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Moving to OAG - any advice?

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#1 pgs/sdg

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Posted 13 August 2019 - 02:03 PM

Next clear night I'm going to make the jump to off-axis guiding using my 8" Edge HD and my T5i.

I have worked on the spacing and I think I have the spacing close - although I do expect some challenges in getting both cams to focus.

I'll be using the Celestron OAG with the F/7 focal reducer. I am very close (1-2mm)  to the 105mm back focus distance for the image train:

Edge HD>F/7 reducer>OAG>T5i.

The guide cam is a new ZWO ASI174 mini. I am putting a lot of trust in its larger sensor to help me with finding guide stars.

Provided I can get everything focused..will create dark library, calibrate in PHD2 and hopefully be off and running.

Any tips or advice for someone starting out with OAGuiding in this configuration would be appreciated.

 

 



#2 bobzeq25

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Posted 13 August 2019 - 02:10 PM

Next clear night I'm going to make the jump to off-axis guiding using my 8" Edge HD and my T5i.

I have worked on the spacing and I think I have the spacing close - although I do expect some challenges in getting both cams to focus.

I'll be using the Celestron OAG with the F/7 focal reducer. I am very close (1-2mm)  to the 105mm back focus distance for the image train:

Edge HD>F/7 reducer>OAG>T5i.

The guide cam is a new ZWO ASI174 mini. I am putting a lot of trust in its larger sensor to help me with finding guide stars.

Provided I can get everything focused..will create dark library, calibrate in PHD2 and hopefully be off and running.

Any tips or advice for someone starting out with OAGuiding in this configuration would be appreciated.

Set it up in daytime.  Be very careful to avoid the Sun.  The Moon can also work.  PhD2 is annoyingly sensitive to misfocus.

 

Be sure to correct your guidecamera's focal length in PhD2.


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#3 DuncanM

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Posted 13 August 2019 - 02:25 PM

Use a two or three hole Hartmann mask for focusing your guide camera. You can make one out of cardboard and it makes it really easy to tell which way to move focus and gets you close enough for the guide camera to work.


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#4 pgs/sdg

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Posted 13 August 2019 - 02:41 PM

Set it up in daytime.  Be very careful to avoid the Sun.  The Moon can also work.  PhD2 is annoyingly sensitive to misfocus.

 

Be sure to correct your guidecamera's focal length in PhD2.

Thank you, yes, may try to set it up tomorrow morning and see how it goes or maybe tom. night using the moon. If I was able to get the old SSAG to focus and guide pretty well I'm hoping I can do so with this new setup. Much better guide camera. 

Have created a new equipment profile in PHD2 with correct pixel size and focal length of 1422mm.



#5 pgs/sdg

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Posted 13 August 2019 - 02:54 PM

Use a two or three hole Hartmann mask for focusing your guide camera. You can make one out of cardboard and it makes it really easy to tell which way to move focus and gets you close enough for the guide camera to work.

I already have a Bahtinov mask - would that work just as well?

Or would the Hartmann be better? 



#6 Hawkdl2

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Posted 13 August 2019 - 03:29 PM

Here's a write up of 5 or 6 different focusing masks and their limitations and sensitivities.  There's a reason the Bahtinov mask is the most prevalent today.

 

http://astro.i-net.hu/node/56


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#7 rkinnett

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Posted 13 August 2019 - 04:20 PM

Your OAG provides some fine adjustments to help you dial in the guider focus while your primary imager is in focus, but the adjustments are very limited and not easy to make in real time.  A helical focuser between your OAG and guider will make focusing much easier.


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#8 DuncanM

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Posted 13 August 2019 - 05:18 PM

I already have a Bahtinov mask - would that work just as well?

Or would the Hartmann be better? 

Bahtinov masks are generally too dim for OAG use, IM experience. YMMV but it's easy to make a two hole mask and the holes don't need to be very accurate to provide focus assistance, and larger holes are better than too small holes. Extremely critical focus is not required for the guide camera.


