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Autoguiding without separate guide scope and camera?

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#1 ajaxuk

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Posted 06 December 2019 - 03:04 AM

I'm a complete beginner with about a week of practice under my belt, so please bear that in mind.

 

The subject of autoguiding appears quite frequently and I think I now understood why it is useful. However, I am trying to figure out why it is not possible to use the same camera and lens used to take the "lights" instead of separate scope and camera. In my case I use a DSLR and a telephoto lens up to 300mm with an Ioptron SkyGuider that supports guiding on the RA axis only.

 

With appropriate software, couldn't the actual subs be used to provide a correction that would feed back into the mount?



#2 Stelios

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Posted 06 December 2019 - 03:25 AM

No. To be effective, guiding must be done at short intervals (typically 1"-5"). Your main camera is busy taking images at that time, and the exposure intervals will typically be of the order of minutes, not seconds.

 

Also, even if you were taking very short exposures with the main camera (a bad idea for many reasons, especially with a DSLR), the exposure would have to be complete in order for the software to analyze and act on it--so guiding would serve no purpose at all. Any deviation would have already happened and be on film. You might prevent a slight drift of the mount, but that is not an issue as stacking software (combining the sub-exposures into one long exposure) works by matching the stars and therefore all that the drift can affect is the edges.

 

Guiding keeps a star centered in the guide camera while the main camera is imaging. This prevents drift *while the shutter is open* which can make stars look like trails and blur detail.

 

If your mount only supports guiding in RA, then you need to get an excellent polar alignment to prevent drift in DEC. 


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#3 james7ca

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Posted 06 December 2019 - 03:26 AM

While you could certainly try something like you've suggested the main problem is that guiding corrections generally need to be done at a frequency (time between samples) that is pretty short unless you have a very good mount that has been calibrated with a pointing model (to correct for various tracking errors). So, assuming that you could find software that would issue guiding commands based upon measurements taken from a captured sub (one of the "lights") you might need to limit your exposure times to only a handful of seconds, since longer exposures of a minute or more would likely show tracking errors before you could analyze the next sub and issue the next guiding command. Kind of a chicken or egg thing, you have to issue a guiding command before any tracking errors get large enough to show in the "light" subs but you can't measure that error until the "light" has already been captured and presented for guiding analysis.

 

However, if you are just taking a lot of short exposures that may or may not show any tracking problems then you really don't need to guide since you can just register the subs during your post-processing which would hardly be any different than trying to guide during the actual capture session using the technique I outlined above.

 

So, what auto guiding with a separate camera allows is the issuing of frequent guiding commands (on the order of every five seconds or so) while the other camera continues with a longer exposure. Thus, you correct for tracking errors before they can appear in the "lights."



#4 ajaxuk

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Posted 06 December 2019 - 03:52 AM

Thanks Stelios and james7ca

 

Understood. The guding takes place while the main camera is taking the exposure. Makes sense now, thank you for explaining it to me.



#5 happylimpet

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Posted 06 December 2019 - 05:24 AM

But look up OAGs, or off axis guiders. They use the same scope/lens but a separate camera.



#6 rgsalinger

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Posted 06 December 2019 - 09:29 AM

If the OP uses a dslr and a camera lens then I don't see that an OAG is really an option. Even if you could find one that had proper Canon/Nikon/Sony flanges on both sides, wouldn't the back focus be off because the back focus is more or less fixed? 

Rgrds-Ross


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#7 TareqPhoto

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Posted 06 December 2019 - 10:26 AM

I have a problem with guiding as it keeps correcting, i don't know why, it always drift in guiding, even with perfect PA, i hope this thread can give me ideas about what's going on with guiding, someone told me i have to match guiding speed with the mount, i have zero idea about how to do that mainly because i don't use a hand controller but PC control.



#8 Stelios

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Posted 06 December 2019 - 12:18 PM

I have a problem with guiding as it keeps correcting, i don't know why, it always drift in guiding, even with perfect PA, i hope this thread can give me ideas about what's going on with guiding, someone told me i have to match guiding speed with the mount, i have zero idea about how to do that mainly because i don't use a hand controller but PC control.

Really you should start your own thread here, but... read *this* for your mount. This tells you how to set guide rate. You then set the guide speed in PhD2 to the same number. (The guide speed is set in Detailed Calibration Parameters--the "Advanced" button in the Guiding tab of the "brain" reveals this). 


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#9 halx

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Posted 06 December 2019 - 12:26 PM

To the OP. Don't forget about the manual guiding option! It is workable too as soon as you have some cheap visual scope with the focal length of at least 3x of your lens. The small guiding scope is cheaper than any OAG for a DSLR, it's the guiding camera (+ computer hookup) a chore and a PITA :)


Edited by halx, 06 December 2019 - 12:32 PM.

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#10 SonnyE

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Posted 06 December 2019 - 12:37 PM

If you get started now with a separate guiding system, when you advance into a different mount the tools can move with you.

My view on guiding was always that it gives my mount an "eye" on the sky so it can keep the mount tracking on a chosen object.

So I never questioned its value to the overall effort.

