Focusing the Guide Camera - Celestron OAG

[farzad.k...@gmail.com]

Hello,

With the ZWO 1600 focused behind a Celestron OAG attached to a C8 Edge HD, I wasn't able to bring my QHY guide camera into focus. The range of vertical motion of the guide camera was well within the distance of the OAG's pick-off prism to the ASI's sensor.

Inside PHD2, I experimented with noise reduction settings of None, 2x2 and 3x3. I also had the camera binning set to 2. I realizing that I needed dark and bad pixel map associated with these and I had them available and in use through the profile.

Exposure lengths were varied between 1.5 seconds and 2.0 seconds with camera gain set to 40 or higher.

Since no guiding took place I am assuming the log and debug files do not exist.

While moving the guide camera in and out to see if I can find a star, the loop display is obviously noisy. Occasionally, the display would go black with what appears as being a couple of stars in the display, but soon static would return, and my attempts to reproduce that condition by gently moving the camera position would fail. I would then continue searching by re-positioning the camera.

I understand that the noisy loop screen means that PHD2 is trying to locate a star, and once it does, most of the noise should go away. I have had success with this before, but the periods of clear skies in my area are so widespread that I can't recall what contributed to my success.

I am wondering:

[1] if there is anything else that I need to do in order to bring my OAG camera into focus. I know it is practical to do this during daytime, and I plan on doing that.

[2] Why does the loop screen suddenly go black and then quickly return to noisy again?

Thanks for any suggestions on improving my chances of bringing the guide camera into focus, and an explanation on the loop screen behavior.

Farzad

[Brian Valente]

Hi Farzard

 

When PHD is gaining it up, it’s because it’s unable to find stars (in your case because they are too out of focus). When the display goes black with a few points of stars, that’s when you know you are in focus (or close to it).

 

If you were to go really slowly, you see a typical star-coming-into-focus transition where there is a big white circle that gets progressively smaller, and as the star comes into focus, PHD reduces its gain so the background becomes darker

 

If you go more quickly, that transition seems to be either noisy or dark.

 

 

Regarding your specific question:

 

[1] if there is anything else that I need to do in order to bring my OAG camera into focus. I know it is practical to do this during daytime, and I plan on doing that.

 

I think you have a good understanding of what’s going on. Getting your OAG camera into rough focus during the day will help you with that starting point

 

A couple adds would be

 

1.     Turn off subframes when focusing, so you don’t lock on to a star and easily lose it

 

2.     go slowly on adjusting your guide camera, and when you make and adjustment during the night with stars, let go of the camera and let phd loop a few times. The guide star may not Touching it or moving it

 

3.     select a star and view the star profile to get more accurate focusing

 

 

 

[2] Why does the loop screen suddenly go black and then quickly return to noisy again?

 

See above explanation.

 

 

Thanks

 

Brian

 

portfolio https://www.brianvalentephotography.com/

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[mj.w...@gmail.com]

SharpCap should give you a better display of stars to focus on.

Michael
Wiltshire UK

[Farzad Khosrownia]

Hi Brian, and thanks for explanation.

What is superior about the Celestron OAG versus other OAGs is its helical focus adjustment knob that allows for gentle adjustments, and I have been using that very slowly to try and find that black/clean loop screen. It is good that you are verifying the screen that appears momentarily is indeed the position where the camera is in focus, but I still don't understand why it should revert back to the noisy screen when I have not moved the camera from that position. However, I will keep trying.

Farzad

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[gerrit...@gmail.com]

Hi Farzad,

The brief black display is not always because you're near focus.  If you have a bright speck for any reason in the FOV it will darken the display for one frame.  That could be cosmic rays, intermittent hot pixels, or I suppose enough of a statistical fluctuation in the noise level.

I've been bothered by this for a long time, too, but I'm warily used to it now.  Look for a bright speck somewhere when you see a sudden single dark frame.