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#9 DuncanM

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Posted 13 August 2019 - 05:25 PM

Here's a write up of 5 or 6 different focusing masks and their limitations and sensitivities.  There's a reason the Bahtinov mask is the most prevalent today.

 

http://astro.i-net.hu/node/56

Focusing an OAG guide camera is very different from focusing the imaging camera. When focusing the OAG camera we are often far out of focus; the two hole Hartmann mask will show two images of the same star that appear reasonably sharp because the holes act to increase the F ratio,  and the stars will move apart or come together as you adjust focus. This makes it relatively easy to make the coarse adjustments necessary during the initial setup of the OAG and guide camera:

 

2stars.gif

 

 

http://spazioinwind....focus/focus.htm

 

The above works OK even using when using sliding barrel focus of the guide camera. The ideal is place a parfocalizing ring on the guide camera prior to placing it in the OAG, with parfocalizing ring all the way up on the barrel to allow for maximum focus travel, then focus the imaging camera first (Bahtinov mask), and then place the hartmann mask on the scope and then focus the guide camera. When the guidecamera is focused, move the parfocalizing ring so that it's snug against the OAG guide camera port and tighten the setscrews and leave the parfocalizing ring permanently in place. You never have to adjust OAG focus again: you focus the imaging camera and drop the guide camera (with parfocaling ring in place) into the OAG and you're ready to go.


Edited by DuncanM, 13 August 2019 - 05:47 PM.

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#10 pgs/sdg

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Posted 13 August 2019 - 09:16 PM

Focusing an OAG guide camera is very different from focusing the imaging camera. When focusing the OAG camera we are often far out of focus; the two hole Hartmann mask will show two images of the same star that appear reasonably sharp because the holes act to increase the F ratio,  and the stars will move apart or come together as you adjust focus. This makes it relatively easy to make the coarse adjustments necessary during the initial setup of the OAG and guide camera:

 

2stars.gif

 

 

http://spazioinwind....focus/focus.htm

 

The above works OK even using when using sliding barrel focus of the guide camera. The ideal is place a parfocalizing ring on the guide camera prior to placing it in the OAG, with parfocalizing ring all the way up on the barrel to allow for maximum focus travel, then focus the imaging camera first (Bahtinov mask), and then place the hartmann mask on the scope and then focus the guide camera. When the guidecamera is focused, move the parfocalizing ring so that it's snug against the OAG guide camera port and tighten the setscrews and leave the parfocalizing ring permanently in place. You never have to adjust OAG focus again: you focus the imaging camera and drop the guide camera (with parfocaling ring in place) into the OAG and you're ready to go.

Thanks for the link & info!

I just made a Hartmann out of some cardboard! (its a real work of art) Really, looks just like the one in the link you posted. Should work.

I have gotten the distance from prism to image sensor VERY close to the same distance as prism to guide cam sensor (~82mm).. it will still be "off" I'm sure, but hopefully not WAY off. The mask will help!

I've also got the parfocalizing ring ready to go on the guide cam. 


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#11 kisstek

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Posted 14 August 2019 - 01:11 AM

Thank you, yes, may try to set it up tomorrow morning and see how it goes or maybe tom. night using the moon. If I was able to get the old SSAG to focus and guide pretty well I'm hoping I can do so with this new setup. Much better guide camera. 

Have created a new equipment profile in PHD2 with correct pixel size and focal length of 1422mm.

I switched recently to using an OAG (ZWO). I strongly urge you to do the initial focus matching during the daytime. I tried at night after thinking I had the two close based upon the math. I wasted a couple of nights trying to get them both in focus. Finally I did what others had suggested and tried it out in the middle of the afternoon. I used the neighbor's tile roof down the street. Took about 15 minutes to get both the main camera and the OAG in focus. That night I was able to do my first successful OAG guiding.

 

Trying to match the focus on a moving target (Moon) just adds an extra complexity to an already touchy situation.


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#12 pgs/sdg

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Posted 14 August 2019 - 12:05 PM

Set it up in daytime.  Be very careful to avoid the Sun.  The Moon can also work.  PhD2 is annoyingly sensitive to misfocus.

 

Be sure to correct your guidecamera's focal length in PhD2.

