46 PM

Another guiding solution is the On Axis Guider where you are guiding using the same light path as

the imaging camera. It's pricey, but the reviews are all very good.



www.innovationsforesight.com/



Don

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

From: "Michael Herman mherman346@... [Losmandy_users]" Losmandy_users@yahoogroups.com>

To: Losmandy_users@yahoogroups.com>

Sent: Friday, February 24, 2017 11:39 AM

Subject: Re: [Losmandy_users] Re: Polar Alignment Frustrations





Hi Tim,



F10 poses challenges in precision...I can't get around them without an OAG

type of autoguider for deep sky long exposures. Autoguiding with a

piggyback scope never proved 100% reliable, only off axis guiding was far

better in my f10 work. As Paul noted, any gradual sag in your image tubing

or focusser could cause star streaks. If gradual, that could be handled by

the OAG, but could not be detected by a piggyback guider.



I polar align using stars nearest I can get to the celestial equator.

These give the maximum shift per unit time spent drift aligning. With my

guide camera on the imaging scope (f10 shall we say, coming automatically

from the Off Axis Guider) I use the PHD2 drift tool. It does a nice job

of showing the drift graphically, and separates out the critical DEC drift

from the ignorable RA movement. The polar scope is not good enough, of

course, for deep sky long exposures. Not changing my imaging camera also

reduces the chance I will jiggle the mount and somehow affect the polar

alignment.



As Paul says, if there is PE in your drive causing RA to oscillate, you

will see that at f10. The Gemini systems have PEC built in, but you must

first "train" the PEC with your Video camera and say PHD2, and then you

must always tell the Gemini to turn on OEC since it defaults to off (I

think Paul has got some Gemini.net feature to put it on at boot up...if you

use Gemini.net). Autoguiding will usually clean up any residual gradual

polar or drive variations. You can run one of several programs to measure

the G11 PE...PecPrep is one of those.



Stacking programs ought to still work for minor movement between sub

frames, but perhaps you were saying that your stars are streaked in each

sub. Those would be unrecoverable. If the stars are sharp, but there is

sub frame movement, I'd hope the stacking software would still work (if

video, try Registax...?)



Very best of luck,



Michael







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

#58818 Feb 24 7:47 PM

I polar align using stars nearest I can get to the celestial equator.

> These give the

> maximum shift per unit time spent drift aligning.



Nope... sorry, but that's not correct! At least not for the azimuth drift alignment adjustment. :-) You can move the declination axis significantly toward the zenith and it will hardly affect the declination drift (if think about it I think you will understand why).

> Guider) I use the

> PHD2 drift tool. It does a nice job of showing the drift graphically,

> and separates out

> the critical DEC drift from the ignorable RA movement.



Yes, but it doesn't account for refraction, which unfortunately leads to an imprecise altitude alignment. But maybe that's ok if you don't mind a little field rotation.

> gradual polar or drive variations.

> You can run one of several programs to measure the

> G11 PE...PecPrep is one of those.



I think that my new free analysis tool, the PEMPro Log Viewer, is much easier to use and allows the user to pick the PHD2 log segment. The latest build can even approximate the periodic error from corrective autopuider moves (that is it will construct periodic error from an autoguided session using PHD2). You can download it from this link, and again it is freeware:



ccdware.infopop.cc/eve/forums/a/tpc/f/401101098/m/3217021086



-Ray Gralak

Author of APCC (Astro-Physics Command Center): www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc

Author of PEMPro: www.ccdware.com

Author of Astro-Physics V2 ASCOM Driver: www.gralak.com/apdriver

Author of PulseGuide: www.pulseguide.com

Author of Sigma: www.gralak.com/sigma



> -----Original Message-----

> From: Losmandy_users@yahoogroups.com

> [mailto:Losmandy_users@yahoogroups.com]

> Sent: Friday, February 24, 2017 8:39 AM

> To: Losmandy_users@yahoogroups.com

> Subject: Re: [Losmandy_users] Re: Polar Alignment Frustrations

>

>

>

> Hi Tim,

>

> F10 poses challenges in precision...I can't get around them without an OAG type of

> autoguider for deep sky long exposures. Autoguiding with a piggyback scope never

> proved 100% reliable, only off axis guiding was far better in my f10 work. As Paul

> noted, any gradual sag in your image tubing or focusser could cause star streaks. If

> gradual, that could be handled by the OAG, but could not be detected by a piggyback

> guider.

>

> I polar align using stars nearest I can get to the celestial equator. These give the

> maximum shift per unit time spent drift aligning. With my guide camera on the imaging

> scope (f10 shall we say, coming automatically from the Off Axis Guider) I use the

> PHD2 drift tool. It does a nice job of showing the drift graphically, and separates out

> the critical DEC drift from the ignorable RA movement. The polar scope is not good

> enough, of course, for deep sky long exposures. Not changing my imaging camera

> also reduces the chance I will jiggle the mount and somehow affect the polar

> alignment.

>

> As Paul says, if there is PE in your drive causing RA to oscillate, you will see that at

> f10. The Gemini systems have PEC built in, but you must first "train" the PEC with

> your Video camera and say PHD2, and then you must always tell the Gemini to turn

> on OEC since it defaults to off (I think Paul has got some Gemini.net feature to put it

> on at boot up...if you use Gemini.net). Autoguiding will usually clean up any residual

> gradual polar or drive variations. You can run one of several programs to measure the

> G11 PE...PecPrep is one of those.

>

> Stacking programs ought to still work for minor movement between sub frames, but

> perhaps you were saying that your stars are streaked in each sub. Those would be

> unrecoverable. If the stars are sharp, but there is sub frame movement, I'd hope the

> stacking software would still work (if video, try Registax...?)

>

> Very best of luck,

>

> Michael

>

>

>

> Fri, Feb 24, 2017 at 7:49 AM yh@... [Losmandy_users]

> Losmandy_users@yahoogroups.com> wrote:

>

>

>

>

>

> .Tim,

>

>

> .A drift an image could be caused by a number of things. Certainly polar

> alignment is one. Flexure and slop in the imaging train is another. Mount periodic (or

> non-periodic) error is yet another.

>

> .If you want to take long exposure images at longer focal lengths, you'll want to

> use an autoguider. You can also try to program PEC to reduce the periodic error.

>

> .Regards,

>

> . -Paul

>

>

>

> --

>

> Michael Herman

> mobile: 408 421-1239

> email: mherman346@...

>

>

>



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

#58823 Feb 26 5:07 PM

Last night I fired up my Orion Awesome Autoguider. ��I've had it for some time but never did more than play with it. �� ��After some fiddling I got PhD2 and the autoguider working and I was able to take long exposures with pinpoint stars. ��Being able to take multiple 2 - 3 minute subs and stack them greatly improved the quality of my images of faint Herschel galaxies. ��Thanks to all for their suggestions.

You can see a couple of my images at my Flickr album:www.flickr.com/photos/114903644@N04/albums/72157678547765712

Tim







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

#58824 Feb 26 7:17 PM

Very nice work, and very good progress, Tim.

You are on the right path...

In the future, you might find the PHD2 tool for polar alignment asist a help. I now use that often. .

.I discovered PHD2 also has a spectroscopy "slit" tool that helped me a lot to get that project underway. .

Stay well and have fun,Michael.



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