18 replies to this topic

[X1m]

hello guys, i have a very troubling problem. so i have a Astro tech rc6. i have made it my imaging scope, i bought a zwo oag small, not L and the upgraded focuser for it. i tested it during the day to see if i could get it in focus on a far tree and it went great, so once it became night time I went to m81 and tried to find a star there and focus, couldn't find a star there and began going all around the night sky look for a star, no matter what i did i could never find a hint of a unfocused star NOTHING. I was doing 7s exps and 70 gain. so i packed it up and did some research and i have some problems and conclusions that I would like to share. number 1. my oag is not getting enough light mixed with not very sensitive sensor. I Have a asi120mm mini that i use and my main scope is a F/9. so my thought was that with this lower end sensor and high f stop not enough light is reaching the camera. Number 2. way too out of focus, i measured my distance from oag prism and oag camera and lined it up with oag prism and main camera and got it pretty close, but maybe somehow i got it wrong? something in my setup is wrong. FIXES:: my fix for problem number one would be return my oag and get the L version and get a newer guide camera that is more sensitive. fix for number 2. well I'm not sure. ALSO i have my oag prism all the way in. i have a asi533mc pro so it does not block the sensor at all. i tried all the way sliding my sensor up and down the focuser and never found a hint of focus. My sensor is getting light, i can shine a light down the tube and it picks it up, i can even shine a flash light on a far tree and point my scope at it and my oag will pick it up. im like in bortle 3/4 so I'm not to bad in that department. there is probably other things i have tried but forgot about at the time of typing this up.

[macdonjh]

Before you return anything: 

 

Set your scope up and take a few flats.  Do a quick screen stretch of the flats: you may see the shadow of your OAG.  If you do, light reaching your OAG is not the problem.  If you can see a shadow from your OAG, slide it out a bit and take another flat.  Repeat that process until the OAG shadow JUST disappears from your flat image.  Then you'll know your OAG prism is as close to your imaging sensor as possible without obstructing it.

 

To confirm your OAG "geometry", you can make a scale drawing of your light cone and include the locations of your OAG prism and imaging sensor.  With that you'll be able to see if you're fully illuminating your main camera and if you can fully illuminate your OAG without causing a shadow on your main camera.  I made a similar drawing for my f/15 scope and my OAG had to block part of my main camera to be fully illuminated.  You wrote your RC-6 is f/9, so that may not be an issue for you.

 

i measured my distance from oag prism and oag camera and lined it up with oag prism and main camera and got it pretty close,

I'm not sure I understand what you did here.  The distance between your prism and guide camera compared to the distance between your prism and main camera doesn't help you focus.  You need to be sure the imaging chips are both at the correct back focal distance.  If you get your main camera in focus, measure the distance between the back plate of your scope and the imaging chip.  Then get your OAG set up such that the distance between your scope's back plate and the guide cam imaging chip is the same.  Then both cameras will be in focus. 

 

Note: the above process is pretty tedious.  Your cameras' chips are set back from the "faces" of the cameras, so you need to know those offsets.  you also need to know the distance between your OAG prism and the guide camera when it's assembled.  Then you'll have to figure out if you need any spacers between your OAG and filter wheel/ camera.  

[AstroFromHome]

I had similar issues too when switching to an OAG, especially the small ZWO one. On this on the stalk is pretty long and some people also had to cut it off a bit.

 

How I managed to get focus with the ZWO OAG.

I took off the plate with the 1.25" connector from the stalk. Then I positioned the prism a close to the sensor edge as possible (I have IMX571 cameras).

Then I slided the plate with the 1.25" connector all the way down and fixed the grub screws as good as possible. The stalk will not be flush with the base plate in the 1.25" connector anymore.

On the 120mm I have removed all extensions except the on that holds the AR glass. Put the camera all the way in. My 120 had to touch the stalk as the IMX571 sensor is also a bit bigger than the 533 sensor.

 

So saying you need to make sure that you get as much in with the stalk and the camera as possible. I think the back focus on the 120mm mini is 9.5 mm. The stalk is 38- 40mm or so. Adding half of the body length of the OAG makes it quite difficult to reach the 55mm backfocus your main camera needs.

That is why I do not use the ZWO-L OAag because I think it brings the same difficulties. A friend was unable to get the guide camera deep enough into the L Version to reach focus.

I see better alternatives with the OAGs from Touptek an. Player One. Also with the OAG from Askar that I have but with that one you need a narrower extention tube/ filter drawer or filter wheel as it is 3mm longer than the ZWO OAGs.

Touptek and Player One OAG have the same size like the ZWO ones.

[macdonjh]

AstroFromHome's post made me think: do you have an ASI120 (the "pancake" body), or an ASI120 mini?  The "pancake" body may not come to focus at all.  I tried and couldn't get it far enough into the OAG housing.  Switching to a camera with a mini housing solved that right away. 

 

ASI120: https://industrially...AyABEgK2fPD_BwE

 

ASI120 mini: https://www.bhphotov...AiABEgLC-_D_BwE

[Alex McConahay]

The way I read it, during the day, you can focus the guide camera on a distant something and get a clear clean picture out of the guiding software-----right?