 

I did make the mistake of trying to use an OAG with my refractor for about a month. (I'm tenacious even when I'm doing dumb things. wink.gif lol.gif  )

When I sent back the two different OAG's I had tried, and got a guide scope, I made leaps and bounds of progress.

 

Still, as with all of this, there is a long steep learning curve. But a step at a time, and an occasional back and go around will get you there.

 

I'm going to be getting a new mount soon, but my rig from the clamp up can simply sit on top of the new mount and do all it has from before.

Using the same guiding I already have learned.

 

Just remember what Dory said, "Just keep swimming."


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#11 fewayne

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Posted 06 December 2019 - 12:55 PM

Just remember what Dory said, "Just keep swimming."

It's kind of scary how perfect a role model Dory is for imagers. Especially those of us who, in the cold, late, and dark, seem to share her memory issues.

 

Personally, I seem to be more like Bruce, though. "I'M HAVIN' M42 TONIGHT!!!" <CRASH> <RATTLE> <BANG> <tinkle>


Edited by fewayne, 06 December 2019 - 12:57 PM.

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#12 SonnyE

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Posted 06 December 2019 - 01:40 PM

It's kind of scary how perfect a role model Dory is for imagers. Especially those of us who, in the cold, late, and dark, seem to share her memory issues.

 

Personally, I seem to be more like Bruce, though. "I'M HAVIN' M42 TONIGHT!!!" <CRASH> <RATTLE> <BANG> <tinkle>

LOL!

Well, the 3 youngest Grandkid's and I have fun when they are here.

And Finding Nemo is a piece of the history.

 

I use to throw wrenches, and generally "Bruce around", too.

But I just walk away and put up my equipment if it decides to be a PITA anymore.

Which use to be every night.

 

Now that song is stuck in your head, too. LOL! lol.gif wink.gif



#13 TareqPhoto

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Posted 06 December 2019 - 02:50 PM

Really you should start your own thread here, but... read *this* for your mount. This tells you how to set guide rate. You then set the guide speed in PhD2 to the same number. (The guide speed is set in Detailed Calibration Parameters--the "Advanced" button in the Guiding tab of the "brain" reveals this). 

Sorry for hijacking it here, but i think i don't need to start anymore, i watched one video that really explained it better way, i think i rushed things and i missed few steps i must do in guiding with PHD, so now i got corrected i will try that next time and see if it works, later if nothing happen and i still have issues then i can start my own thread, so please i am sorry and apologizing about it here, keep going.



#14 *skyguy*

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Posted 06 December 2019 - 05:31 PM

Starlight Xpress used to sell a gizmo called the "Star 2000". It allowed some of their cameras to image and autoguide at the same time. Unfortunately, there was no free lunch with this device since the camera would image half the time and guide the other half of the time. This resulted in the need to double the overall exposure time to achieve the same image detail found in an unguided image.


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#15 psandelle

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Posted 06 December 2019 - 05:43 PM

Starlight Xpress used to sell a gizmo called the "Star 2000". It allowed some of their cameras to image and autoguide at the same time. Unfortunately, there was no free lunch with this device since the camera would image half the time and guide the other half of the time. This resulted in the need to double the overall exposure time to achieve the same image detail found in an unguided image.

Yeah, I'd always wished there were some other way to accomplish this. It was a neat idea in theory, not so much in practice.

 

Paul



#16 james7ca

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Posted 06 December 2019 - 06:59 PM

You'll eventually be able to guide with the same camera since things like the jot or Quanta image sensor will read out their data thousands of times per second. However, when that happens you really won't need to guide since you should be able to register all of the data during the post processing. That also means you won't have to track the target that well, as long as the target stays within the field of view of the camera and scope.

 

I suspect that this (or a similar sensor technology) will be the future of astrophotography, no guiding, no tracking (to speak of), just a stream of data that can be processed at a later time. But, this isn't going to happen within the next few years, maybe a decade or two in the future.



#17 klaussius

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Posted 08 December 2019 - 08:58 AM

I'm a complete beginner with about a week of practice under my belt, so please bear that in mind.

 

The subject of autoguiding appears quite frequently and I think I now understood why it is useful. However, I am trying to figure out why it is not possible to use the same camera and lens used to take the "lights" instead of separate scope and camera. In my case I use a DSLR and a telephoto lens up to 300mm with an Ioptron SkyGuider that supports guiding on the RA axis only.

 

With appropriate software, couldn't the actual subs be used to provide a correction that would feed back into the mount?

 

I have actually tried that. The most you can accomplish is something pretty much equivalent to PEC, where you can predict drift for the duration of the next exposure, and issue "predictive corrections" to compensate, then adjust the model based on the outcome. But that is only able to handle periodic error and PA error, kinds of errors that are predictable.

 

But other tracking errors that are less predictable will still happen and this kind of guiding isn't able to compensate. I'm currently using that technique and losing quite a few subs, especially for longer exposures of say 1'. When the model and PE get out of sync, I can lose half a dozen subs before it syncs up again.

 

That's why I'm going to start properly guiding soon. The technique is neat, but unable to match the performance of proper guiding with a secondary camera.




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