Re: Other possibility for oblong stars
>>>
When you get to very long focal lengths however OAG becomes technically challenging.?
In my experience it's not all that challenging. I'm imaging at 3454mm with a monster moag and haven't bothered with star selection (yet?).?
I had to borrow a quite old SX superstar camera, which adds technical issues, but overall it's pretty much same as in my driver?@ 840mm
I have an obs partner Martin Pugh who uses AO and definitely has to think about guidestars due to the very short exposure times of the AO unit
Tom, We'd need to see the images to see the orientation of the movement and if it's consistent for different parts of the frame and in different parts of the sky. What was the exposure time and what is the pixel size? What is the focal ratio?? You mentioned two different cameras, the ASI1600 and the ASI120. Both of these cameras have similar pixel sizes (3.8 micron vs 3.75) As others have said, when you go from a 400mm to 180mm f/l? with this size sensor it does not provide sufficient resolution. The image scale is around 4.5 arc-seconds per pixel. these cameras are better matched to the 1000mm scope. Even at 400 mm your image scale is over 2 arc-seconds per pixel which is still slightly undersampled in all but poor seeing conditions . You are better off with the asi178 at 2.8 microns or the ASI294mm at 2.3 microns. On the other hand the ASI120 should be ok with an OAG at 1000mm.? Image scale is about .78 and you are slightly oversampled.
It could also be the 180 guider's optical axis wasn't precisely aligned with the main telescope/imaging system. Orthogonal alignment errors can look like polar alignment or cone error. The guide software, by correcting the misaligned guide camera imparts an error on the main imaging system. The differential gets multiplied by the ratio of the effective focal lengths of the two optical systems and by ratio of the pixel sizes (imaging precision).
The impact to your images (oblong or 'stretched' stars) like other tracking errors is still dependent on the effective focal length of the imaging optical system it's precision, where in the sky you are pointing and exposure time. These are the same factors that we normally consider when imaging, so if you see this problem when you're collecting data and don't have an option to reconfigure you should shorten exposure time and take more subs.
1000 mm F/L in combination with today's popular imaging cameras is typical for when these issues start being noticed.
There's a general recommendation in the astrophotography community these days to use an off axis guiding system rather than a guide scope when imaging using long focal lengths over about 1000 mm. This eliminates the focal length ratio multiplier and solves most of the alignment precision challenges with guide scopes that need to have fairly long focal lengths themselves and common mounting rail systems. Ironically there's an argument for using OAG on shorter focal length compact refractors too because of the mechanics of alignment and reduction of equipment load. When you get to very long focal lengths however OAG becomes technically challenging. The longer the focal length the smaller the field of view which can make selecting guide stars difficult. Pixel sizes need to be larger to match the focal length. The new ZWO monochrome planetary cameras using the new Sony sensors have very low noise and are extremely sensitive. Very small pixels can be binned by the low level driver to optimize pixel size.? Precision mounting systems and rails, higher mass, increased stiffness and eliminating dissimilar metals with different thermal coefficients? ?and careful installation followed by testing to confirm orthagonal alignment in is time consuming??
-- Brian?
Brian Valente portfolio
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Dear Paul K., as the starter of this topic, I say thank you, thank you, for your most practicable list you have started, as a long term astrophile and owner of a TMBss130 and the new owner of a Stellarvue SVX 140T ( Wednesday). I am in good shape for most astronomical terms, but Losmandy mount terms and abbrevations, my data base is currently very limited, however, what Paul has started is very helpful, and it may be a while before I can add a term or two, and I will make a effort to make some additions. And I am grateful for Paul's effort to fill a need that will help many a member. Respectfully submitted.
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Re: Other possibility for oblong stars
Michael,
my point was that my imaging scope is 1000mm f3.9 using an asi1600? (3.8 microns/pixel). I used an ST80 400mm refractor as my guide scope using an old SBIG ST4 tracker with rectangular approx 13 x 15 microns/pixel. This resulted in poor tracking as you would expect due to the large imager-to-guider pixel ratio. I then switched to the asi120 (3.75 microns/pixel)? for the guider resulting in round stars.
I also tried switching to a lighter guide scope (50mm 180mm f/l) with the asi120 and found the f/l ratio between the 1000mm and the 180mm to be too much for good guiding.
Tom
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Re: Other possibility for oblong stars
Why would you care about ¡°orthogonal mounting¡±. You need to ensure no differential flexure between main scope and guide scope over the course of one exposure. You do not need to ensure they are pointing at the same RA/DEC
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Edward
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Michael wrote (elsewhere)..
