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Issues with OAG Guiding at Long Focal Length

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#1 The Cat

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Posted 18 June 2021 - 09:40 AM

All-

I continue to struggle with off axis guiding at long focal lengths. I am eliminating one problem after another, but I continue to lose the guide star. This is with an Edge 9.25, 0.7 focal reducer, on a CEM70 mount. The usb cables are now shorter and don’t drop the signal. My new laptop doesn’t freeze. I’ve got the Prism tube at 90 degrees to the camera sensor. I’ve foot quality focus in the zwo 290 mm guide cam. Dark library, calibration and focal length/exposure settings in Phd2 are good. But the app keeps choosing guide stars that it then says have low SNr or loses even during the initial steps.

Yesterday I tries to guide near the Wild Duck Nebula. Dozens of potential guide stars! Still no luck with attaching to a star and staying on it.

Any thoughts?

#2 kozmo

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Posted 18 June 2021 - 09:52 AM

What OAG are you using?  I struggled with the ZWO OAG and switched to the Celestron OAG.  It has a larger prism (8mm vs 12mm).

Are you saying you have plenty of visible stars in PHD but it's just not staying latched to the stars?

Does the guide star show a nice peak?

 

Bin 2x2 to increase sensitivity.

Don't be afraid to increase the exposure up to 5s if necessary.

Change the camera gain to the lowest read noise.



#3 rgsalinger

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Posted 18 June 2021 - 10:22 AM

Not sure why your experience is so different from mine using the same camera. Using that camera I bin it to get a better signal and I use 3 second or more exposures. That should get you more good stars than you are getting now. You might also want to look at the star mass detection parameter in PHD. Make sure that it's not set to some value that causes PHD to think it's lost the star when it hasn't. 

 

You might be a candidate for multi-star guiding. Since you're getting stars, I don't see that a larger prism is going to help with this problem but it will help you in the long run and you have the back focus to handle it .  



#4 f300v10

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Posted 18 June 2021 - 10:23 AM

I concur with Ryan.  Make sure the gain on the guide camera is 40% or higher, and bin 2x2.  Created a new dark library of the guide camera in PHD2 using the 2x2 bin setting.  If 'auto select star' keeps selecting a star with marginal SNR, try manually selecting one with a higher SNR if one is available.  You can also up the minimum SNR for autofind to help with this issue.

 

You could also try increasing the 'search region' under the brain/guiding tab, which will make the area PHD2 looks for the guide star larger, I have mine at 40 pixels.  Also be sure the 'minimum star HFD' is set to something > 1, I use 2.0 and my C11 setup is similar to yours.



#5 DuncanM

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Posted 18 June 2021 - 10:56 AM

All-

I continue to struggle with off axis guiding at long focal lengths. I am eliminating one problem after another, but I continue to lose the guide star. This is with an Edge 9.25, 0.7 focal reducer, on a CEM70 mount. The usb cables are now shorter and don’t drop the signal. My new laptop doesn’t freeze. I’ve got the Prism tube at 90 degrees to the camera sensor. I’ve foot quality focus in the zwo 290 mm guide cam. Dark library, calibration and focal length/exposure settings in Phd2 are good. But the app keeps choosing guide stars that it then says have low SNr or loses even during the initial steps.

Yesterday I tries to guide near the Wild Duck Nebula. Dozens of potential guide stars! Still no luck with attaching to a star and staying on it.

Any thoughts?

Set your guide rate to .25x, use maximum binning, and the multi-star guiding release of PHD2. Use a 2 - 4sec integration time. 

 

I have struggled with the same issues with PHD2 (with a CEM60 and 120) and found that Astroart 7's autoguider control to perform much more reliably especially for faint guidestars. 



#6 The Cat

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Posted 18 June 2021 - 03:57 PM

Hi --

 

Thank you for all the great tips!  I am using the Celestron OAG with the Celestron Edge 9.25. I'm using an extender for the guide cam so that it is focused: tested it on the moon. I can't imagine that PhD2 was struggling due to a lack of stars. I was targeting the Wild Duck Cluster so there were tons of stars in PhD2's viewer. 