I have the Celestron OAG also -- it's very nice.  You might need to put extenders of a couple mm in the image or guide path to make them parfocal.  Also check to make sure your prism is far enough into the optical path (without extending into the imaging light path).  You might consider rotating your guide pick-off relative to the image frame so that the prism extends into the short side of your image rectangle, too.  You can extend it further in that way.

Gerrit

[Farzad Khosrownia]

Thanks a lot. I have heard of cosmic rays, and what you are saying makes a lot of sense. I have drawn up the focal distances and am sure I have enough distance, but tonight I am going to take some measurements of it. My fov is pretty narrow but I get plenty of stars in the imaging camera.

Farzad

--

[William Hansen]

Farazad,

Do not feel bad. I have been having the same problem with my Celestron OAG. I will follow on once I am able to work it out with Celestron. I have tried everything, less using a 1.25 inch eyepiece extension.

Perhaps someone else in this group with a Celestron OAG can assist?

Captain Wm. Hansen

510.410.0494

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[Vince]

I use the Celestron OAG with an Edge HD 1100 scope.  My required backfocus distance to the imaging sensor is 146.05mm, and the same backfocus distance is needed to the guide camera sensor to achieve focus of both cameras at the same time.  As a result I have had to add a total of 2.5" (63mm) in extensions to my guide camera (QHY5III224).  Your 8" Edge HD scope has a required backfocus distance of 133.35mm, so while you will not need as much extension as I do you will still need a fair amount.  So I would try with at least 2" of extensions, achieve focus on a star with your imaging camera, then achieve focus with the guide camera on the guide star, first by moving the guide camera in and out of the OAG to achieve a rough focus, then with the OAG's focuser.

To get set up initially, I situated the guide camera perfectly vertically in the OAG, focused on a star with the imaging camera, then moved the scope in RA (East I think, but you'll have to use trial and error) until the guide star appeared in the guide camera.  You should use as high gain as possible to make it show up in case it is a diffuse donut due to lack of focus at that point.

Vince

[Vince]

BTW my guide camera's backfocus distance is 10-12mm from the front of the camera to the camera sensor.  The total extensions you will need to achieve your 133.35mm backfocus distance will also depend on your camera's backfocus distance.

Vince

[sarg314 sarge]

I have a Celestron OAG and was never sure I was getting the guide camera well focused.  I started using a Bahtinov mask (from highpoint scientific) to focus both the main imaging camera and the guide camera, quickly.

To focus the main imaging camera I use one of the bright calibration stars during the alignment process. Just slip the mask over the front of the telescope and focus the DSLR camera in "live view mode."  It takes less than a minute during the alignment process.

After alignment is done, I just have to get a bright star (brighter than about 3.5 mag for my QHY 5L guide camera) onto the guide camera. The simplest way is to set the guide camera at position angle 0 or 180 (due north or south)  and then center the main camera on the bright star that you are going to use for guider focus (again, using Live View).  

Then start PHD2 scanning the guide camera and offset the telescope north or south until the bright star appears in the guide camera field. Then slip the Bahtinov mask on the telescope again and focus the guide camera with the OAG focus ring.  You will have to experiment to find out how much the telescope needs to be offset to move the star from the field center to the OAG prism.  For my 2032 mm fl telescope it is 31 arcminutes. But that also depends on how the prism's radial position has been set (it's adjustable with a set screw).  Once you know the offset you can do it in a single step.  The actual focusing takes just 10 seconds.  I wish Celestron hand controller had a real offset command available. It would simplify this and a number of other operations, but you can get by without it.

I've written software that allows me to precompute RA and Dec of field center for guide stars arbitrary position angles   (not just 0 or 180 ).  If you interested, let me know about your set up and I may be able to make some suggestions.

--

Tom Sargent

[Stu Beaber]

I've been watching this train for a few days. I got a Celestron OAG about 3 days ago and tried it the first night without any success. I'm using a 10" Meade ACF in an observatory with the added complication of a moveable mirror. (does have a mirror lock) and a Moonlite electric focuser.