 

Focusing an OAG guide camera is very different from focusing the imaging camera. When focusing the OAG camera we are often far out of focus; the two hole Hartmann mask will show two images of the same star that appear reasonably sharp because the holes act to increase the F ratio,  and the stars will move apart or come together as you adjust focus. This makes it relatively easy to make the coarse adjustments necessary during the initial setup of the OAG and guide camera:

 

2stars.gif

 

 

http://spazioinwind....focus/focus.htm

 

The above works OK even using when using sliding barrel focus of the guide camera. The ideal is place a parfocalizing ring on the guide camera prior to placing it in the OAG, with parfocalizing ring all the way up on the barrel to allow for maximum focus travel, then focus the imaging camera first (Bahtinov mask), and then place the hartmann mask on the scope and then focus the guide camera. When the guidecamera is focused, move the parfocalizing ring so that it's snug against the OAG guide camera port and tighten the setscrews and leave the parfocalizing ring permanently in place. You never have to adjust OAG focus again: you focus the imaging camera and drop the guide camera (with parfocaling ring in place) into the OAG and you're ready to go.

 

I switched recently to using an OAG (ZWO). I strongly urge you to do the initial focus matching during the daytime. I tried at night after thinking I had the two close based upon the math. I wasted a couple of nights trying to get them both in focus. Finally I did what others had suggested and tried it out in the middle of the afternoon. I used the neighbor's tile roof down the street. Took about 15 minutes to get both the main camera and the OAG in focus. That night I was able to do my first successful OAG guiding.

 

Trying to match the focus on a moving target (Moon) just adds an extra complexity to an already touchy situation.

Just installed the new image train on the 8" Edge and rebalanced.

Focused cameras on a pine tree a couple hundred yards away and got both in good focus.

Should make things easier tonight.

I only had to turn the helical focuser on the Celestron OAG about 3 turns and things came into focus.

Will use the Bahtinov and Hartmann masks tonight on a bright star to dial it in close.

 

I think I'm going to be pleased with this setup. ASICAP software for the 174 mini is nice, easy to use.

So far so good.... thanks for help guys ...

I'll update 


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#13 pgs/sdg

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Posted 16 August 2019 - 02:55 PM

Set it up in daytime.  Be very careful to avoid the Sun.  The Moon can also work.  PhD2 is annoyingly sensitive to misfocus.

 

Be sure to correct your guidecamera's focal length in PhD2.

 

 

Focusing an OAG guide camera is very different from focusing the imaging camera. When focusing the OAG camera we are often far out of focus; the two hole Hartmann mask will show two images of the same star that appear reasonably sharp because the holes act to increase the F ratio,  and the stars will move apart or come together as you adjust focus. This makes it relatively easy to make the coarse adjustments necessary during the initial setup of the OAG and guide camera:

 

2stars.gif

 

 

http://spazioinwind....focus/focus.htm

 

The above works OK even using when using sliding barrel focus of the guide camera. The ideal is place a parfocalizing ring on the guide camera prior to placing it in the OAG, with parfocalizing ring all the way up on the barrel to allow for maximum focus travel, then focus the imaging camera first (Bahtinov mask), and then place the hartmann mask on the scope and then focus the guide camera. When the guidecamera is focused, move the parfocalizing ring so that it's snug against the OAG guide camera port and tighten the setscrews and leave the parfocalizing ring permanently in place. You never have to adjust OAG focus again: you focus the imaging camera and drop the guide camera (with parfocaling ring in place) into the OAG and you're ready to go.

 

 

I switched recently to using an OAG (ZWO). I strongly urge you to do the initial focus matching during the daytime. I tried at night after thinking I had the two close based upon the math. I wasted a couple of nights trying to get them both in focus. Finally I did what others had suggested and tried it out in the middle of the afternoon. I used the neighbor's tile roof down the street. Took about 15 minutes to get both the main camera and the OAG in focus. That night I was able to do my first successful OAG guiding.

 

Trying to match the focus on a moving target (Moon) just adds an extra complexity to an already touchy situation.