 

I will check binning. I tried various points along the gain range. No luck. It does select a guide star. When doing the various East, West, North steps, it will lose the star momentarily. It will then start guiding and the graph actually looks okay. But, again, star will be lost. 

 

My polar align is strong. Even at the long focal length, I am able to get 90 seconds of exposure on the DSLR with minimal star trails. 

 

I will change the guide rate. 

 

I tried different exposures, ranging from 1 second to 6 seconds. Still: star would be lost. 

 

Any further thoughts? Anyone initially have this experience with the ZWO 290 in the Celestron OAG and then make adjustments to cure the problem? 

 

Thanks!



#7 TelescopeGreg

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Posted 18 June 2021 - 04:12 PM

Is the loss of the star happening during the calibration or after? 

 

I ask because I had the problem during calibration, and the fix appears to have been a combination of a factory reset of the mount, and a reset of the guiding parameters in the Advanced page under the Guiding tab, followed by another calibration pass.  

 

I still don't understand what happened.  Basically the guiding steps had changed to something like 10x what they used to be, so the first step was too far to track. There was a software update in there, along with some experimental changes theoretically unrelated to PHD2. 



#8 The Cat

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Posted 18 June 2021 - 04:24 PM

And here is the link to the guide log:  https://www.dropbox....002403.txt?dl=0. Love your thoughts.



#9 DaveB

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Posted 18 June 2021 - 05:10 PM

I'm wondering if you need to tweak one of the top four parameters on this dialog:

 

PHD.png

  • Make sure that your search region isn't too small. At long focal lengths, you might want to make it bigger than the default. PHD will consider your guide star lost if it goes outside of this region. If it's too small, you increase the likelihood of "losing" your star.
  • Make sure that your Minimum HFD isn't too small. I don't know if the default is 1 or something else, but try setting it to at least 2 or 3 pixels. If it is set at 1, it could try to guide on a hot pixel (but that doesn't sound like your problem).
  • Make sure that the Tolerance isn't too high. If it is at 90% or more, it would only take a 10% drop in brightness to consider the star "lost".
  • Or, consider turning off star mass detection altogether.

 



#10 michael8554

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Posted 19 June 2021 - 04:00 AM

Pretty much what DaveB has already said.

 

From your GuideLog:

 

You are getting Star Lost messages because the Star Mass is frequently dropping below 50% - disable Star Mass Detection and go for a HFD = 2 setting instead.

 

Bin x 2 to increase sensitivity, and don't go below 2 secs exposure.

 

You Calibrated at Dec = 68.5, you are supposed to Cal at Dec = 0 for an ASCOM setup.

 

Your Minimum Moves are set very low, the equivalent of 0.25 arcsecs.

 

 I don't see you doing a PHD2 Guide Assistant run on target ?

 

Have a read of this:

 

https://openphdguidi...best-practices/



#11 The Cat

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Posted 23 June 2021 - 03:13 PM

Hi -- 

 

I have turned off tolerance and tuned up the HFD to 2. 

 

Does the ZWO 290 allow for binning x2? I looked at the helpful Best Practices guidance but don't see a page that allows for binning with my camera. 

 

Also, for Dec=0 I basically have to point the scope straight up at the sky, right?  I callibrated on the Wild Duck Cluster because I wanted to be sure that I had stars in the OAG. The last thing I want to do is struggle for calibration in a black area of the sky. 

 

Thanks!

Felix



#12 KTAZ

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Posted 23 June 2021 - 03:34 PM

Yes, the 290 can be binned 2x2 and should be. The setting is in PHD2 under Advanced Settings>Camera. It is available once you have connected to the camera.

 

In the top menu bar select Darks. Are you using Dark Library or Bad Pixel Map? You should try Use Bad-pixel Map.



