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Guiding without a guiding scope/camera

Astrophotography Imaging Software
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#51 Midnight27

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Posted 11 December 2022 - 05:54 PM

Let me give it another shot :-)

First, am sticking to current camera technology.

Second, definitions.

"Guiding" (to me), is making minor mount movements during a main exposure to ensure that the mount movement over the length of the exposure stays within (say) a pixel. Main objective -keep single subs sharp.

"Recentering" is after a frame has been taken - it plate solves the previous image and moves the mount so that the target goes back to the centre for the next image. Main objective - keep target incentre over a session so that you don't get huge black borders after stacking, recover from a wind gust, animal bump, or whatever.

With typical set of ZWO/QHY etc hobby astro cameras, guiding and imaging simultaneously with the same camera is not possible.

What you can do, in NINA, Sharpcap, Firecapture and others, is recentering with the same camera. You do need to set a tolerance for drift - after which recentering is triggered.

If the frame exposure is long, say 2 min like DSO, you can recenter with the same camera but the mount will get a correction signal only after 2 min. It has no way of correcting within those 2 min.

If the frame exposure is very short, like 10ms in planetary, Sharpcap or Firecapture will recenter much faster, in maybe a hundred or milliseconds (depending on tolerance set). Because of the very short times involved, this may be practically indistinguishable from guiding, but is technically recentering as the mount move happens only after the previous frame is analysed and not during an exposure.

Planetary capture has another feature called ROI centering. here the software moves the ROI to ensure the planet is centered in it even if it is moving all over the sensor. Again, it will look like the planet is steady in the centre, but this is a third animal, neither guiding nor recentering.

Hope that helps :-)

Novice here, however I'm hoping to do something along these lines as mentioned in the original post.

 

I have an idea and not sure if it will pan out but going to try it when my equipment arrives (AP 1100).

 

I've read some people saying that with a high end mount and good camera it may be possible to do unguided exposures for up to 2 minutes or longer.  Is this correct?

 

I think this would work best if you can pre-build a model of the sky area you are intending to transit over so that any mount variations during the viewing session can be taken into account "in advance".  Luckily there is exactly such a plugin for NINA called Astro Physics Tools (for AP mounts, see and  https://www.youtube.com/watch?app=desktop&v=JMcEd1_FpjI ).  By modeling the section of sky in advance and putting that model into the software it can predict the guiding needed and do it while doing exposures.  Then you can also plate solve and recenter between exposures if needed.

 

Secondly, by using a good camera is it possible that you don't need 2 minute exposures?  Can you get by with 30 sec or 1 min exposures?  I'm not clear on this, however why can't you just combine 4 or 2 such images and get effectively the same as a single 2 minute exposure?  There are probably some "reasons" I'm not aware of.

 

Currently I don't have a guide camera.  I'm hoping to somehow get by without one, not because of the cost but because it seems wonky to me, especially if using a whole other scope which isn't inline with the optical train.  I may add an OAG setup if I need to, but first I thought to try and figure out something more elegant.

 

Does anyone have advice on software that works better in combination with guiding software so that the problem of one program "grabbing control" of the camera is not an issue?  For instance PHD2 is using the camera, then if I connect the camera in NINA it said connection to camera is lost in PHD2.  I guess the expectation is you use a separate camera in PHD2, but is there some way around this using a pass-through plugin or something so both systems can see the same picture?



#52 Alex McConahay

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Posted 11 December 2022 - 06:20 PM

Last June, Richard Wright did an interesting show on The Astro Imaging Channel which discussed "Why Guiding Needs to Die and How It Is Going to Happen."

 

https://www.youtube....h?v=p4bpFJSdHDY

 

It discussed many of the ideas you are talking about in post #51

 

Alex


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#53 WadeH237

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Posted 11 December 2022 - 08:27 PM

I've read some people saying that with a high end mount and good camera it may be possible to do unguided exposures for up to 2 minutes or longer.  Is this correct?

The AP1100 itself is capable, especially when you use a tracking model with APCC Pro.

 

Be aware, though, that even if the mount's tracking is absolutely perfect, it's possible (or even common) that microscopic flex, or other mechanical shifting, in the system can make unguided imaging work poorly.  In particular, SCT's don't work well, since the primary mirror tends to move enough to cause issues.