I think it is best to use a high friction clutch pad and
keep the axial forces below extreme levels, yet still grip the clutch
firmly.??
***
?
I have the older two piece
worm blocks, all of Michael¡¯s mods, and everything is adjusted nicely.. I
am sure it could be better at any given time, as it is generally ¡°loose¡±,
but yields 0.7¡± rms total.. Pointing and modeling ¡°sucked¡±..
Previous time out, things were bad.. But I ¡°live¡± for Saturn
Opposition and had several hours to ¡°make it work¡±.. Note I have
never completed a model..
?
I ¨C Mr Warm Restart ¨C
did a cold boot with a quite light clutch pressure on Michael¡¯s high
friction clutch discs.. Went to the first alignment star with a low powered
wide field eyepiece and it was in the FOV.. Swapped eyepieces and recentered
until it was centered at high power.. I ¡°added to model¡±.. I did
NOT synch on the first star, I aligned..
?
Then I neglected to switch
back to a low power eyepiece, yet I actually completed a full model at 250 power..
4 stars in both the East and the West, with each model having a star across the
meridian.. I have said many times before that stiction is a problem, and I of
course have no idea why this worked, but a high friction clutch disc and low
clutch pressure just worked perfectly for an idiot like me.. It might work for
you too..
?
Derek
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Re: 76sec error on new OPW-11
On Mon, Aug 9, 2021 at 07:37 AM, Michael Ben-Yehuda wrote:
The only way stored PEC works is if the drive component positions remain known to the software. Without indexing, this can only happen if nothing is moved and the system is never cold started which resets the CWD position.?
No, clutches can be loosened and the mount moved manually without losing PEC index. Cold starting is also not going to affect it or changing CWD position. The important thing is not to rotate the worm manually, without using Gemini motors. As long as the worm remains in the same position PEC playback will be sync'ed correctly to the worm. Regards, ? ? ?-Paul?
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Re: 76sec error on new OPW-11
Typically when using PEMPro, we use a hour or less of data to create a PEC curve which is then uploaded to the Gemini unit. Therefore we largely measure the behavior of the worm, and possibly some of the profile found on that section of ring gear. It would be nice to have data over the entire circumference of the ring gear, but that would take 24 hours of data, and some method of rotating through various sections of the ring. But how to maintain using the preferred best mating parts and only those parts is something I'm not sure the average user could do in practice. Since you can't see inside during operations, this may be difficult. But regardless of the random ring position which may be at CWD at a Cold Start, the PEC curve plays back with the same worm oscillations being repeated, and PE should be mostly negated for better guiding. Without PEMPro I can readily see the sawtooth pattern in PHD2 during guiding. I think we would all like to see better guiding performance without using a correction curve if possible
John,
I understand this. The problem is that Gemini has no way of knowing the physical position of the worm. My understanding is it either assumes the drive train is in the same position that is was or that it's where it was at CWD,? To have these positions be consistent the RA clutch must not be released or slipped. because it changes the drive train's orientation when the axis is at CWD..?
The only way stored PEC works is if the drive component positions remain known to the software. Without indexing, this can only happen if nothing is moved and the system is never cold started which resets the CWD position.?
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Re: Other possibility for oblong stars
Tom, We'd need to see the images to see the orientation of the movement and if it's consistent for different parts of the frame and in different parts of the sky. What was the exposure time and what is the pixel size? What is the focal ratio?? You mentioned two different cameras, the ASI1600 and the ASI120. Both of these cameras have similar pixel sizes (3.8 micron vs 3.75) As others have said, when you go from a 400mm to 180mm f/l? with this size sensor it does not provide sufficient resolution. The image scale is around 4.5 arc-seconds per pixel. these cameras are better matched to the 1000mm scope. Even at 400 mm your image scale is over 2 arc-seconds per pixel which is still slightly undersampled in all but poor seeing conditions . You are better off with the asi178 at 2.8 microns or the ASI294mm at 2.3 microns. On the other hand the ASI120 should be ok with an OAG at 1000mm.? Image scale is about .78 and you are slightly oversampled.
It could also be the 180 guider's optical axis wasn't precisely aligned with the main telescope/imaging system. Orthogonal alignment errors can look like polar alignment or cone error. The guide software, by correcting the misaligned guide camera imparts an error on the main imaging system. The differential gets multiplied by the ratio of the effective focal lengths of the two optical systems and by ratio of the pixel sizes (imaging precision).