#13 Lead_Weight

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Posted 23 June 2021 - 04:02 PM

The 290 is not the most sensitive guide camera. While it works great on a guide scope with a wide field of view, at slow speeds on an SCT and limited field of view, you’ll find that it has trouble picking up stars. There are some galaxy areas where you will literally not be able to find a single guide star. Moving to a 174MM camera would be a better long term solution, and likely better than getting a larger OAG which also introduces it’s own issues because it doesn’t fit nicely into the image train the way the ZWO option does. I’ve used all the various guide cams with my Edge11 setup, and it wasn’t until I got to the 174 that the issue went away. It’s a lot more sensitive camera, and can more easily pick up fainter stars in the available space of the limited FOV.


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#14 The Cat

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Posted 24 June 2021 - 01:03 PM

Thanks for the comments!! I will use Bad Pixel Map and set the binning to 2x2.

Regarding the 174 mm, I could easily buy it. But I hesitate to splurge knowing that it will not eliminate all the other problems associated with guiding off axis and essentially having a guide scope with a massive focal length. I am not thrilled about still having to rotate the OAG to find a star or struggling with long guide exposures. What about a 60 mm guidescope with the zwo 290? That guide scope would not count as wide fov, but it’s still a much wider fov than the Edge 9.25 with reducer. What do you think?

#15 DuncanM

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Posted 24 June 2021 - 01:22 PM

Thanks for the comments!! I will use Bad Pixel Map and set the binning to 2x2.

Regarding the 174 mm, I could easily buy it. But I hesitate to splurge knowing that it will not eliminate all the other problems associated with guiding off axis and essentially having a guide scope with a massive focal length. I am not thrilled about still having to rotate the OAG to find a star or struggling with long guide exposures. What about a 60 mm guidescope with the zwo 290? That guide scope would not count as wide fov, but it’s still a much wider fov than the Edge 9.25 with reducer. What do you think?

I am able to find usable guidestars in about 80-90% of random frames using a 10in SCT at F7 via a Starizona SCT FR, and a 120mm mini/OAG.  I never rotate the OAG and I would advise that you not do so either.  If I can't find a usable guider star, I usually just move the mount slightly in RA/Dec until I find one. It really helps to use platesolving to determine the location of the imaging sensor, and I can use the Astroart star chart and the USNO A2 star catalog (or Aladin) to see if there is a suitable guidestar nearby at the OAG frame location, and by going to 4secs I can usually find one without difficulty. 



#16 choward94002

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Posted 24 June 2021 - 01:23 PM

Guiding is always tricky ... lot's of factors involved!

 

If you're guiding with a guidescope (which I do, with a C6 and an SSAG) then you get big fields of view but you have to keep flexure in mind (so hardlock your scope, no little plastic nubbin ring things) and shorter exposures are best (even without flexure, you'll get slight field rotation), for me about 10min ...

 

If you're guiding with an OAG (which I do, with a Celestron OAG and an Orion G3 mono) then you can expose for longer than with a guidescope (my longest to date is 27min which was a bear!) but you have a much limited field of view and need a more sensitive camera ...

 

For OAG: Get an OAG with the biggest, fattest pick prism you can find ... bigger pick prism means more light going into the guide scope.  If your camera sensor is diagonal then you're really good (put the prism at the long diagonal, where there's no sensor to block), if it's square then put it in a corner (where you've probably got coma) but you want as much light as possible hitting that guide scope.  Regarding guide camera's you want something really sensitive; I use the Orion G3 for unrelated reasons (got a box of them as payment for some automated observatory work I did for a remote observatory), I've heard really good things about the LodeStar ... there are threads here about that ... binning down the camera is also a good idea, if things are still just too dim ...

 

For guidescope:  Get the biggest one you can get/ fit (I use a C6, that should tell you something) and make sure that the connection to the mount is hardlocked; vixen plate to losmandy plate ... there should not be a thumbscrew or a plastic nubbin anywhere ... same deal on guidescope, sensitive is your friend ...