 

Rather than go into details here, I would suggest that you start a new topic to discuss this.  This topic is about doing actual guiding without a separate guide camera.


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#54 freestar8n

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Posted 11 December 2022 - 08:39 PM

This came up recently in another thread - but in principle you can take a normal long exposure and then take another one and use the plate solves to know what correction rates are needed in RA and Dec to keep the target from drifting during an exposure.  And then keep adjusting as you take more exposures and do plate solves - to hone in on it and dynamically adjust over time.

 

But the killer for mid-range equipment is that it is very hard to move accurately in dec. at a slow rate.  RA isn't really a problem because it is always moving and this would just slightly alter the rate.

 

So this is an example of a "blind" guiding method that isn't actually getting info from the stars during an exposure.  And any slop in dec. will make slow corrections a challenge.

 

If instead you have a high end mount, then you should be able to model the sky well so you don't need to guide or do any of this - at least for an acceptable result.

 

It is at least feasible and I don't know anyone who implemented it like this.  It may be worth a try - if it is well polar aligned there may not be much dec. drift.

 

These various approaches are much more viable with low noise cmos cameras that don't require a long exposure.

 

But for best results I would always guide.

 

Frank


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#55 WadeH237

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Posted 12 December 2022 - 09:31 AM

This is a reminder to please stay on topic.

 

I have removed a number of threads discussing unguided imaging.  This thread is about guiding without a separate guide camera.


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#56 dciobota

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Posted 13 December 2022 - 09:58 AM

So, it can be done. Sorta.

As mentioned, you cannot read an image during exposure, so the only time you can guide is in between exposures.
Guiding commands need to be around 1-3 seconds in order to properly correct tracking errors.
Traditionally, exposures are much longer for two main reasons. One is to "swamp" read noise, and the second is to "swamp" sky noise. There are other noise sources but these two are important here. With increasing exposure time, the signal from the object increases faster than the noise from those two sources. This is why exposures are in the order of minutes, not seconds.
Also mentioned in these forums, and true to an extent, is that total imaging time is the driver, not necessarily the individual exposure times.
So, it's entirely feasible to set your main camera exposure time to say one second, then read those exposures by some guiding software while they're also saved on disk.
The disadvantages are twofold. One, you'll be saving and dealing with thousands of images when you calibrate and integrate. Not a problem if you have enough space and computing power, but then again, those cost money
Second big disadvantage is noise. Read noise gets generated with every image, so for a given total exposure the signal to noise of 1000 one second exposures vs say 10x100second exposures will be lower. There's a lot of math to show why that is true but I'm not going to go into that.
So has this been done anyway? Yep. There was a thread on short exposure imaging with some pretty nice examples of images done from 1 second exposures.
There was also a guy on here who wrote a piece of software that would take short exposures (I think it was one second) and integrate them on the fly line sharpcap live and also guide on them. It would only save the integrations on disk. I think you could set how long an integration to sacmve, say one minute. And his sample results looked pretty good

With today's low read noise cameras it is doable if the software is there. That guy never followed up with any public release, but I don't see why someone else can't do it.
Again, with this approach you will lose some information due to the increased noise, but if budget is tantamount and the software free, I see that as a viable alternative for beginners.

#57 dciobota

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Posted 13 December 2022 - 11:32 AM

Btw, for those of you that want to dive into the deep end of the very short exposure imaging (also called lucky dso imaging), take a look at this article:

https://www.cloudyni...y-imaging-r3220

 

There are actual advantages to doing short exposure imaging of say one second or so, despite some of the disadvantages.  And the article also mentions doing full image guiding, which means examining the full image, not just a few stars, to determine mount corrections.  I think this may be the same guy that posted the CN topic I was refering to earlier.

 

And actually, if you don't mind some image drift, with one second exposures you may not need to guide anyway.  That short exposure thread I mentioned before showed images taken on a tripod, and not just short focal length milky way images either.  One stunning one was the Rho Ophiucci complex, I think it was 135mm.  

 

So there are many possibilities nowadays, and hardware is finally getting to the point where such type of imaging becomes feasible indeed.




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