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Guide Scope?

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#1 rrbailey89

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Posted 03 October 2022 - 11:21 PM

Hello all, I have recently gotten my SV165 in the mail.. I plan to pair it with my AT60ED and my 533MC-Pro (imaging) and 174MM-Mini(guide camera).

What I actually really don't understand is what exactly will auto guiding do for me? The reason I purchased these things is because people here said "if your not guiding you should be". I have the EQ6 R pro and it seems to do a great job of tracking if properly polar aligned allowing me to take (IMO) very long exposures of 2+ minutes (I have not tried taking longer yet) and they stars show no trailing.

I suppose I'd just like to know what exactly I should expect from a guide scope, what the benefit is for astrophotography etc.

#2 sctbrd

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Posted 03 October 2022 - 11:33 PM

If your mount tracks well enough you may not need guiding. It tends to be for people that set it up to track a target all night, so if youre only doing a short period of time (1-2 hours), it isnt needed.

If you have a long focal length, and use high power eyepieces or barlows, and want to observe a smaller target, guiding will help keep it in the eyepiece.

 

Guiding doesnt necessarily apply to only photography, it also helps with observing.

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Have to be careful here- alot of people like what they have or feel that it works for them so everyone should use it (or if something doesnt, you should have it).

In this hobby, you have to work with what you have, figure out what YOU need, and take opinions here with an amount of thinking.



#3 Protheus

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Posted 03 October 2022 - 11:37 PM

Well, first, when not used off-label, say, as a normal, standalone telescope, the only use for a guide scope is photography.  This isn't the forum for that, so I'd expect the post to get moved to somewhere more appropriate.

 

That out of the way, the guide scope is generally strapped to the main Telescope and aligned much like a finder.  You frame your shot so that there's something in the guide scope you can keep in the same spot easily, you watch that (or have something watch it automatically), and you make (or have something make) minor corrections so that nothing drifts over time.  That's it.  What a guide scope gets you is a way to do this.  There are a couple other ways as well.

 

Chris


Edited by Protheus, 03 October 2022 - 11:38 PM.


#4 F.Meiresonne

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Posted 04 October 2022 - 02:34 AM

Guiding is needed for AP. Mounts have PEC, so their tracking  is compromised. With good polar alignment one can take exposures perhaps to 2 minuts, yes (i never succeeded, 90 sec was about my limit) but for longer exposures like 3 minutes or if one uses a mono cam even longer exposures are needed like 6-20 minutes and then your stars will notlook that good anymore.



#5 rrbailey89

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Posted 04 October 2022 - 03:24 AM

Guiding is needed for AP. Mounts have PEC, so their tracking is compromised. With good polar alignment one can take exposures perhaps to 2 minuts, yes (i never succeeded, 90 sec was about my limit) but for longer exposures like 3 minutes or if one uses a mono cam even longer exposures are needed like 6-20 minutes and then your stars will notlook that good anymore.


This might be what confuses me. To allow for longer exposures that implies that the auto guiding software... Nina or PHD2 will be moving the mount in addition to the mount moving itself... I don't understand how this extra movement (potentially in an opposite direction) wouldn't make images worse

#6 rgsalinger

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Posted 04 October 2022 - 03:50 AM

First of all, your mount has periodic error of at least 10 arc seconds during one revolution of the worm. That means if the revolution takes 5 minutes, then in a two minute exposure your stars will be elongated by 5 arc seconds. They will look awful. If you MEASURE one two minute exposure when guiding and one when not guiding the guided one will be measure better because some of the PE will be eliminated. That's the number one reason why we guide. 

 

Second, no polar aligment is perfect. That means that your mount cannot track without have some DEC axis drift. So, over the same period that will give you even worse looking stars. And the entire frame will drift over the course of a few hours.

 

Third, there is wind to consider. Wind will blow the mount off target and without guiding it will never return to the target. Same with minor cable pulls - there's no way to correct for any movement that they might cause. 

 

Fourth, post a link to some full resolution images that are two minutes so we can see what you think is acceptable. That's another factor. Some of us are really really picky. Other's just want to get a picture and minor imperfections just don't matter. 

 

Fifth, if you are, as I am, imaging for 5-8 hours the cumulative tracking error of the mount will cause your target to move out of the frame. The movement of stars is not even across the sky due to diffraction. You won't notice that in a two minute exposure but over the course of a full night without guiding you'll have a lot of cropping to do. 

 

You are using a tiny short focal length scope and short exposures. Those two factors help cover up both the mount's PE and the DEC drift. The minute you increase your focal length or exposure time you'll see that's the case. The image scale will no longer cover up all these issues. With guiding you can do much longer exposures without those issue ruining them. 

 

What the guiding does is to move the mount so that all these errors are reduced to a minimum. Since some of these errors can push the system in different directions you need to make corrections in different directions. It won't make things worse. 

 

I sugesst that since you are a beginner (?) that you buy a copy of the Deep Sky Imaging Primer by Charles Bracken and join your local astronomy club. We are guiding because it's fun. We're doing it to get really high quality data from out systems. 

 

Rgrds-Ross 


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#7 acrh2

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Posted 04 October 2022 - 03:54 AM

This might be what confuses me. To allow for longer exposures that implies that the auto guiding software... Nina or PHD2 will be moving the mount in addition to the mount moving itself... I don't understand how this extra movement (potentially in an opposite direction) wouldn't make images worse

1) https://en.wikipedia...wiki/Autoguider

 

2) You need guiding for dithering, which is the preferred way of removing walking noise. 

https://www.cloudyni.../#entry10793724



#8 Drothgeb

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Posted 04 October 2022 - 04:16 AM

Hello all, I have recently gotten my SV165 in the mail.. I plan to pair it with my AT60ED and my 533MC-Pro (imaging) and 174MM-Mini(guide camera).

