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Major OAG issues. Help please.

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#1 dcm_guitar

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Posted 07 March 2021 - 04:47 PM

I have an OAG that I'm trying to incorporate into my rig, and I am continually failing. 

 

I have the ZWO M68 OAG.  It has a built-in focus adjustment, so getting the focus for the guide camera and the focus for the imaging scope to align SHOULD be easy.  But it isn't (for me).  I have verified that the prism is facing the correct direction.  The flat side of the prism is facing towards the telescope.  The prism is in a perfect position for getting light and not interfering with the camera sensor.

 

  • All of the stars in the guide camera image are spikes.  Nothing is round.  I've played with the focus of both the OAG as well as the main telescope focus, but I can't seem to get anything from the guide camera to show round stars.  They're all vertical lines.
  • Because the stars are not round, guiding simply does not work.

I'm at a complete loss as to what to do.

 

Here's the image train:

  1. WO GT81IV
  2. WO Flat 6AIII flattener which is also a 0.8 reducer (currently set to 7.5mm to get to the correct 62mm of back space)
  3. ZWO M68 OAG
  4. ZWO EFW
  5. ZWO 6200MM Pro

I have downloaded all of the manuals from ZWO.  I have validated that my connections are correct and correspond exactly to the images in the manual.  But no matter what I do I can not get the OAG to produce a round star will work with guiding.

 

Can anyone with more knowledge than I please provide a suggestion?  I'm at a loss.

 

THANKS!!!!!

 



#2 happylimpet

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Posted 07 March 2021 - 05:06 PM

if you can, move the prism down, closer to the optical axis. Better star shapes are found there. As close as you can without shadowing the sensor.

 

I had some success by tilting my prism slightly. Might be worth trying. Very small angle obviously.

 

What do the guidestars look like a little out of focus? do they switch to a line going at right angles, maybe passing through a 'blob' stage?


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#3 dcm_guitar

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Posted 07 March 2021 - 05:12 PM

A little out of focus they look like fuzzy lines.  As the focus improves, they become less fuzzy.  No right angles; just spikes instead of stars.

 

I'll see what I can do with the prism position.



#4 PirateMike

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Posted 07 March 2021 - 06:30 PM

I just have to ask...

 

Is there a finger print, scratch or some other stuff on the prism?

 

Probably not the issue, but one never knows. flowerred.gif

 

 

 

Miguel   8-)

 

.


Edited by PirateMike, 07 March 2021 - 06:37 PM.

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#5 happylimpet

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Posted 07 March 2021 - 07:16 PM

A little out of focus they look like fuzzy lines.  As the focus improves, they become less fuzzy.  No right angles; just spikes instead of stars.

 

I'll see what I can do with the prism position.

I think either your prism is somehow introducing distortion (very unlikely) or this represents the shape of stars this far off axis, and you either need a better flattener (but that would be daft, just for guidestar images) or just move the prism much further in. If its situated square with the long edge of the sensor that gets you the closest without shadowing.



#6 Peter in Reno

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Posted 07 March 2021 - 07:23 PM

Did you save an image taken with guide camera and OAG?

 

I'm assuming you used PHD2 for guiding, PHD2 generally does not care for shapes of guide stars, it uses centroid calculation.

 

Happylimpet gave very good explanation of what might be going on with your setup.

 

Peter 


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#7 dcm_guitar

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Posted 08 March 2021 - 01:30 AM

I worked on this issue a lot tonight, and I'm 100% certain I know what the issue is.  I simply can not get round stars at the edge of my optics.  I have a Williams Optics 6AIII flattener, but I can not get it to actually give me flat stars at the edge.

 

The appropriate back space for my scope and this flattener is 62.1mm  This means I have to adjust the flattener to 7.1mm of backspacing to go with the 55mm of backspace already available from the camera (12.5mm), EFW (20mm), tilt plate (5mm) and OAG (17.5mm).  Tonight I adjusted the 6AIII in 1/2mm increments from 6mm to 10mm and looked at images from the OAG as well as images from the main imaging camera at 1:1.  I was able to get improvements in the star shape, but never got them round on the edge.  Since the OAG only sees stars at the edge of the image, it only sees these non-round stars.