I started 2.5 days ago in the day time and used a tree about 200-300 feet away. I used a pair of Ha filters in both the main camera (ST-10XME) and the guide camera (SBIG RGH) so I could take 1 second shots and not be bothered by the Sun. Once I could see pine needles in both cameras I knew I was in the ballpark.

I also know the focus will need adjusting when ever the nights clear enough to use it because of the greater distance...but I'm close. I had to use a 5mm spacer between the OAG and the main camera. I had  set my electric focuser at mid range as well as the helical focus on the OAG before I started. This way I'll hopefully have enough in/out travel on both to tweak both focusers on a star.

It is a lot of luck mixed with an educated guess or 2 once you start to see changes in the test images. Hope it will be worth the trouble. Once you find dual focus it should only need a tweak every once in awhile.

Good luck and hang in there,

Stu

[Brian Valente]

Hi Stu

 

>>> I also know the focus will need adjusting when ever the nights clear enough to use it because of the greater distance

 

Once you get the guide camera parfocal with the main camera, you shouldn’t need to adjust the focus of the guide camera at all.

 

The focusing of the main camera should take care of it, right?

 

 

Thanks

 

Brian

 

portfolio https://www.brianvalentephotography.com/

 

[Stu Beaber]

I think you are right Vince... what you will be doing is moving the pair of cameras in/out of the light cone equal amounts...and will effect both cameras the same. Good point

Stu

Stu

[farzad.k...@gmail.com]

Thanks. I will definitely try to bring a star into the guide camera instead of hoping it will have one in it for focusing purposes.

Farzad

On Friday, April 26, 2019 at 10:37:59 AM UTC-7, sarg314 sarge wrote:

I have a Celestron OAG and was never sure I was getting the guide camera well focused.  I started using a Bahtinov mask (from highpoint scientific) to focus both the main imaging camera and the guide camera, quickly.

To focus the main imaging camera I use one of the bright calibration stars during the alignment process. Just slip the mask over the front of the telescope and focus the DSLR camera in "live view mode."  It takes less than a minute during the alignment process.

After alignment is done, I just have to get a bright star (brighter than about 3.5 mag for my QHY 5L guide camera) onto the guide camera. The simplest way is to set the guide camera at position angle 0 or 180 (due north or south)  and then center the main camera on the bright star that you are going to use for guider focus (again, using Live View).  

Then start PHD2 scanning the guide camera and offset the telescope north or south until the bright star appears in the guide camera field. Then slip the Bahtinov mask on the telescope again and focus the guide camera with the OAG focus ring.  You will have to experiment to find out how much the telescope needs to be offset to move the star from the field center to the OAG prism.  For my 2032 mm fl telescope it is 31 arcminutes. But that also depends on how the prism's radial position has been set (it's adjustable with a set screw).  Once you know the offset you can do it in a single step.  The actual focusing takes just 10 seconds.  I wish Celestron hand controller had a real offset command available. It would simplify this and a number of other operations, but you can get by without it.

I've written software that allows me to precompute RA and Dec of field center for guide stars arbitrary position angles   (not just 0 or 180 ).  If you interested, let me know about your set up and I may be able to make some suggestions.

On Thu, Apr 25, 2019 at 4:17 PM William Hansen <captb...@gmail.com> wrote:

Farazad,

Do not feel bad. I have been having the same problem with my Celestron OAG. I will follow on once I am able to work it out with Celestron. I have tried everything, less using a 1.25 inch eyepiece extension.

Perhaps someone else in this group with a Celestron OAG can assist?

Captain Wm. Hansen

510.410.0494

On Apr 25, 2019, at 2:57 PM, Farzad Khosrownia <farzad.k...@gmail.com> wrote:

Thanks a lot. I have heard of cosmic rays, and what you are saying makes a lot of sense. I have drawn up the focal distances and am sure I have enough distance, but tonight I am going to take some measurements of it. My fov is pretty narrow but I get plenty of stars in the imaging camera.