 

At night--without changing anything at all in your system (including the focus position you got on the distant object during the day), you cannot see any stars in the guiding software---right?

 

 

Obviously, you should have gotten something at night if you had something during the day. It should have had a soft focus, but should have been something. If anything, when you changed from a daytime (terrestrial) object to nighttime stars, you might need to move the focuser slightly inward to focus on a celestial object. How much depends on how far away the terrestrial object was compared to the celestial (infinity). 

 

To get around the focus problem, try focusing your guider on the moon. 

 

So far, you can ignore the main camera. You do not care at all about what you see in the main camera. You are just trying to get your guide camera focused so far.

 

I'll stop here, and let you get back to us on whether I have the situation correct and what you fine. 

 

Alex

[Alex McConahay]

By the way.....spend some time measuring things as well as you can. But, realize that because it is difficult to define what you are measuring from and to, this can only get you so close. The final test is in the final movements, that are actual slight movements, and not easily movable. 

 

The problem I am speaking of is that you cannot get the rulers, micrometers, and all that in just the right positions to take the measurements as accurately as they eventually need to be. 

Alex

[klangwolke]

Oops, I responded in your other thread, just checking, after focusing your scope, you are also moving the mini in and out of the OAG tube to get in focus?

[Alex McConahay]

McDonjh is accurate in Post #2

 

>>>>>>>Note: the above process is pretty tedious.  Your cameras' chips are set back from the "faces" of the cameras, so you need to know those offsets.  you also need to know the distance between your OAG prism and the guide camera when it's assembled.  Then you'll have to figure out if you need any spacers between your OAG and filter wheel/ camera.

 

 

It is pretty tedious. 

 

If you are using a focuser on the back end of your scope to focus (and not an internal focuser) it can be a lot easier. 

 

Focus your main camera (ignoring the guide camera). Measure how far out from the rear plate, or some easily measured point, to the camera. (Best yet, if there are markers on your focuser tube showing its position, simply note the position where the main camera is in focus. 

 

Now, focus the guide camera (ignoring the main camera). Note again where your main camera is positioned by measuring those same markers as you measured before. 

 

When your OAG and main camera are properly positioned, by definition, these two measurements will be the same. So, if they are different, subtract the shorter distance from the longer.  This is the distance you must move the camera that focuses at the shortest distance outward. (Or, the one that focuses at the farthest distance inward.) So, add or remove spacers (or if the camera can shift in its mounting, or by using a separate helical focuser use those changes) until you have changed the appropriate distance as needed. ("Distance as needed" means inward or outward as far as the difference you found between their focus positions.) 

 

Of course, this will not help you until you get an image in both your cameras at the same time. I believe it will work, however, during the day. If it is not precise during the day, it will get you awfully close for at night. 

 

Alex

[rgsalinger]

When in deep OAG trouble I rely on the moon if I can. It's easy to see no matter how far out of focus you might be. A second, not so good, alternative is to make sure that the scope is pointing to somewhere in the MIlky Way. And play with the contrast a bit if you are using PHD to see large out of focus (and hence dim) stars. 

[AstroFromHome]

You always focus the main camera first.

The guide camera second by moving it in the 1.25" connector. Fine tuning for the focus via helical focuser if you have one on the OAG.

After tightening the locking screws for the guidecam you just do anything on the OAG when you see the stars are well out of focus after a few sessions and having the main camera in focus. 

[TXLS99]

Another thing that may be causing you problems is that when you position a star in the center of your imaging camera field of view, that same star will not show up on your guide camera field of view.  Your guide camera is picking up light from off to the side of the light cone, so it does not see what your main camera sees.

[archiebald]

hello guys, i have a very troubling problem. so i have a Astro tech rc6. i have made it my imaging scope, i bought a zwo oag small, not L and the upgraded focuser for it. i tested it during the day to see if i could get it in focus on a far tree and it went great, so once it became night time I went to m81 and tried to find a star there and focus, couldn't find a star there and began going all around the night sky look for a star, no matter what i did i could never find a hint of a unfocused star NOTHING. I was doing 7s exps and 70 gain. so i packed it up and did some research and i have some problems and conclusions that I would like to share. number 1. my oag is not getting enough light mixed with not very sensitive sensor. I Have a asi120mm mini that i use and my main scope is a F/9. so my thought was that with this lower end sensor and high f stop not enough light is reaching the camera. Number 2. way too out of focus, i measured my distance from oag prism and oag camera and lined it up with oag prism and main camera and got it pretty close, but maybe somehow i got it wrong? something in my setup is wrong. FIXES:: my fix for problem number one would be return my oag and get the L version and get a newer guide camera that is more sensitive. fix for number 2. well I'm not sure. ALSO i have my oag prism all the way in. i have a asi533mc pro so it does not block the sensor at all. i tried all the way sliding my sensor up and down the focuser and never found a hint of focus. My sensor is getting light, i can shine a light down the tube and it picks it up, i can even shine a flash light on a far tree and point my scope at it and my oag will pick it up. im like in bortle 3/4 so I'm not to bad in that department. there is probably other things i have tried but forgot about at the time of typing this up.