The impact to your images (oblong or 'stretched' stars) like other tracking errors is still dependent on the effective focal length of the imaging optical system it's precision, where in the sky you are pointing and exposure time. These are the same factors that we normally consider when imaging, so if you see this problem when you're collecting data and don't have an option to reconfigure you should shorten exposure time and take more subs.
1000 mm F/L in combination with today's popular imaging cameras is typical for when these issues start being noticed.
There's a general recommendation in the astrophotography community these days to use an off axis guiding system rather than a guide scope when imaging using long focal lengths over about 1000 mm. This eliminates the focal length ratio multiplier and solves most of the alignment precision challenges with guide scopes that need to have fairly long focal lengths themselves and common mounting rail systems. Ironically there's an argument for using OAG on shorter focal length compact refractors too because of the mechanics of alignment and reduction of equipment load. When you get to very long focal lengths however OAG becomes technically challenging. The longer the focal length the smaller the field of view which can make selecting guide stars difficult. Pixel sizes need to be larger to match the focal length. The new ZWO monochrome planetary cameras using the new Sony sensors have very low noise and are extremely sensitive. Very small pixels can be binned by the low level driver to optimize pixel size.? Precision mounting systems and rails, higher mass, increased stiffness and eliminating dissimilar metals with different thermal coefficients? ?and careful installation followed by testing to confirm orthagonal alignment in is time consuming??
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Re: Other possibility for oblong stars
Ah, but one of the best ways to have good guiding is to have good seeing :). Please see PHD2 screenshot from tonight below. This is as about as good as it gets for me with the G11G2. I have my Esprit 100ED side saddled with an Orion CT-80 cheapo spotting scope with 400mm FL. Using the ZWO ASI290Mini guide cam with 2.9 micron pixels gives me 1.5 arcsec/pixel, which is almost the same as for the main scope and camera (ASI2600MM-P). The Esprit doesn't have enough backfocus for both a filter wheel and OAG. But despite some high thin clouds tonight (with new moon!), guiding numbers are good and my stars are super round.?
It is nice to know that under good air the mount will perform. With these results, don't think I will be doing any rebuilds or tune-ups for a while:).
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Re: Gemini-2 momentarily went haywire
Michael. ?Try use Gemini ASCOM to see if the fault is the same as with HC.?
--
Brendan
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Re: Other possibility for oblong stars
PHD2 can only tell you how well it is really doing if the guide scope and guide camera have reasonably good resolution. Then the resolution is so low that PHD2 cannot see what is really happening it is a user error.?
--
Chip Louie Chief Daydreamer Imagination Hardware?
? ?Astropheric Weather Forecast - South Pasadena, CA?
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Re: Other possibility for oblong stars
The reason pixel comparison comes into play here is that the guider is extremely undersampled. 13x15 microns pixel size is 20 microns on a diagonal. With a 480mm focal length, the image scale for the guider is 8.6"/pixel.? The computed RMS error with such an image scale is extremely inaccurate, and can be off by many arcseconds from the actual guiding error -- there's no 1/10 of a pixel precision here.
Assuming normal seeing conditions, of say 2-3" FWHM, the guide star will have all of its flux captured inside a?single?guider pixel. When this happens, no software, including PHD, can calculate the center of the star with anything approaching a 1/10 of a pixel precision. A pixel can't be further sub-divided, and there's simply no information coming from the neighboring pixels that is normally used in the centroid calculation. This lowers the RMS error value, since the star can move a good number of arcseconds before PHD will even recognize that it has moved.
Regards,
? ? ? -Paul
toggle quoted message
Show quoted text
On Sun, Aug 8, 2021 at 09:29 PM, Brian Valente wrote:
Paul yes i think we're on the same page
?
The only thing i'd add here is just to skip the whole pixel comparison and just look at the RMS in arcseconds. That will tell me exactly how much error there is and I can translate to the imaging camera
On Sun, Aug 8, 2021 at 3:50 PM Paul Kanevsky < yh@...> wrote:
That¡¯s what I meant, Brian. 1/10 of a guider pixel is possible with a bright, well focused star, but is lower with lower SNR. When pixel size and image scale are such that 1/10 of a guider pixel is still a large error on the main imager, PHD error number will indicate good guiding while the main imager nay have elongated stars.
On Sun, Aug 8, 2021 at 06:34 PM, Brian Valente wrote:
>>>PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
I am not sure exactly what you mean by this, but PHD has sub-pixel accuracy. I've seen it guide down to 0.10 pixel and even lower
?