 

In fairness, my backspacing was off by a couple of millimeters the first time I tried to use the OAG and this may be why I couldn't get successful guiding.

 

I honestly did not trying guiding tonight with the best star shapes I could muster.  I probably should have, but I was so darn tired of taking off and putting on my camera as I adjusted the back spacing that when I realized I would never get fully round stars on the edge of my image, I just shut everything down and went back inside.

 

I'll try to guide with the backspacing at 7.1mm (or as close as I can get) the next time I go out to see if the shapes I was able to achieve are good enough.  Tomorrow night may have too many clouds.  We'll see.

 

I'm not sure whether to even keep the flattener on the scope since it doesn't actually give me round stars at the edge.  I have to crop out these stars whether the flattener is on the scope or not.

 

Thoughts?



#8 happylimpet

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Posted 08 March 2021 - 06:15 AM

I worked on this issue a lot tonight, and I'm 100% certain I know what the issue is.  I simply can not get round stars at the edge of my optics.  I have a Williams Optics 6AIII flattener, but I can not get it to actually give me flat stars at the edge.

 

The appropriate back space for my scope and this flattener is 62.1mm  This means I have to adjust the flattener to 7.1mm of backspacing to go with the 55mm of backspace already available from the camera (12.5mm), EFW (20mm), tilt plate (5mm) and OAG (17.5mm).  Tonight I adjusted the 6AIII in 1/2mm increments from 6mm to 10mm and looked at images from the OAG as well as images from the main imaging camera at 1:1.  I was able to get improvements in the star shape, but never got them round on the edge.  Since the OAG only sees stars at the edge of the image, it only sees these non-round stars.

 

In fairness, my backspacing was off by a couple of millimeters the first time I tried to use the OAG and this may be why I couldn't get successful guiding.

 

I honestly did not trying guiding tonight with the best star shapes I could muster.  I probably should have, but I was so darn tired of taking off and putting on my camera as I adjusted the back spacing that when I realized I would never get fully round stars on the edge of my image, I just shut everything down and went back inside.

 

I'll try to guide with the backspacing at 7.1mm (or as close as I can get) the next time I go out to see if the shapes I was able to achieve are good enough.  Tomorrow night may have too many clouds.  We'll see.

 

I'm not sure whether to even keep the flattener on the scope since it doesn't actually give me round stars at the edge.  I have to crop out these stars whether the flattener is on the scope or not.

 

Thoughts?

WHats the advertised image circle? If your OAG prism is WAY outside that you shouldnt expect good stars. Having said that, my OAG is outside my coma corrcetors corrected field but the stars are still pretty darn good, certainly good enough. Can you move the prism closer to the optical axis as I suggested above? ie push the stalk inwards until it starts to (or just before) shadow the sensor? Thats the best thing you can do for sure.


Edited by happylimpet, 08 March 2021 - 06:16 AM.

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#9 michael8554

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Posted 08 March 2021 - 03:25 PM

My OAG guidestars are crescent shaped due to coma, but I can guide sub 1arcsec at 1280mm Focal Length with the PHD2 Multistar algorithm.



#10 Der_Pit

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Posted 09 March 2021 - 09:21 AM



I'm not sure whether to even keep the flattener on the scope since it doesn't actually give me round stars at the edge.  I have to crop out these stars whether the flattener is on the scope or not.

 

Thoughts?

Not sure if I understand you correct - you say you don't even have proper star shapes in the main camera?  That would mean your BF regarding the corrector is way off.   So I'd first try to get that dialed in, to make sure that the corrector is at least close to proper settings - else you cannot expect it to give good star images.

As was already entioned, in default settings the prism is 19mm off the optical axis.  That's similar to the corners of your 6200.  

 

One thing you might check:  take out the guide camera, and look (at daytime) in while pointing at something light (while wall etc.).  Can you see the full aperture?  (Get as close as possible to the hole in the prism stalk).