Farzad

On Thu, Apr 25, 2019 at 2:41 PM <gerrit...@gmail.com> wrote:

Hi Farzad,

The brief black display is not always because you're near focus.  If you have a bright speck for any reason in the FOV it will darken the display for one frame.  That could be cosmic rays, intermittent hot pixels, or I suppose enough of a statistical fluctuation in the noise level.

I've been bothered by this for a long time, too, but I'm warily used to it now.  Look for a bright speck somewhere when you see a sudden single dark frame.

I have the Celestron OAG also -- it's very nice.  You might need to put extenders of a couple mm in the image or guide path to make them parfocal.  Also check to make sure your prism is far enough into the optical path (without extending into the imaging light path).  You might consider rotating your guide pick-off relative to the image frame so that the prism extends into the short side of your image rectangle, too.  You can extend it further in that way.

Gerrit

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Tom Sargent

[farzad.k...@gmail.com]

I have done the same thing except that I did not use a filter and instead I set the imaging to very high speed to bring my imaging camera into nearby focus after installing a focus motor on it.

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[farzad.k...@gmail.com]

Thanks

On Friday, April 26, 2019 at 10:48:35 AM UTC-7, Stu Beaber wrote:

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[farzad.k...@gmail.com]

Thanks. I will have to slowly move the scope north-south, east-west to see if I can find that bright star with my guide. My setup is C8 at f10, Zwo ASI 1600 QHY guide camera.

I am introducing a spacer to help bring the guide camera a bit more in.

Farzad

On Friday, April 26, 2019 at 10:37:59 AM UTC-7, sarg314 sarge wrote:

I have a Celestron OAG and was never sure I was getting the guide camera well focused.  I started using a Bahtinov mask (from highpoint scientific) to focus both the main imaging camera and the guide camera, quickly.

To focus the main imaging camera I use one of the bright calibration stars during the alignment process. Just slip the mask over the front of the telescope and focus the DSLR camera in "live view mode."  It takes less than a minute during the alignment process.

After alignment is done, I just have to get a bright star (brighter than about 3.5 mag for my QHY 5L guide camera) onto the guide camera. The simplest way is to set the guide camera at position angle 0 or 180 (due north or south)  and then center the main camera on the bright star that you are going to use for guider focus (again, using Live View).  

Then start PHD2 scanning the guide camera and offset the telescope north or south until the bright star appears in the guide camera field. Then slip the Bahtinov mask on the telescope again and focus the guide camera with the OAG focus ring.  You will have to experiment to find out how much the telescope needs to be offset to move the star from the field center to the OAG prism.  For my 2032 mm fl telescope it is 31 arcminutes. But that also depends on how the prism's radial position has been set (it's adjustable with a set screw).  Once you know the offset you can do it in a single step.  The actual focusing takes just 10 seconds.  I wish Celestron hand controller had a real offset command available. It would simplify this and a number of other operations, but you can get by without it.

I've written software that allows me to precompute RA and Dec of field center for guide stars arbitrary position angles   (not just 0 or 180 ).  If you interested, let me know about your set up and I may be able to make some suggestions.

On Thu, Apr 25, 2019 at 4:17 PM William Hansen <captb...@gmail.com> wrote:

Farazad,

Do not feel bad. I have been having the same problem with my Celestron OAG. I will follow on once I am able to work it out with Celestron. I have tried everything, less using a 1.25 inch eyepiece extension.

Perhaps someone else in this group with a Celestron OAG can assist?

Captain Wm. Hansen

510.410.0494

On Apr 25, 2019, at 2:57 PM, Farzad Khosrownia <farzad.k...@gmail.com> wrote:

Thanks a lot. I have heard of cosmic rays, and what you are saying makes a lot of sense. I have drawn up the focal distances and am sure I have enough distance, but tonight I am going to take some measurements of it. My fov is pretty narrow but I get plenty of stars in the imaging camera.