 

Connection.jpg
 
And realize that just like a carpenter will have more than one kind of hammer in the toolchest, there are some objects that are best guided with a guidescope, some that are best with an OAG ... flexibility is your friend ...


#17 Lead_Weight

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Posted 24 June 2021 - 01:31 PM

Thanks for the comments!! I will use Bad Pixel Map and set the binning to 2x2.

Regarding the 174 mm, I could easily buy it. But I hesitate to splurge knowing that it will not eliminate all the other problems associated with guiding off axis and essentially having a guide scope with a massive focal length. I am not thrilled about still having to rotate the OAG to find a star or struggling with long guide exposures. What about a 60 mm guidescope with the zwo 290? That guide scope would not count as wide fov, but it’s still a much wider fov than the Edge 9.25 with reducer. What do you think?

A 60mm guide scope with the 290 will find plenty of guide stars to guide on, no matter where you're aimed. However, it will not solve the issue of mirror flop or flexure.

 

There are two main issues you have to overcome when using an SCT. One is flexure of the (typically long and heavy) image train, and the other is mirror flop. Flexure is when the weight of your imaging train flexes and bends just slightly enough to move around compared to a rigid guide scope. So while you might be guiding well with a guide scope, flexure will do in your images at long focal length (ie, oval stars). The second issue, mirror flop, happens because the built in focuser moves the primary mirror, and because the primary mirror shifts when moved (or even with gravity when you flip over the meridian), it will not match your guide scope, and you'll lose images to the flop.

 

To overcome flexure, you use an OAG, this way you're guiding on the same image train that is flexing, and no matter how much it flexes, you'll always match between the primary imaging camera and guide camera. To overcome the mirror flop, you need to have an external focuser. The external focuser allows you to lock down the primary mirror so that it doesn't move, and focus with an external focuser that is more precise and stable. 

 

Here's a link to a thread showing a common setup for the SCT

 

The last reason you might use an OAG over a guide scope, is that at long focal lengths, your image will show all tracking error introduced by your mount. You can track better with guiding, and the closer you guide to your imaging focal length, the more accurate your tracking will be. 

 

In my case, the 174mm solved all star finding issues with guiding and the ZWO OAG. I do not have to rotate the camera system around in order to find a suitable guide star. But with the 290, I did. And in some cases, I could never get a guide star to guide on. The area around M51 was like this.



#18 idclimber

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Posted 24 June 2021 - 01:41 PM

I also use a 174mm as a guide camera through my Celestron OAG. It definitely helps to have more sensitivity as well as a wider field of view that this sensor provides. 

 

Even so, at f/7 I still have to up the gain to 200 and bin 2x2 and use 3 second subs. This is on my 12" SCT. Otherwise the stars really are hard to see. 


Edited by idclimber, 24 June 2021 - 01:41 PM.

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#19 Peregrinatum

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Posted 24 June 2021 - 01:45 PM

welcome to the club, struggling with guiding at F10 is normal, took me several months to work out the kinks...

 

some ideas:

-Celestron OAG has a huge prism which can find more stars

-ASI174mm mini has a huge sensor as far as guide cams go

-make the search region box in IS PHD large, like 40 pixels

-lost stars come from low SNR, make sure gain is kicked up and exposure is long enough

-don't use the auto exposure feature in PHD, it favors shorter exposures and can cause low SNR

-do not pause guiding during auto focus, i know this doesn't make sense but sometimes if you pause it will lose the guide star



#20 KTAZ

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Posted 24 June 2021 - 01:46 PM

Interesting input from folks.

 

I have been using the Celestron OAG for about a year with the ASI290MM Mini. I like the COAG because of it's large pickoff prism (12mm) and I have never had a problem finding a guide star (yet). However, I am sure that time will come. When it does, the COAG has thumb screws allowing me to rotate the guide cam or primary cam at will without taking anything apart.