 

A note: With the M68 OAG from ZWO you have to be careful when changing the prism position:  That does not move the guide camera, i.e., changing the prism positon also changes focus, and probably a lot...


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#11 John Tucker

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Posted 09 March 2021 - 09:40 AM

I had this same problem with a Celestron six inch SCT.  Tried two different OAGs, futzed with the spacing endlessly, and the best I ever go in the FOV of the guidescope was distorted, diffuse stars.  It would guide OK (ca. 1 arc sec) once I got it locked onto a star that it wouldn't lose every 30 seconds, but this generally took about 5 minutes of trying different stars and futzing with the gain and exposure duration.  Of course guiding never picked up again after the automated meridian flip at 2 am.

 

I switched over to a guidescope, which works fine at ~1000mm (with the focal reducer).  You're shooting at around 400mm if I've understood correctly.  Why struggle with this when you can buy an Orion Mini Guidescope for $80 and you'll never be able to tell the difference?  With my 400mm Newt I can almost get by without guiding at all.


Edited by John Tucker, 09 March 2021 - 09:46 AM.

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#12 happylimpet

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Posted 09 March 2021 - 09:57 AM

I had this same problem with a Celestron six inch SCT.  Tried two different OAGs, futzed with the spacing endlessly, and the best I ever go in the FOV of the guidescope was distorted, diffuse stars.  It would guide OK (ca. 1 arc sec) once I got it locked onto a star that it wouldn't lose every 30 seconds, but this generally took about 5 minutes of trying different stars and futzing with the gain and exposure duration.  Of course guiding never picked up again after the automated meridian flip at 2 am.

 

I switched over to a guidescope, which works fine at ~1000mm (with the focal reducer).  You're shooting at around 400mm if I've understood correctly.  Why struggle with this when you can buy an Orion Mini Guidescope for $80 and you'll never be able to tell the difference?  With my 400mm Newt I can almost get by without guiding at all.

All fair, but in my case, I bought an OAG some years ago, got it set up and working the same night, and its worked perfectly ever since. I've had slight issues with the star shapes (due to prism tilting on my cheap OAG) but thats about it. I know my experiece has been a  walk in the park but i hope it suggests this doesnt have to defeat anyone.


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#13 dcm_guitar

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Posted 09 March 2021 - 11:28 AM

It's been cloudy the last two nights, so no more new experiments.

 

THANKS for all the suggestions.  To be clear, all of my distorted star shapes are on the edge of my image.  The stars in the central part of the image are lovely and round.  

 

I'm using a full frame sensor and the image circle for my scope is advertised as "greater than full frame".  Realistically, I do get some vignetting that get taken care of with flats.  The vignetting is from shadows rather than an actual blocking of the image circle.

 

Whether I use the flattener or not, I have to crop my images to get rid of the distorted stars on the edge.  I bought the flattener to get clean stars all the way to the edge of my frame, but the stars on the edge of my frame and the stars in my OAG view are distorted even with the flattener.

 

The OAG I have has the helical focuser.  The prism is already right at the edge of my image sensor and any further movement would start to block the image sensor.  I don't think moving the prism is a good idea.  I like its placement relative to the image sensor, I just want to get more round stars.

 

The stars in my OAG view MAY BE good enough for guiding.  I haven't tried guiding since I did my work on getting the stars in the OAG field of view as round as I could.  When the clouds clear, I'll give it another try, but the weather says that's going to be Thursday at the earliest.

 

I don't NEED the OAG right now given my current imaging set up.  With the flattener, my scope focal length is only 382 and my image scale with the full frame camera is 2.02" per pixel.  Generally, I guide with a 50mm guide scope.

 

When I bought my full frame camera I got it as part of a package that included the camera, filter wheel, LRGB filters and the OAG.  Because finding gear is so hard right now, the ONLY way I could get a camera and filters was to buy the whole bundled package.  So, I got an OAG by default.

 

I want to get a longer focal length scope for galaxy imaging.  The scope I'm looking at would move my image scale to 0.7" per pixel.  In order to make this work, I need to improve my guiding.  I've been working on getting my guiding to <=0.7"RMS and I just can't seem to get there.  I've plateaued at ~0.8" RMS.  Through my reading I'm curious if I'm getting a bit of some flexure that's causing some guiding error.  I decided to try the OAG I already had "laying around" to see if it would improve my overall guiding RMS.  And here we are......