 

So please excuse the long post - but perhaps this info may be helpful to someone with a similar situation. This turned into a 2 night affair. 

 

To sum up - I've been working on switching from 50mm guidescope with a SSAG over to OAG using the Celestron OAG and a new ASI174mini guide cam. When I finally got things to work - I got some of the best guiding numbers I have ever gotten -  BUT - it really looks like unless there is another solution that I have missed - I am going to have to image at F/10 instead of with FR at F/7 as long as I'm using the T5i.

 

Here's why - (and if anyone can shed any light on this/has found a solution - I am all ears because I'd like to be able to image at F/7 - but can't see right now how to do it)

 

Back focus requirement using the F/7 reducer for the 8" Edge is 105mm (from the front of the reducer to the imaging sensor) 

So.. I set it up this way:

Edge > F/7reducer > Bluefireball adapter > 48mm adapter (skinny one NOT the SCT adapter) > OAG > 48mm adapter > T-ring > T5i

This got me to within a mm or so of the 105mm. (~8 + 29 + 12 + 55 = 104mm)

 

I set it up this way because using the SCT adapter (which is 25mm) + 55mm of distance for the T5i + the adapters + the width of the OAG - was putting me way over the 105mm. (Switching eventually to a dedicated astro cam with less back focus will eventually solve this problem I hope - but that's another conversation)

So I connected up the above train - and tried to dial in both cameras. Got perfect focus with the T5i. No problem there.

But I could not bring the ASI174 to good focus. Could not get a good image. Tried the Hartmann, the Bahtinov, used the moon, and bright stars - but the image was just bad. So I took things apart - looked down the tube and it really seemed to me that NOT using the SCT adapter and substituting the narrower M48 adapter was what was causing the problem. Despite the large size of the 174 sensor - it was just not grabbing enough of the light path. And I had the prism all the way down. The inner edge of the reducer really appeared to be too close to the prism and was cutting off a lot of the light path. The physical opening at the end of the reducer was (so it appeared) just too close to the prism and seemed to be what was causing problems with the guide cam image. But that was the only way to get the proper spacing.  

 

I have seen some other posts here and on Stargazer's Lounge - where several folks have reported having focus problems when trying to use the F/7 reducer with a DSLR. The back focus dilemma. 

 

So, next night, I switched things out to this (which put me back at F/10):

 

Edge > SCT adapter > OAG > adapter > T-ring > T5i

 

Reset all the parameters in PHD2 - did dark library - did a calibration and Voila! - beautiful guide cam image (more guide stars than I ever saw with my SSAG by the way) It appears to me that skinnier adapter was the problem. When I ditched the FR used the SCT adapter instead - the opening was larger and further away from the prism - and the prism was able to utilize the full area of the light path rather than be blocked by the edges of the reducer. 

 

So, it looks like as long as I continue to use the T5i - I'm going to have to image at F/10. Not the end of the world - but I was hoping to image @ F/7.

At least until I can purchase a good imaging camera and ditch the DSLR.

After the switch - the guiding appeared to be much improved. Although I did not track it for an hour or so - it was much better than I ever got with the guidescope and SSAG. Best I could usually do with that set up was about 1 - 1.20 RMS or worse. 

After the switch back to the second image train above - I was getting .5 - .6 RMS. Tried it in different parts of the sky - and it stayed pretty consistent. (I still need to run PEC on my mount - just haven't gotten around to it yet)

 

Anyway - here's a screen shot of the guiding:

 

Screenshot (7).png

 

I did not guide for a real long time - but I did slew the scope around to different quadrants and it seemed to guide well all over. 

I really like this ASI 174 - easy to focus, easy to find stars.

 

So that's where I'm at - I was very pleased to get good guiding first try.

If there's no work around to this - I guess I'll have to be satisfied with F/10 for now. Any thoughts or insights - much appreciated. 

 

Paul 


Edited by pgs/sdg, 16 August 2019 - 02:59 PM.

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#14 pgs/sdg

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Posted 16 August 2019 - 02:56 PM

And here's another screen shot:

 

Screenshot (6).png




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