I think the more relevant spec is the image scale of the guider setup vs. the imaging. The general rule of thumb is 4:1 max image scale, where the guide camera image scale should not be more than 4x coarser than the imaging camera's image scale.
?
?
On Sun, Aug 8, 2021 at 3:28 PM Paul Kanevsky < yh@...> wrote:
Not odd at all. Guider has to be able to detect guide star deflection from center. If the resolution/image scale of the guider is low, it will detect and correct errors too late, after they have become?already?sufficiently large to be visible on the main imager. PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
On Sun, Aug 8, 2021 at 06:01 PM, wa1vta01452 wrote:
Oddly, PHD2 showed nominal numbers but the images would still be smeared. I first started guiding with my ASI1600mm using my old ST4 autoguider in the ST80 400mm guider and had similar streaky results on stars till I realized I was guiding 3.8 micron pixel size with approx. 13 x 15 micron ST4 pixels. That's when I purchased an ASI120mm for guiding and all was fine.
Tom
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Brian?
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Brian Valente
portfolio
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--
Brian?
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Brian Valente
portfolio
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Re: Gemini-2 momentarily went haywire
Yes. The solution for me was a new G2 PC board right from Losmandy. I just swapped out and put in the new battery. After making an Ethernet connection, I updated the standard firmware with the Beta Ware from August 2018. Then I simply transferred all my personal settings from the Gemini app. Easy!
Since then, about 6-8 weeks now, I have had no high rev runway Dec slews. All running normally. Michael was sent the old board as he has said.?
Many thanks to Michael, Brendan, Brian, and all who helped with the troubleshooting and recommendations.
Best Regards,
John
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Re: Other possibility for oblong stars
Paul yes i think we're on the same page
The only thing i'd add here is just to skip the whole pixel comparison and just look at the RMS in arcseconds. That will tell me exactly how much error there is and I can translate to the imaging camera
On Sun, Aug 8, 2021 at 3:50 PM Paul Kanevsky < yh@...> wrote: That¡¯s what I meant, Brian. 1/10 of a guider pixel is possible with a bright, well focused star, but is lower with lower SNR. When pixel size and image scale are such that 1/10 of a guider pixel is still a large error on the main imager, PHD error number will indicate good guiding while the main imager nay have elongated stars.
On Sun, Aug 8, 2021 at 06:34 PM, Brian Valente wrote:
>>>PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
I am not sure exactly what you mean by this, but PHD has sub-pixel accuracy. I've seen it guide down to 0.10 pixel and even lower
?
I think the more relevant spec is the image scale of the guider setup vs. the imaging. The general rule of thumb is 4:1 max image scale, where the guide camera image scale should not be more than 4x coarser than the imaging camera's image scale.
?
?
On Sun, Aug 8, 2021 at 3:28 PM Paul Kanevsky < yh@...> wrote:
Not odd at all. Guider has to be able to detect guide star deflection from center. If the resolution/image scale of the guider is low, it will detect and correct errors too late, after they have become?already?sufficiently large to be visible on the main imager. PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
On Sun, Aug 8, 2021 at 06:01 PM, wa1vta01452 wrote:
Oddly, PHD2 showed nominal numbers but the images would still be smeared. I first started guiding with my ASI1600mm using my old ST4 autoguider in the ST80 400mm guider and had similar streaky results on stars till I realized I was guiding 3.8 micron pixel size with approx. 13 x 15 micron ST4 pixels. That's when I purchased an ASI120mm for guiding and all was fine.
Tom
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--
Brian?
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Brian Valente
portfolio
-- Brian?
Brian Valente portfolio
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Re: Gemini-2 momentarily went haywire
Thank you all...very good pointers.? I'll test each step and see what is likely to be wrong.??
I think it can't be the H bridge...it's driving the motors fine (unless it's flaw is it won't shut off!).? I think it must be the optical encoder detector chip....maybe.? Too many maybes!
Anyways it's a spare board. My own G2 is still working fine.??
Stay well!!!!
Michael
Or H bridge.?
You can compare CRO signals at Ra+ on the PIC and H bridge against moving sidereal west at same points but Ra- pins. ?
As others have said. It¡¯ll be the DSPIC or H bridge.?
--
Brendan
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Re: Gemini-2 momentarily went haywire
Or H bridge.?
You can compare CRO signals at Ra+ on the PIC and H bridge against moving sidereal west at same points but Ra- pins. ?