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#14 Der_Pit

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Posted 09 March 2021 - 11:57 AM

Flexure will probably ruin long exposure images, but it will not affect guide RMS.  Also, switching from guide scope to OAG will not give you better guiding RMS per se.  It rather helps you against mirror flop effects (and flexure).  I.e., you consider such a switch if your guide RMS is good, but still the images you get are bad.

So rather dig into why the guiding isn't great.  That can be the mount, the seeing, or the settings you use.



#15 John Tucker

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Posted 09 March 2021 - 11:59 AM

All fair, but in my case, I bought an OAG some years ago, got it set up and working the same night, and its worked perfectly ever since. I've had slight issues with the star shapes (due to prism tilting on my cheap OAG) but thats about it. I know my experiece has been a  walk in the park but i hope it suggests this doesnt have to defeat anyone.

I think its good if we all share our individual experience. I suspect that the difference between doing X with Telescope A, guiding setup B, and mount C and doing the same thing with Telescope A', guiding setup B', and mount C' is often larger than we suspect.  And maybe this is even true with different examples of the same equipment.  


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#16 dcm_guitar

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Posted 09 March 2021 - 12:15 PM

Flexure will probably ruin long exposure images, but it will not affect guide RMS.  Also, switching from guide scope to OAG will not give you better guiding RMS per se.  It rather helps you against mirror flop effects (and flexure).  I.e., you consider such a switch if your guide RMS is good, but still the images you get are bad.

So rather dig into why the guiding isn't great.  That can be the mount, the seeing, or the settings you use.

Makes sense.  I'm new to the hobby.  I read several reports from others stating their guiding improved substantially when switching to an OAG.  Since I had one I wasn't using I thought, "let's give it a shot".

 

I run GA prior to all of my imaging sessions, so I have a sense of what my seeing conditions are like.  Usually, my seeing conditions are in the .2 to .3 to RMS high frequency star motion.  I always implement the guiding assistant recommendations.  I just can not get guiding RMS <=.7RMS which makes me wary of increasing my focal length.

 

What I have noticed during my guiding work is that my mount has a lot of RA drift.  DEC drift is low, but my RA drift seems very high.  My RA drift rate as reported by GA is ~15 to 18 pixels per minute.  I do work on my balancing.  I've tried both "perfect" static balance and a slight east bias balance.  Both balance options result in the same RA drift reported by GA.

 

My RA error tends to run about 2x my DEC error.

 

I run the PPEC guiding algorithm for RA.  I'm running multi-star guiding.  Even with this, I generally get really nice guiding and then "wrestle mania" hits and I get HUGE errors, and then it settles down to really nice guiding, and then more "wrestle mania".  Rinse and repeat.  The huge error excursions do not happen with regular periodicity.

 

It's all so confusing.


Edited by dcm_guitar, 09 March 2021 - 12:16 PM.


#17 happylimpet

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Posted 09 March 2021 - 06:29 PM

THANKS for all the suggestions.  To be clear, all of my distorted star shapes are on the edge of my image.  The stars in the central part of the image are lovely and round.  

 

I'm using a full frame sensor and the image circle for my scope is advertised as "greater than full frame".  Realistically, I do get some vignetting that get taken care of with flats.  The vignetting is from shadows rather than an actual blocking of the image circle.

 

Whether I use the flattener or not, I have to crop my images to get rid of the distorted stars on the edge.  I bought the flattener to get clean stars all the way to the edge of my frame, but the stars on the edge of my frame and the stars in my OAG view are distorted even with the flattener.

 

Sounds like theres something amiss with the OTA/etc then, and the poor OAG stars are a symptom. Sounds like the flattener could do with adjusting the spacing....



#18 petercoxphoto

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Posted 10 March 2021 - 08:16 AM

It's been cloudy the last two nights, so no more new experiments.