What I actually really don't understand is what exactly will auto guiding do for me? The reason I purchased these things is because people here said "if your not guiding you should be". I have the EQ6 R pro and it seems to do a great job of tracking if properly polar aligned allowing me to take (IMO) very long exposures of 2+ minutes (I have not tried taking longer yet) and they stars show no trailing.

I suppose I'd just like to know what exactly I should expect from a guide scope, what the benefit is for astrophotography etc.

Typically, to ensure round stars, you want drive error to be less than your imaging scale. Your image scale is around 2” per pixel. So you should get round stars at 2” drive error or less. The EQ6 is rated at +/- 5” over an 8 minute period. Based on that, there will be periods when you’ll be within acceptable drive error, and get round stars without guiding, but it won’t be with every exposure.

 

You said you don’t get trailing, but do you get round stars? And if they are round, are they small and tight, or are they a little bloated? Similar tracking errors for RA and DEC, will yield more or less round stars, but as the tracking error increase, the size of the star will increase, or bloat. The tracking error that leads to bloated stars, also lead to a loss of detail. That’s not good. 

Think of what happens when your shoot with a long lens at too slow a shutter speed. On a tripod the image is nice and sharp, but when you hand hold it’s just not as sharp. There aren’t necessarily trails, but is just isn’t sharp. That’s what guiding will give you, tighter stars and a sharper image. 


Edited by Drothgeb, 04 October 2022 - 04:19 AM.


#9 idclimber

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Posted 04 October 2022 - 07:25 AM

If your mount tracks well enough you may not need guiding. It tends to be for people that set it up to track a target all night, so if youre only doing a short period of time (1-2 hours), it isnt needed.

If you have a long focal length, and use high power eyepieces or barlows, and want to observe a smaller target, guiding will help keep it in the eyepiece.

 

This is simply incorrect information. As Ross and others have pointed out guiding is essential for deep sky imaging, even if you are only image for an hour. Any of the modern acquisition software is capable of re-centering the image without guiding. They are not related. 

 

Guiding is needed to correct errors in the mount during an interval in time that is shorter than the main exposure. These tiny corrections are typically measured and made every one or two seconds and are quite helpful for exposures as short as 30 seconds. This is needed if image scale is lower than the periodic error of your mount, which it is the case for the vast majority of imagers. 

 

There are premium mounts precise enough that guiding is not needed. They are a lot more expensive than the typical entry level mounts like the EQ6. They typically require software and modeling that corrects for what little error their drive systems have. There are limits though and these mounts may need guiding if the focal length or exposure is long enough. 

 

 

2) You need guiding for dithering, which is the preferred way of removing walking noise. 

https://www.cloudyni.../#entry10793724

 

You do not need guiding for dithering. All that is needed is a command to move the mount and time to allow it to settle. I do this routinely when doing unguided imaging with my 4" refractor. This is typically when capturing LRGB subs which are one or two minutes. I typically guide narrow band. 



#10 acrh2

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Posted 04 October 2022 - 08:27 AM

 

 

You do not need guiding for dithering. All that is needed is a command to move the mount and time to allow it to settle. I do this routinely when doing unguided imaging with my 4" refractor. This is typically when capturing LRGB subs which are one or two minutes. I typically guide narrow band. 

 

I stand corrected.



#11 TelescopeGreg

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Posted 04 October 2022 - 10:52 AM

What they all said.

 

Bottom line, the #1 rule:  This is a hobby where microns matter, and microns are really really really small.



#12 bbasiaga

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Posted 04 October 2022 - 11:01 AM

This might be what confuses me. To allow for longer exposures that implies that the auto guiding software... Nina or PHD2 will be moving the mount in addition to the mount moving itself... I don't understand how this extra movement (potentially in an opposite direction) wouldn't make images worse

It just sends a correction right along with the normal tracking signal.  I just set this up on my scope as well.  The mount is just tracking along,and may get told to bump West a little faster, or east a little to, but it knows to keep generally going West.  The guider just sends little guide corrections that add or subtract to the normal tracking signal.  Kinda like staying centered in your lane while going around a curve.  you might be turning right, but you can correct a bit to the left without losing track of where you are generally going. 

 

Brian


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#13 fuadramsey

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Posted 04 October 2022 - 02:26 PM

The op's setup has a focal length of 360mm. 

 

The EQ-6 is a pretty nice mount, with good polar alignment and assuming the mount is tracking well, and balanced, then guiding may not show much of an advantage for subs less than 5 minutes. -unless of course there are issues, which there usually is . . .

 

I have a CGEMII which is not as nice as an EQ-6 in my opinion, and it tracks pretty well (but I did hypertune it). But I can get 5 minute subs that look good with my Z61 (360mm focal length). I found this out by forgetting to start PHD2 during an imaging session. 

 

But since you have stuff in the mail . . . go for it! You'll eventually step up to more focal length and your standards will get higher so yes, you will eventually start guiding. And then everyone will tell you, you need to go OAG. Which you will once you need more precision or go with more focal length, and then a better camera and then a mono camera, and then the filters . . .

 

It'll never end! 




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