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OAG, do you always find guide stars? Need help on OAG.

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#1 Jinux

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Posted 04 May 2021 - 08:04 PM

Hi,

 

It's widely accepted wisdom to use OAG when you go with long focal, especially with SCT due to mirror flops. 

Despite of practically everyone's despise on my 3D printed finder converted guider, it works pretty well for my setup, I think. :)  But there is always something to make it better, right? So, I'm doing another push on OAG, which I tried several times but has been miserably failing.

This time, it went kind of okay. Find focus(ish - how do you confirm it's really focused? Bhatinov mask is impossible to use. Any tips?) and PHD2 found handful stars for multi-star guiding. 300s sub shows good round stars. Guiding numbers are worse than guider setup but image it takes matters not PHD2 number. So I kept going. 

I can't find it apparently better than guider but at least it was good start. 

 

Problem happened when I tried to setup for overnight automation. Loaded auto sequencer in NINA and started. Plate solved, centered, focused, all good then failed to start guiding. Why? There is no stars detected. The target was on deserted area. Increased PHD2 time to 5sec. Still no stars. Increased to 15sec, now barely faint stars showed up and I could start guiding. Typically this could be resolved by rotating OAG to find guiding stars but I can't do that for overnight automation. 

 

Prescription I can expect would be to use bigger prism OAG (currently ZWO 8mm one) with bigger sensor guide cam like 174mm (currently 290mm). And better focus might reveal some fainter stars, I guess. Would this really lead to 'always' finding guide stars? 

Probably this situation was aggravated by dualband NB filter for OSC but I can't image without it under B6+ sky. 

 

Can you share your wisdom on using OAG for long focal AP? Currently it's at F=1780 (C11/0.63)

 

Thanks!

Jinux



#2 dswtan

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Posted 04 May 2021 - 08:24 PM

Intuitively, you are being over-ambitious with that setup. It’s a different topic, but see thread below for some good examples for why a larger sensor guide camera is warranted in your scenario.

 

FWIW, I use a 174MM at 1135mm with no issues with 6nm filters *in front of the imaging camera only, NOT the guidecam* (and large prism Celestron OAG), but I would never expect a 290MM to reliably see stars. 

 

https://www.cloudyni...ag-sensitivity/


Edited by dswtan, 04 May 2021 - 09:38 PM.

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#3 davidmalanick

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Posted 04 May 2021 - 08:24 PM

The 174 camera is incredible! So glad I spent the money on it.

In a short time in this hobby I've learned to spend on the best right away or you'll end up doing it later and wasting money on the initial costs.

Here is a shot from the first night I was working on focusing the OAG.

With a QHY-M OAG (8mm Prism), not intruding in frame at all.

Untitled

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#4 KTAZ

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Posted 04 May 2021 - 09:04 PM

I have a C9.25 XLT and use the 290MM on the Celestron OAG and have never failed to find stars at 1350mm f/5.7. I routinely run through meridian flip and have never had PHD2 fail to find a star after the flip.

 

Is it realistic to expect that to never happen? Of course not. But for now, I am perfectly happy the C .63 reducer, the 290mm, and the COAG.

 

The small prism on that ZWO is definitely a problem for you, but so is the FL. The longer the FL you are using, the harder to find stars. If I had a C11, I’d probably be opting for the 174 as well.


Edited by KTAZ, 04 May 2021 - 09:06 PM.


#5 Alex McConahay

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Posted 04 May 2021 - 09:12 PM

While the OAG fanboys (like me) claim that you can always find a guide star, it ain't necessarily so. There are a few (very few) targets that do not offer some guide star near enough. (I'm talking about maybe every couple of years I cannot find a guide star just by slewing to the target, no fiddling around.) 

 

In order to make it work, you need a good guide camera. One with sufficient sensitivity. 

 

You may need longer exposures. Although, really, PHD2 does not take much.

 

You may need to rotate the OAG to find a guide star (I never have!!!). 

 

You may need to move the target a few bits left, right, up, or down to find a guide star. I do this maybe once every two years--what I do when I cannot find the guide star mentioned above. I have never lost more than maybe five to ten per cent of the original field of the main imaging chip, and I always have enough room for the target on the main imaging chip.

 

Most importantly, if I read you right, you need to put the OAG in front of the filters, especially any narrowband filters. You sound like you have put it behind a dualband NB filter. That will do it absolutely no good. 

 

 

>>>>>>>>how do you confirm it's really focused?

Check out the star profile and the FWHM readings in PHD2. They should be a nice pointy thing (profile) and the smallest number possible (FWHM) when focused. The hard part is making sure that happens at the same time your main imaging scope is also in focus. May take some patience. But, once done, it is done for good.

 

 Good Luck. OAG is the right choice for any reflector system, and many refractors.

 

Alex


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#6 dswtan

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Posted 04 May 2021 - 09:36 PM

Most importantly, if I read you right, you need to put the OAG in front of the filters, especially any narrowband filters. You sound like you have put it behind a dualband NB filter. That will do it absolutely no good. 

Alex makes a great point (and the other things he said :-)), and I inadvertently misled in my own post — I’ll edit.



#7 Jinux

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Posted 04 May 2021 - 10:11 PM

Thanks for the nice tips! 

NB filter is the obviously the one I should try but it's not that simple matter when we're actually doing it.

My image train looks like

 

SCT 3.25" -- Baader 3.25 to 2" Click-lock --(slide in) -- (2" filter threaded) Starizona 0.63 - M48 - OAG - M48- rings - M42- Camera

 

And I don't have filter slider to put in...