 

The difference between the 290 and the 172 is significant, so budget can be a deciding factor here. It was for me.

 

Personally, I would not use a guide scope on an SCT unless it was smaller than 8". Flexure and mirror flop possibilities are removed with the OAG. Why have to worry about them? At long Fl, you want as stable a system as possible to maintain even guiding.



#21 Lead_Weight

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Posted 24 June 2021 - 01:46 PM

A larger prism like in the case of the Celestron OAG, will get you about 20% more area to view, but it will not make your 290mm more sensitive. The reason you need a more sensitive guide camera is because at F7 and F10 stars appear more dim, and when imaging galaxies that are not against the Milky Way you have a much more limited set of stars to see around a given target. You will often have to guide on higher magnitude stars that are extremely dim. 

 

I would caution against using a Celestron OAG because it takes up a lot of your working backspace, and makes getting focus on the OAG and guide camera a little tricky. The ZWO setup locks you in to only taking up 55mm, and is matched to be in focus at the guide camera and main camera when put together. If you are not using an external focuser, you have more room to play with using the Celestron OAG, and it becomes less an issue of back space requirements, and more that you need to get both the OAG and main camera in focus at the same time.



#22 KTAZ

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Posted 24 June 2021 - 02:42 PM

A larger prism like in the case of the Celestron OAG, will get you about 20% more area to view, but it will not make your 290mm more sensitive. The reason you need a more sensitive guide camera is because at F7 and F10 stars appear more dim, and when imaging galaxies that are not against the Milky Way you have a much more limited set of stars to see around a given target. You will often have to guide on higher magnitude stars that are extremely dim. 

 

I would caution against using a Celestron OAG because it takes up a lot of your working backspace, and makes getting focus on the OAG and guide camera a little tricky. The ZWO setup locks you in to only taking up 55mm, and is matched to be in focus at the guide camera and main camera when put together. If you are not using an external focuser, you have more room to play with using the Celestron OAG, and it becomes less an issue of back space requirements, and more that you need to get both the OAG and main camera in focus at the same time.

Reasonable point that the COAG might cause back-focus limitations when trying to use a barrel focuser instead of the mirror focuser.

 

Since I have never used the ZWO OAG, in the event that you cannot find a guide star, what is the process to reposition the OAG in a spot where there is one?



#23 Lead_Weight

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Posted 24 June 2021 - 02:56 PM

Reasonable point that the COAG might cause back-focus limitations when trying to use a barrel focuser instead of the mirror focuser.

 

Since I have never used the ZWO OAG, in the event that you cannot find a guide star, what is the process to reposition the OAG in a spot where there is one?

I've never not been able to find a guide star. But I suppose if this became an issue, my Moonlite focuser can rotate, and I would just rotate accordingly.



#24 KTAZ

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Posted 24 June 2021 - 03:32 PM

I've never not been able to find a guide star. But I suppose if this became an issue, my Moonlite focuser can rotate, and I would just rotate accordingly.

Fair enough. But between the delta in cost on the 290 versus 174, and a moonlight focuser, aren't we talking over $1,000?

 

Back to the OP; there is simply no reason why this problem cannot be solved with what you have. Several of us use the exact same setup; 9.25, OAG, 290mm Mini, and have good guiding results. The several really good tips here should give you what you need to correct the issue; just run through them one by one;

 

Bin 2x2

Increase exposure time to 2-3 seconds.

Bump the gain up

Increase the star Search Region size

Drop the Minimum star HFD size

Drop the Minimum star SNR for Autofind

Try bumping Noise Reduction to 2x2

Try using the Bad Pixel Map instead of the Dark Library



#25 Lead_Weight

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Posted 24 June 2021 - 04:07 PM

Yeah, it's not a cheap proposition. I'm not saying it won't work. Just that it's not going to work 100% of the time. And to truly solve the problems inherent in an SCT, you need to consider what others have done. But at the end of the day, it's your money and your time. 




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