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OAG/Camera Can’t Focus

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#1 dum-dum

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Posted 25 September 2021 - 11:08 PM

My Celestron OAG and ASI290 mini were working just fine.

I then decided I wanted to add a barlow to my main camera, and installed it.

My was then unable to focus the ASI290 and just got a ton of noise.

For the life of me I cannot figure out what is wrong. Its almost like something is obstructing it. Frankly, it doesn't even seem like light is getting to it.

I have no idea where to start, so any help is appreciated.

Edited by dum-dum, 25 September 2021 - 11:08 PM.


#2 rgsalinger

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Posted 25 September 2021 - 11:53 PM

When you use a barlow not only does the focal length change but also the focal point. If memory serves me when you use a barlow, the focal point moves out. My guess is then that you don't have enough room to move the 290 away from the prism to get it in forcus. Remember you've also halved the light it's getting. When I start setting up an OAG I'm using 10 second exposures binned two so I can hopefully see something.

 

See this thread which seems to confirm my memory. 

 

Rgrds-Ross



#3 dum-dum

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Posted 26 September 2021 - 12:13 AM

Thanks, but the barlow is behind the OAG. Nothing should have affected the ASI290. There is nothing new in front of the ASI290.

So its Scope/OAG/Barlow/main camera, and the ASI290’sits in top of the OAG.

Edited by dum-dum, 26 September 2021 - 12:14 AM.


#4 t-ara-fan

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Posted 26 September 2021 - 01:11 AM

It would be smart-smart to say what kind of scope you are using.

 

I doubt when you added the barlow the main camera remained in focus. So you probably changed the scope's focus a bit to focus the main camera.  Guess what: that put the '290 out of focus.



#5 rkinnett

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Posted 26 September 2021 - 01:18 AM

The distance from the prism to each camera sensor needs to be identical or you'll never find focus on both cameras at the same time.  This is fundamental with OAGs.



#6 dum-dum

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Posted 26 September 2021 - 02:00 AM

The distance from the prism to each camera sensor needs to be identical or you'll never find focus on both cameras at the same time. This is fundamental with OAGs.

I haven’t tried it, but that has to be it. And of course {slaps head.}

So is it possible to use a barlow and an OAG? Would the barlow go in front of the OAG? Or, move to a guide scope?

Its an EdgeHD 11. Wonderful scope. With the reducer and Hyperstar, I really can’t think of too many objects you couldn’t use it on.

PS, thanks to everyone for helping. Its so much appreciated.

Edited by dum-dum, 26 September 2021 - 02:08 AM.


#7 imtl

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Posted 26 September 2021 - 02:09 AM

You have the EdgeHD 11'' and you want to add a barlow to that???

You have 2800 FL with the native F/10. How much more do you need? What are you trying to image?

#8 dum-dum

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Posted 26 September 2021 - 02:13 AM

Any small object.

#9 imtl

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Posted 26 September 2021 - 02:15 AM

Have you imaged with your 11'' at F/10?

Adding a barlow for DSOs is the wrong way to go. For planets you don't really need guiding much. So, where's the problem actually?

#10 dum-dum

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Posted 26 September 2021 - 02:34 AM

Oh yeah, I’ve done a ton with it. The Dumbbell is perfect for it, as are a tone of galaxies. But I’m still getting used to the OAG as I’ve never used one before. I’ve been doing some short exposure, no guiding on a CGX-L, and though not perfect, its not bad with the right post processing.

I also have an 80mm, 127mm, and 8”. But the 11” is my favorite. I don’t have a hyperstar, so wide field is out for now. I have plenty to play with right now so I’m in no hurry to get a hyperstar, or a reducer for that matter.

#11 imtl

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Posted 26 September 2021 - 02:38 AM

Got it. So, drop the barlow. You don't use a barlow for DSOs. You really don't want to try and image a faint fuzzy at F/20...

#12 MikeECha

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Posted 26 September 2021 - 10:18 AM

Thanks, but the barlow is behind the OAG. Nothing should have affected the ASI290. There is nothing new in front of the ASI290.

So its Scope/OAG/Barlow/main camera, and the ASI290’sits in top of the OAG.

But when you added the barlow you had to refocus the main camera right? And that changed the focal plane of the guider with respect to the scope. 

 

I bet that if you pull the 294 out with your hand while looking at say PHD screen you will see an inprovement in focus.

 

If that is the case adding a focal reducer on your guider will bring your 290 back inside the OAG holder.

 

Note now the equal distance for both cams may no longer be true. You would have to focus the main first and then find good/decent focuse with the guider, spacers and reducer. The amount of reduction you end up with is not that important for the guider.

 

I have one on my setup and works great.



#13 rgsalinger

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Posted 26 September 2021 - 04:19 PM

First, just to clarify. Has the OP actually focused the main camera yet? If not then that has to be done first. Then when you start trying to get the OAG in focus take long binned 2x images - I use 10 seconds and look closely at each shot as you move the guide camera in and out. 

 

Second, trying to do long exposure imaging at 5.6 meters of focal length is going to be difficult and frankly a waste of time. At the image scale with any camera I can think of you're way over sampled to the point where you cannot get more detail than you would at 2.8 meters unless you can put the telescope in outer space.

 

Finally, trying long exposure imaging at F20 will require very long exposures (like 30 minutes or more) to get sufficiently above the read noise.

 

The barlow is pointless. I image at a Bortle 3 site with 1.5 arc second seeing on good nights. Scopes about about 10" don't show more detail. Aperture is always helpful if you can match the image scale to the seeing, but more aperture just over sampling gets you nothing. 

 

Rgrds-Ross




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