 

THANKS for all the suggestions.  To be clear, all of my distorted star shapes are on the edge of my image.  The stars in the central part of the image are lovely and round.  

 

I'm using a full frame sensor and the image circle for my scope is advertised as "greater than full frame".  Realistically, I do get some vignetting that get taken care of with flats.  The vignetting is from shadows rather than an actual blocking of the image circle.

 

Whether I use the flattener or not, I have to crop my images to get rid of the distorted stars on the edge.  I bought the flattener to get clean stars all the way to the edge of my frame, but the stars on the edge of my frame and the stars in my OAG view are distorted even with the flattener.

 

The OAG I have has the helical focuser.  The prism is already right at the edge of my image sensor and any further movement would start to block the image sensor.  I don't think moving the prism is a good idea.  I like its placement relative to the image sensor, I just want to get more round stars.

 

The stars in my OAG view MAY BE good enough for guiding.  I haven't tried guiding since I did my work on getting the stars in the OAG field of view as round as I could.  When the clouds clear, I'll give it another try, but the weather says that's going to be Thursday at the earliest.

 

I don't NEED the OAG right now given my current imaging set up.  With the flattener, my scope focal length is only 382 and my image scale with the full frame camera is 2.02" per pixel.  Generally, I guide with a 50mm guide scope.

 

When I bought my full frame camera I got it as part of a package that included the camera, filter wheel, LRGB filters and the OAG.  Because finding gear is so hard right now, the ONLY way I could get a camera and filters was to buy the whole bundled package.  So, I got an OAG by default.

 

I want to get a longer focal length scope for galaxy imaging.  The scope I'm looking at would move my image scale to 0.7" per pixel.  In order to make this work, I need to improve my guiding.  I've been working on getting my guiding to <=0.7"RMS and I just can't seem to get there.  I've plateaued at ~0.8" RMS.  Through my reading I'm curious if I'm getting a bit of some flexure that's causing some guiding error.  I decided to try the OAG I already had "laying around" to see if it would improve my overall guiding RMS.  And here we are......

If you're getting distorted stars at the corners of your frame, your spacing is off. The advertised backfocus for your flattener is not necessarily the same as the actual backfocus required by your specific one. I would treat the published figure as a place to start. If the edge stars are pointing in towards the center then you likely need to add spacers - go 1mm at a time, refocus and test. If the problem gets worse, then remove spacers, also 1mm at a time until the problem goes away.

 

While it's possible your flattener may have a problem, it's more likely to be this - so do a little testing now and you may well sort it. It's slow, methodical, boring work to get it right, but worth it in the end.

 

Cheers,
Peter


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#19 dcm_guitar

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Posted 10 March 2021 - 11:25 AM

I mentioned earlier that I spent an entire evening adjusting my flattener back spacing in 1/2mm increments.  The published back space for my setup is 62.1mm.  I have 55mm of backspace with the image train, so I set the adjustment threads on the flattener to 7.1 (or as close as I can get).

 

I looked at the edge stars in both the imaging camera as well as the view from the OAG camera in 1/2mm increments from 6mm of adjustment through 10mm of adjustment.  The best result was at 7mm and 7.5mm of backspace adjustment.  The stars were almost round at these backspace values, but not entirely.  I could probably get closer by adjusting the tilt screws, but at that point I was tired of pulling my camera/FW/OAG off the scope and adjusting the backspace threads.


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#20 petercoxphoto

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Posted 10 March 2021 - 11:30 AM

Ah, I missed that. Yes, I would be getting on to the vendor or manufacturer at this point and asking them to help diagnose the issue and/or issue replacements as it does seem like you've done your due diligence on this.



#21 Der_Pit

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Posted 10 March 2021 - 03:40 PM

OK, left then is maybe to check how symmetric your FOV is, regarding distortions.   I.e., if there is a clear tilt, and/or if you're on the optical axis.  But seeing how careful (waytogo.gif ) you went through the BF adjustment I'm afraid it really is just the reducer that is at its limits, and you just cannot do better.  