Ah... I just have an idea. I have Hyperstar filter slider which has M48 in front and M42 on the back. So I can put this in place of M48 - M42 ring area with other rings to keep 90.3 backfocus. Ok. I can try this tonight.

 

 

Check out the star profile and the FWHM readings in PHD2. They should be a nice pointy thing (profile) and the smallest number possible (FWHM) when focused. The hard part is making sure that happens at the same time your main imaging scope is also in focus. May take some patience. But, once done, it is done for good.

Theoretically yes. And I've done that with guider and moved to B-mask. The problem of SCT's OAG is that there is huge astigmatism and coma on off-axis after FR/FF. So there is no such thing as pointy stars. But I got that idea. Maybe filter location makes some difference. 

 

The latter part is not that difficult in practice. Just use auto-focuser on main camera then only use OAG focuser to adjust OAG camera focus. So can make both at the same time easily. 



#8 bobzeq25

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Posted 04 May 2021 - 10:32 PM

Answering the title question.  Always.

 

I've _never_ not had my choice of guide stars, even far away from the Milky Way.  The reasons are simple, it's just that people would rather struggle than spend a bit of money.  I'm old, have not a lot of money, but even less time.  <smile>

 

174 camera.  Celestron OAG (big prism).  The OAG is the first thing in the optical train after the telescope.  No problems.

 

My "guiding" philosophy in everything I do in DSO AP.

 

This is hard.  If I can make it easier, that's what I'm going to do.


Edited by bobzeq25, 04 May 2021 - 10:32 PM.


#9 Jinux

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Posted 04 May 2021 - 10:39 PM

 

I can try this tonight.

Change of plan. Tonight is the first night with 'excellent' seeing in this year. I can't blow the night with experiment.

I'll update next week how it went.



#10 freestar8n

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Posted 05 May 2021 - 03:11 AM

Not only do I have a star but it is bright enough to guide on with video - at 5-10 fps.

 

I do what used to be fairly standard practice - and that is to rotate the OAG to find the best available guidestar.  The Hubble telescope does the same - except it needs two guidestars to avoid rotation.

 

I used to do it manually with a field of view  indicator (FOVI) in TheSky that showed where the oag sensor is relative to the main sensor.  I had degree markings on the OAG and I could dial in the star - and there it would be once I centered the target.

 

Now I use a rotator and I have software to automatically set the rotator for the best star so I don't have to think about it.

 

And I also use a large prism and 174 sensor.  I got one for the purpose as soon as they were available in 1.25" format many years ago.

 

But even then - since I mainly want stars 9th mag or greater, sometimes they are hard to find.  I may need to shift the scene a bit and select the star semi-manually.  But for people who are willing to expose 5, 10 even 15 seconds for a guidestar they wouldn't need to do that.  I'm aiming for more like 100 ms.

 

Frank


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#11 michael8554

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Posted 05 May 2021 - 05:47 AM

I have a 8mm OAG on a 1400mm SCT.

 

The OAG is not located as close to the camera as it could be due to a Coma Corrector:

 

Canon - Coma Corrector @ 57mm - OAG - SCT microfocuser - SCT

 

I plan my sessions with a FOV in my Planetarium, rotating the whole OAG plus camera:

 

FOV02.JPG

 

The image is accurately showing  the spacing that gives correct target and guidestar positions and least shadow, it only appears to be too far from the imaging sensor on the planetarium

 

Note so although the prism was adjacent to the top (north) edge of the imaging camera, in fact I found Vega to be south of the imaging camera FOV !

 

So watch out, that's optics for you !

 

I have just received the TS Optics thin OAG with a Canon bayonet, so I can now go Canon - OAG - Coma Corrector @ 57mm - SCT Microfocuser - SCT

 



#12 Jinux

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Posted 05 May 2021 - 02:34 PM

Thanks for all great tips but anything involves 'manual' or 'tweak' can't be applied to my setup.

Even though the mount is sitting on backyard, I treat it as 'remote' observatory. I need full automation process overnight and can't tweak it as targets can be anywhere in the sky. I take several targets over month night after night, so repeatability without changing anything on image train is really important, otherwise I have to deal with multiple flats which takes my manual process next to the scope and increased the amount of data & process. 

 

So, I'm taking larger prism route by picking up Celestron OAG - scored last one on Amazon probably from Adorama and I didn't like ZWO OAG anyway - and moving NB filter next to camera using filter slider.

90mm backfocus of FR/FF is long enough to allow non-mini version of guide camera on OAG, so I can try 178 instead of 290. It's 2x sensor area than 290 not as big as 174 but Celestron OAG can't fully illuminate 174 anyway. 

All in next week. 

 

Thanks for your great advices!

Jinux



#13 AstroBruce

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Posted 05 May 2021 - 05:05 PM

Have you tried binning? With an OAG, you are guiding at the same focal length as your imaging camera. Binning won't hurt your guiding accuracy.

 

Bruce



#14 NoWhereButOutThere

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Posted 05 May 2021 - 05:05 PM

Curious that I see people lauding the ASI174 for guide camera.   I picked in the ASI290 because other threads/forums emphasized its increased sensitivity, even though more expensive.    But I did not expect (read: did not research) the sensor size implications for the ASI290.  It's a narrow field, has led to challenges.  Bortle 7/8 does not help.  Tracking is picky depending on my target.



#15 Jinux

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Posted 05 May 2021 - 05:41 PM

Have you tried binning? With an OAG, you are guiding at the same focal length as your imaging camera. Binning won't hurt your guiding accuracy.

 

Bruce

At F=1760, binning is a "must". PHD complains if I don't. At 2x2binning, image scale is 0.7arcsec/px. I think it's too much and I may try 4x4 binning.




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