The ASI120 is not suitable for use as an OAG camera - it generally has a tiny FOV that is exacerbated if you are aiming outside of the galactic plane (less stars to guide with).  You really need something with a larger sensor and larger pixels.

 

I have an ASI120 that I keep for use with a guidescope, but for my OAG I use the Player One Xena-M.

 

You also REALLY need a helical focuser on the OAG - trying to adjust it by hand is a lesson in frustration.

 

Also, as mentioned above, the OAG does not see what the main camera sees, it is more like this....

 

Attached Thumbnails

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[Scott Badger]

You always focus the main camera first.

The guide camera second by moving it in the 1.25" connector. Fine tuning for the focus via helical focuser if you have one on the OAG.

After tightening the locking screws for the guidecam you just do anything on the OAG when you see the stars are well out of focus after a few sessions and having the main camera in focus. 

Once you first focus the scope/main camera on something, star, moon, tree, whatever, then focus the OAG/guide camera, shouldn't that be it? From there, any adjustments to the scope's focus should also correct the OAG focus, or so I thought. Seems to work that way for me

 

Cheers,

Scott

Edited by Scott Badger, 27 March 2025 - 09:00 PM.

[Alex McConahay]

Once you first focus the scope/main camera on something, star, moon, tree, whatever, then focus the OAG/guide camera, shouldn't that be it? From there, any adjustments to the scope's focus should also correct the OAG focus, or so I thought. Seems to work that way for me

 

Cheers,

Scott

Yup......Once they both focus at the same time, you never need to adjust again as long as you keep them together (or reassemble them the same way). 

 

Moving the focuser focuses both of them.

 

Alex 

[klangwolke]

Yep, that’s it, I’ve got a helical on mine because I can’t stop myself from tweaking the focus every now and then, smile. Wonder if they got it working?

[AstroFromHome]

Yup......Once they both focus at the same time, you never need to adjust again as long as you keep them together (or reassemble them the same way). 

 

Moving the focuser focuses both of them.

 

Alex 

Usually it should work. I had the case that I focused the main camera on a building about 1 mile away and then also brought the guide cam in focus on the same building.

During the night when I wanted to start my session nothing to see in the guide camera via the OAG. That's why I don't do this anymore at daytime. Since then the very first focus of a new OAG and guide cam combination is always towards the stars.

Just set the exposure to 0.5 sec or lower, raise the gain and you find stars while slowly moving the guide camera outwards. At the best point lock the camera in place and work with the helical focuser.

Does not take more than 5 minutes.

[Alex McConahay]

Just set the exposure to 0.5 sec or lower, raise the gain and you find stars while slowly moving the guide camera outwards. At the best point lock the camera in place and work with the helical focuser.

Yes, during the day it is hard to get perfect pinpoint focus.  You need stars for that. 
 

One thing to add to your suggestion though is that you first need to focus your main camera before adjusting the guide camera by doing what you recommend. 
 

Alex 

[HubSky]

 

If you are at f/9, then you are at 1390mm.  You will almost always have trouble finding guide stars using a small prism of that OAG and the small sensor of a 120.  Even for me at 1440 FL with a 13mm prism and a 174mm camera I may only get one or two guide stars especially during galaxy season where there is often not many stars to begin with.  The 120 can be a very good guide camera on a guide scope, but it is in my opinion an awful camera for an OAG with long focal lengths due to its tiny sensor.  

[macdonjh]

If you are at f/9, then you are at 1390mm.  You will almost always have trouble finding guide stars using a small prism of that OAG and the small sensor of a 120.  Even for me at 1440 FL with a 13mm prism and a 174mm camera I may only get one or two guide stars especially during galaxy season where there is often not many stars to begin with.  The 120 can be a very good guide camera on a guide scope, but it is in my opinion an awful camera for an OAG with long focal lengths due to its tiny sensor.  

Very good point, I had similar troubles, x3.5.  Some of the posts here provide some details: https://www.cloudyni...-focal-leength/

 

My solution to that problem is an on-axis guider, which I got for Christmas.  I finally installed it and got it working in February and haven't had trouble finding guide stars since.  Another solution I considered was a rotator.  Manual rotators are inexpensive, but every time you change your camera's orientation you'd have to recalibrate your guiding software, which was too frustrating for me to contemplate.  I also considered I'd still have a tiny FOV for my guide camera, my prism would still obstruct my imaging chip, and still be restricted to looking for guide stars around the periphery of the field.  The OnAG  provided more benefits for about the same expense in my case. 

 

Back to the original topic: if X1m wants to use either an OAG or an OnAG, he'll still have to figure out his focus, and maybe field of view, troubles.  There have been several posts here with good advice and methods to do this.  One other recommendation: I pointed at various open clusters when getting my OAG set up.  That way I had high-percentage chances of having guide stars within the tiny field of view of my OAG.