 

Following GA recommendations for sure also is a good idea.  Striking is the 2x difference in RA and DEC RMS.  That is where you really need to start looking.  It means something's bad in RA.  The DEC RMS should (more or less) scale with the seeing.  If RA is much worse that suggests either the gear system of the mount is not smooth enough and produces erratic jumps, or the aggressiveness is too high and you over-correct causing zig-zag.  Maybe get a guiding log and post it in the mount forum.  There's experts in dissecting them to find the root of the problems smile.gif



#22 dcm_guitar

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Posted 12 March 2021 - 12:20 PM

Updates:

 

I've been able to validate that both my imaging camera and my OAG camera can achieve simultaneous focus.  I did this during daylight hours.  My guide camera is a QHY5III462C I bought for planetary imaging and thought it could also double as a guide camera.  However, to get the camera positioned in the OAG low enough to achieve focus I have to unscrew the sensor cover and can not use the IR filter.  The resulting images during daylight hours look FUNKY.  I've ordered a monochrome camera specifically for guiding.  It will hopefully arrive soon.

 

Last night I decided to only guide and ignore imaging.  This was hard because it was completely clear!!!!

 

I got my rig assembled and balanced.  I balanced with a slight east bias.  Then I polar aligned.  After achieving focus with my imaging telescope (using a Bahtinov mask), I slewed to the rosette nebula and then plate solved.  I thought this was as good a place as any to test guiding.

 

I started up PHD2, and went through a new profile wizard.  After going through the profile wizard, I started looping in PHD2 and then worked on getting the OAG camera in focus.  Using the helical focuser on the OAG I got the guide camera as focused as I was able.  Again, the stars were not round; they were "roundish".  These are the best shapes I've been able to achieve after doing a LOT of trial and error work on adjusting back space (in 1/2mm increments).

 

I set my camera exposure to 3 seconds and let it guide for almost an hour with all the default settings from the profile wizard.  The RA algorithm was Hysteresis and it only used single star guiding.  After 53 minutes my total RMS was 1.02", but the overall variance in guiding looked pretty good.  There were no "large excursions" like I've seen in the past.  The RA RMS was 0.82" and the DEC RMS was 0.65".  The RA periodicity was clearly evident in the RA plot.

 

I changed to multi-star guiding and changed the RA algorithm to predictive PE.  I then ran a GA for 10 minutes.  I implemented all of the GA recommendations.  It also suggested I improve focus on my guide camera, but the focus is as good as I can get it.

 

I guided for an additional 40 minutes.  This session produced a total RMS of 0.76".  The RA RMS was 0.61" and the DEC RMS was 0.45".

 

According to the GA results:  RA-HPF:  0.18"; DEC-HPF:  0.07"; Total HPF:  0.21".  This pretty typical for my seeing.

 

My RA RMS was 1.4x greater than DEC RMS.  The GA seeing measurements show RA HPF at 2.6x greater than DEC HPF.  I'm interpreting this to mean my RA RMS is not out of line with my DEC RMS given the seeing conditions.  Is this a correct conclusion?

 

Overall, this is the best guiding I've achieved.  Ideally, I'd get this down to ~0.5" of total RMS.  My first step will be to install the new monochrome guide camera and see what results.  I may also spend some time adjusting the tilt screws on my camera to see if I can improve the edge star shapes a little.  Then, I'll start changing some algorithm parameters.

 

For thise who have gone down this path, where do you start "tweaking" in the algorithm parameters?  Should I start on the RA algorithm or the DEC algorithm or both?

 

This is tedious work, but if I'm going to drop my image scale from ~2.01"/pixel to ~0.8"/pixel I'm going to need to do the work.



#23 happylimpet

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Posted 12 March 2021 - 12:27 PM

Updates:

 

I've been able to validate that both my imaging camera and my OAG camera can achieve simultaneous focus.  I did this during daylight hours.  My guide camera is a QHY5III462C I bought for planetary imaging and thought it could also double as a guide camera.  However, to get the camera positioned in the OAG low enough to achieve focus I have to unscrew the sensor cover and can not use the IR filter.  The resulting images during daylight hours look FUNKY.  I've ordered a monochrome camera specifically for guiding.  It will hopefully arrive soon.