Farzad

On Thu, Apr 25, 2019 at 2:41 PM <gerrit...@gmail.com> wrote:

Hi Farzad,

The brief black display is not always because you're near focus.  If you have a bright speck for any reason in the FOV it will darken the display for one frame.  That could be cosmic rays, intermittent hot pixels, or I suppose enough of a statistical fluctuation in the noise level.

I've been bothered by this for a long time, too, but I'm warily used to it now.  Look for a bright speck somewhere when you see a sudden single dark frame.

I have the Celestron OAG also -- it's very nice.  You might need to put extenders of a couple mm in the image or guide path to make them parfocal.  Also check to make sure your prism is far enough into the optical path (without extending into the imaging light path).  You might consider rotating your guide pick-off relative to the image frame so that the prism extends into the short side of your image rectangle, too.  You can extend it further in that way.

Gerrit

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Tom Sargent

[Vince]

Also, my guide camera has a ring that fits around the 1.25" extension, so when I have the right focus position I tighten the ring and leave it on the camera for next time.  The focus is always very close when I reassemble the scope.  I focus with the primary so once imaging camera and guide camera are focused, refocusing the imaging camera focus will also refocus the guide camera.  I usually need at most to tweak the guide camera focus with the OAG focuser.

As I've mentioned before in this forum, finding a guide star at long focal length with OAG is not easy due to the very small FOV.  When you find the guide star as I have described, that is a good time to key in your planetarium program.  I use CdC and first I sync mount/CDC with the guide star perfectly centered in the imaging camera, using plate solving.  Then when I move the mount in RA and get the star centered in my guide camera, by trial and error I create a finder circle in CdC that intersects the new location of the guide star.  I then add a finder rectangle representing the FOV of my guide camera onto the finder circle, at the same angle as the guide camera (90 or 180 degrees for this initial setup), so that the finder rectangle is now centered on the guide star.  Now you have your finder rectangle at the correct distance to find a guide star.  You might have to redo it a couple of times to get it precisely right.

Then before imaging I locate in CdC a suitable guide star on the finder circle around my target, change the angle of the finder circle in CdC so that it is centered on that star, and rotate the OAG to that angle.  I use a paperboard 360 degree protractor pasted to the back of the OTA, and align the OAG screws opposite the guide camera to the correct angle on the protractor.  For east targets the OAG angle is the same as shown in CdC, for west targets the angle is 180 degrees opposite.  As long I first accurately sync to the target using plate solving, the guide star is always in the guide camera. 

Vince

[gerrit...@gmail.com]

Hi Vince,

That's a nice technique, but what do you do after a meridian flip?  That's been my problem.  I'll get a good guide star, but then nothing after the flip.  If you're at the telescope you can fudge things till a star just enters your guider FOV, but I haven't got a good solution for automated imaging.

Gerrit

On Friday, April 26, 2019 at 12:46:01 PM UTC-7, Vince wrote:

...

[Vince]

I wish there were a way to edit these, in my last para above I meant "change the angle of the finder rectangle in CdC so that it is centered on that star".

Gerrit, after meridian flip I manually rotate the OAG 180 degrees.  As long as I am accurate the guide star will be in my guide camera. With the Celestron OAG this means only the inner ring that turns the guide camera, not the outer ring that turns the imaging camera.

I use APT for imaging, which has a meridian flip module.  Just in case I ever get a rotator, I have asked then developer to add an option to turn the rotator 180 degrees.  He said he would add that at some point, so you might want to keep an eye on that.

Vince

[Vince]

Actually Gerrit, in APT you can create an imaging plan that incorporates scripts, and one of the available scripts is to change focuser position to a predetermined angle.  That might work...

Vince

[gerrit...@gmail.com]

Hi Vince,

I agree about the post editing!  I wish you could too.