As others have said. It¡¯ll be the DSPIC or H bridge.?
--
Brendan
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Re: Things noticed with my G11G regarding recent topics including shaft wear, bearings, worms and miscellaneous
Peter,
A few things come to mind.
-Try to keep decreasing your exposure time and see when you reach a interval where the stars are not trailing. This info could lead to determining the period of the oscillation and where it is coming from. If it is still there at a few seconds, that sounds like clutch slippage.
-Since you changed out the thrust bearings we should assume they are in perfect condition. Now consider the quantity and type of grease used. Perhaps you are not smoothly turning on the bearings but are somehow sliding and chattering. Perhaps a new light coating of the OEM recommended Artic Lube would be best.
-These thrust bearings ride on a flat washer if memory serves correct. If you are using the same worn side as before, you could be riding on the grooves or high spots where the previous bearings cut a track. If you flip the flat washer you should now be on fresh metal, and you can break in properly with the new part.
All for now. Good luck.
John
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Re: Other possibility for oblong stars
That¡¯s what I meant, Brian. 1/10 of a guider pixel is possible with a bright, well focused star, but is lower with lower SNR. When pixel size and image scale are such that 1/10 of a guider pixel is still a large error on the main imager, PHD error number will indicate good guiding while the main imager may have elongated stars.
On Sun, Aug 8, 2021 at 06:34 PM, Brian Valente wrote:
>>>PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
I am not sure exactly what you mean by this, but PHD has sub-pixel accuracy. I've seen it guide down to 0.10 pixel and even lower
?
I think the more relevant spec is the image scale of the guider setup vs. the imaging. The general rule of thumb is 4:1 max image scale, where the guide camera image scale should not be more than 4x coarser than the imaging camera's image scale.
?
?
On Sun, Aug 8, 2021 at 3:28 PM Paul Kanevsky < yh@...> wrote:
Not odd at all. Guider has to be able to detect guide star deflection from center. If the resolution/image scale of the guider is low, it will detect and correct errors too late, after they have become?already?sufficiently large to be visible on the main imager. PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
On Sun, Aug 8, 2021 at 06:01 PM, wa1vta01452 wrote:
Oddly, PHD2 showed nominal numbers but the images would still be smeared. I first started guiding with my ASI1600mm using my old ST4 autoguider in the ST80 400mm guider and had similar streaky results on stars till I realized I was guiding 3.8 micron pixel size with approx. 13 x 15 micron ST4 pixels. That's when I purchased an ASI120mm for guiding and all was fine.
Tom
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?
?
--
Brian?
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Brian Valente
portfolio
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Re: Things noticed with my G11G regarding recent topics including shaft wear, bearings, worms and miscellaneous
>>>
Good guidescope guiding but drifting stars on the main imager are the hallmark of differential flexure. Definitely try an OAG, as that is much less likely to suffer from flexure problems.
I agree, classic symptoms
On Sun, Aug 8, 2021 at 3:40 PM Paul Kanevsky < yh@...> wrote: Good guidescope guiding but drifting stars on the main imager are the hallmark of differential flexure. Definitely try an OAG, as that is much less likely to suffer from flexure problems.
On Sun, Aug 8, 2021 at 05:55 PM, <pcboreland@...> wrote:
Les and Paul,
That's what is strange about this. My guiding was better on the bad image. Is was only just about dark on the good image. Total rms was 0.71". On the second bad image it was about 1 hour later total rms was 0.56". So better guiding as target was higher in the sky and properly dark.
I showed the first bad image in total nights imaging that were all trash.
Boy this is a real hair puller!
Peter
-- Brian?
Brian Valente portfolio
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Re: Things noticed with my G11G regarding recent topics including shaft wear, bearings, worms and miscellaneous
Good guidescope guiding but drifting stars on the main imager are the hallmark of differential flexure. Definitely try an OAG, as that is much less likely to suffer from flexure problems.
On Sun, Aug 8, 2021 at 05:55 PM, <pcboreland@...> wrote:
Les and Paul,
That's what is strange about this. My guiding was better on the bad image. Is was only just about dark on the good image. Total rms was 0.71". On the second bad image it was about 1 hour later total rms was 0.56". So better guiding as target was higher in the sky and properly dark.
I showed the first bad image in total nights imaging that were all trash.
Boy this is a real hair puller!
Peter
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Edward Plumer
Paul Kanevsky
Chip Louie
Michael Herman