 

Last night I decided to only guide and ignore imaging.  This was hard because it was completely clear!!!!

 

I got my rig assembled and balanced.  I balanced with a slight east bias.  Then I polar aligned.  After achieving focus with my imaging telescope (using a Bahtinov mask), I slewed to the rosette nebula and then plate solved.  I thought this was as good a place as any to test guiding.

 

I started up PHD2, and went through a new profile wizard.  After going through the profile wizard, I started looping in PHD2 and then worked on getting the OAG camera in focus.  Using the helical focuser on the OAG I got the guide camera as focused as I was able.  Again, the stars were not round; they were "roundish".  These are the best shapes I've been able to achieve after doing a LOT of trial and error work on adjusting back space (in 1/2mm increments).

 

I set my camera exposure to 3 seconds and let it guide for almost an hour with all the default settings from the profile wizard.  The RA algorithm was Hysteresis and it only used single star guiding.  After 53 minutes my total RMS was 1.02", but the overall variance in guiding looked pretty good.  There were no "large excursions" like I've seen in the past.  The RA RMS was 0.82" and the DEC RMS was 0.65".  The RA periodicity was clearly evident in the RA plot.

 

I changed to multi-star guiding and changed the RA algorithm to predictive PE.  I then ran a GA for 10 minutes.  I implemented all of the GA recommendations.  It also suggested I improve focus on my guide camera, but the focus is as good as I can get it.

 

I guided for an additional 40 minutes.  This session produced a total RMS of 0.76".  The RA RMS was 0.61" and the DEC RMS was 0.45".

 

According to the GA results:  RA-HPF:  0.18"; DEC-HPF:  0.07"; Total HPF:  0.21".  This pretty typical for my seeing.

 

My RA RMS was 1.4x greater than DEC RMS.  The GA seeing measurements show RA HPF at 2.6x greater than DEC HPF.  I'm interpreting this to mean my RA RMS is not out of line with my DEC RMS given the seeing conditions.  Is this a correct conclusion?

 

Overall, this is the best guiding I've achieved.  Ideally, I'd get this down to ~0.5" of total RMS.  My first step will be to install the new monochrome guide camera and see what results.  I may also spend some time adjusting the tilt screws on my camera to see if I can improve the edge star shapes a little.  Then, I'll start changing some algorithm parameters.

 

For thise who have gone down this path, where do you start "tweaking" in the algorithm parameters?  Should I start on the RA algorithm or the DEC algorithm or both?

 

This is tedious work, but if I'm going to drop my image scale from ~2.01"/pixel to ~0.8"/pixel I'm going to need to do the work.

Truth be told, in my experience, the difference between sensible values for everything and truly perfect settings for everything is very marginal. I dont think there are magic settings, the adjustment of which will suddenly improve your guiding hugely. Go with the guiding assistant settings, dont chase the seeing, and you'll be good.

 

I assume you're binning your guide camera as much as possible? Best thing when stars arent perfect.



#24 petercoxphoto

petercoxphoto

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Posted 12 March 2021 - 12:28 PM

Glad you got it to work! I don't know if tweaking the algorithm will give you the sorts of improvement you're looking for. Make sure you're perfectly balanced in both RA, Dec and your 'third axis' (basically making sure your scope's center of gravity is right over the middle of the saddle plate). Then also recognize that guiding is dependent on seeing when you get to this level. Sometimes on my CEM120EC2 I get 0.3" RMS guiding, other times it's 0.6". It just depends on the seeing on the night.

 

Cheers,
Peter



#25 dcm_guitar

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Posted 12 March 2021 - 02:34 PM

Truth be told, in my experience, the difference between sensible values for everything and truly perfect settings for everything is very marginal. I dont think there are magic settings, the adjustment of which will suddenly improve your guiding hugely. Go with the guiding assistant settings, dont chase the seeing, and you'll be good.

 

I assume you're binning your guide camera as much as possible? Best thing when stars arent perfect.

Uh, no I'm not binning my guide camera.  Should I?

 

Thanks for the suggestion.  Now for more reading!!!




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