I think you're right, that a rotator & script is what's required to hang onto that precious guide star.  I used APT up until just recently and liked it a lot, but now I'm a Sequence Generator Pro guy.  You can do rotating and scripts with SGP too, so that's probably what I'll do.

Gerrit

[Farzad Khosrownia]

Yes, sometimes I use Sharpcap. Thanks

On Thu, Apr 25, 2019 at 10:13 AM <mj.w...@gmail.com> wrote:

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[farzad.k...@gmail.com]

Tried the method suggested to bring a bright star into the view of the guide camera. It worked perfectly although I just took a chance and nudged the scope a bit here and there. Thanks for that suggestion.

After focusing, calibrating, and running GA, I slewed to my target  (M81), and suddenly there were no stars. I think I should tweak the FWHR to allow dimmer stars to be recognized as stars.

I am close to ditching the OAG and going with a piggy back guide scope for my C8 though. The OAG, its bulkiness set aside for the time being, has been a pain in the neck. And I know with dedicated guide scope there are issues with flexure, etc. Got to choose one evil versus another. I feel like I spend a lot of time dealing with an OAG.

Farzad

On Friday, April 26, 2019 at 3:34:53 PM UTC-7, Farzad Khosrownia wrote:

Yes, sometimes I use Sharpcap. Thanks

On Thu, Apr 25, 2019 at 10:13 AM <mj.w...@gmail.com> wrote:

SharpCap should give you a better display of stars to focus on.

Michael
Wiltshire UK

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[Bryan]

Farzad

fyi  

I have tried both an Astrodon MOAG and a separate guidescope (ST-80) approach on an A-P 1100 (SBIG mono ST-i guider).  I struggled with the same issues.  Even though the ST-i is quite sensitive, I sometimes had to adjust my pointing to bring a suitable star into the smalll FOV of the MOAG.

Imaging around zero DEC and the meridian, I typically get RMS values from the separate scope of 0.6 to 0.8 a-s compared to 0.5 to 0.6 a-s with the MOAG.  May not even be statistically different.

I go back and forth as to which I prefer.

The one thing I did that significantly improved performance with the separate guidescope was put in a long dovetail and a set of adjustable rings to hold the extension tube in which the ST-i fits. This mitigates (not sure it eliminates) flexure.

See attached image

Bryan

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[Vince]

Farzad, if you're referring to the vertical OAG method I've described, that is only good for co-focusing your guide camera and imaging camera, and for setting the guide camera distance from target center for your planetarium program.  The only way I can get a usable guide star for a particular target is by setting the OAG to the guide star angle indicated by the planetarium program, as I've also described.  Otherwise the FOV is just too small with the Edge HD scopes to find even 12 mag stars, which I've used at times.  And you will of course want to set the OAG angle before calibration. 

Vince

[Farzad Khosrownia]

Yes, it was about getting the guide camera in focus after the imaging camera is in focus already. 

I understand what you are saying about the guide star angle. The reason I am more and more discouraged by OAG and all its adjustments to find a guide star is the problems with automation of imaging. You definitely cannot set your guide star to be selected automatically from slew to slew because you have to be there, babysitting the OAG. I am going to keep trying the OAG because I am not advanced enough for that kind of automation yet, but with a properly selected dedicated guide scope in which many stars are to choose from you can count on PHD to find a guide star automatically. Do you agree?

On Mon, Apr 29, 2019 at 3:05 PM Vince <vprof...@gmail.com> wrote:

Farzad, if you're referring to the vertical OAG method I've described, that is only good for co-focusing your guide camera and imaging camera, and for setting the guide camera distance from target center for your planetarium program.  The only way I can get a usable guide star for a particular target is by setting the OAG to the guide star angle indicated by the planetarium program, as I've also described.  Otherwise the FOV is just too small with the Edge HD scopes to find even 12 mag stars, which I've used at times.  And you will of course want to set the OAG angle before calibration. 

Vince

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[Vince]

Yes I agree, automating with the OAG will be challenging.

Vince