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Manual Guiding Technique and Tolerances

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#26 rwiederrich

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Posted 14 February 2006 - 09:17 AM

And with the help of a friend of mine, hand guided M3

Rob

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#27 928rob

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Posted 29 August 2006 - 12:51 PM

CF,

I have a question regarding having a guidescope.

I have just gotten into this hobby and purchased a Celestron CPC800. I have gone through much of the CN Forums and have read your FAQ about manual guiding.

I am wondering if the Orion scope you had mentioned required some kind of counterbalance and is there any problem with a guidescope being physically longer than the scope used for the imaging ?

Perhaps it is a very stupid question, but I was wondering about it. Also, what is the reason for choosing the focal length of the guidescope as it is?

Thank you very much for your time.

Rob

#28 ClownFish

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Posted 29 August 2006 - 02:00 PM

Hi Rob

I had to add an extra 10 pounds of counterweight to my RA axis to counter the guidescope. You can have a guidescope physically longer then the main scope, many do.

Your goal in guiding is to catch errors in tracking BEFORE they show up in your image. So.. you need to see the error before the camera does, which is why you must guide using a much higher magnification than the imaging system. My camera scope is 812mm focal length, and at prime focus with my camera that is about 16X. My guidescope is 912mm and with a 5mm eyepiece that puts my guideing at 182x or 11 times the imaging scope!

CF

#29 928rob

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Posted 29 August 2006 - 02:41 PM

CF,

Thank you very much for the explanation. That clears it up nicely.

Rob

#30 ClownFish

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Posted 29 August 2006 - 03:38 PM

For those who want to try out Manual Guiding from the comfort of their home, check out my GUIDING TUTORIAL on my website. There's animations and even a simulator to let you try your hand at guiding.

CF

#31 rwiederrich

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Posted 29 August 2006 - 03:42 PM

Hi Rob

I had to add an extra 10 pounds of counterweight to my RA axis to counter the guidescope. You can have a guidescope physically longer then the main scope, many do.

Your goal in guiding is to catch errors in tracking BEFORE they show up in your image. So.. you need to see the error before the camera does, which is why you must guide using a much higher magnification than the imaging system. My camera scope is 812mm focal length, and at prime focus with my camera that is about 16X. My guidescope is 912mm and with a 5mm eyepiece that puts my guideing at 182x or 11 times the imaging scope!

CF



CF,

It appears we image to guide at the same ratio. 11.

Mine: Image, 990fl/camera=20X
Guide, 2286fl/10mm=228X Ratio=11

You: Image, 812fl/camera=16X
Guide, 912fl/5mm=182X, Ratio=11

Would it be better if I used a 2x barlow along with the 10mm IR EP? That would give me 456x, or nearly 23X.
:question:

Rob(the other)

#32 ClownFish

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Posted 29 August 2006 - 04:12 PM

The more magnification you guide at, the larger tracking errors appear in your guiding reticle. This boils down to comfort vs tolerance. Higher magnification allows you to allow small errors occur (what we call guiding tolerance), but it's not a good habit to get into. On the other hand, I do not guide as accurately when piggybacking a 135mm lens vs prime focus at 812mm. There's no need.

Some try to find a guiding magnification to let them stay comfortably within the guiding reticle's box, allowing any movement of the guidestar within the box to go without correction. At the 11X ratio with my setup, I try to keep the guidestar behind a line the whole time.. never letting it peek out for more than a split second. I probably could go 1/4 of a box without the error showing up, but I like to err on the side of over-correcting vs under.

Adding more magnification also means less comfort at the eyepiece. You end up with a dimmer star, and of course a narrower field of view - which makes finding a guidestar that much more difficult. Also, eye relief suffers. Like I said.. it boils down to comfort and tolerance.

Seeing plays an important part too. You can only guide as well as seeing will allow. When the star is jumping all over the place, give it up and try later in the evening or another night.

The argument to support higher magnification is that the better your guiding, the finer the resolution of your image will be. Small tracking errors destroy resolution your tiny stars blur and very fine detail is lost.

I like to think of my guidescope as a MICROSCOPE. I am peering down into a microscope at my film, watching the tiny white spec of light burning an image into a single grain of film. The more the star is allowed to move, the more blur I am allowing.

The 11X ratio works for me. Each astrophotographer has to find the ratio that works best for him/her.

CF

#33 928rob

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Posted 29 August 2006 - 05:24 PM

CF,

Well you sold me. I also like your attitude towards manual guiding. That is just the kind of discipline I would like to acquire (being new to the hobby).

I just ordered the Orion quidescope set to add to my CPC800.

Rob

#34 rwiederrich

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Posted 29 August 2006 - 05:30 PM

The more magnification you guide at, the larger tracking errors appear in your guiding reticle. This boils down to comfort vs tolerance. Higher magnification allows you to allow small errors occur (what we call guiding tolerance), but it's not a good habit to get into. On the other hand, I do not guide as accurately when piggybacking a 135mm lens vs prime focus at 812mm. There's no need.

Some try to find a guiding magnification to let them stay comfortably within the guiding reticle's box, allowing any movement of the guidestar within the box to go without correction. At the 11X ratio with my setup, I try to keep the guidestar behind a line the whole time.. never letting it peek out for more than a split second. I probably could go 1/4 of a box without the error showing up, but I like to err on the side of over-correcting vs under.

Adding more magnification also means less comfort at the eyepiece. You end up with a dimmer star, and of course a narrower field of view - which makes finding a guidestar that much more difficult. Also, eye relief suffers. Like I said.. it boils down to comfort and tolerance.

Seeing plays an important part too. You can only guide as well as seeing will allow. When the star is jumping all over the place, give it up and try later in the evening or another night.

The argument to support higher magnification is that the better your guiding, the finer the resolution of your image will be. Small tracking errors destroy resolution your tiny stars blur and very fine detail is lost.

I like to think of my guidescope as a MICROSCOPE. I am peering down into a microscope at my film, watching the tiny white spec of light burning an image into a single grain of film. The more the star is allowed to move, the more blur I am allowing.

The 11X ratio works for me. Each astrophotographer has to find the ratio that works best for him/her.

CF


Good advice.

OK, I have a question.

If the guide star falls from behind a reticle line, and you put it back, does your correction show?

What I mean, is when you move the star back does your scope jump slightly to move the star?

I am experimenting with a new powerstat control for my clock.

I can manually speed up or slow down the clock. I wondered if I can create a lap controller that I can control the clock motor, via a dial wheel, while I am looking through the EP.

Remember, my scope has a seporate RA drive, and clock drive, so I can either move the RA drive, or simply speed up or slow down the clock itself.
I'm trying to find the simplest solution, before I take another roll.

Rob

#35 ClownFish

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Posted 29 August 2006 - 05:57 PM

The best way to guide is to have a system setup that allows you to both STOP the RA drive and to speed it up x 2.

Stopping the RA drive effectively BACKS up the star, as the Earth moves untracked.
SPEEDING up the mount x2 makes the star move FORWARD at a very slow speed.

You can see this on my simulator too. Set the guidespeed to #1 (guiding speed) and watch both the screen and the motor drive's gears.

The scope should NOT jump at any time. The guidestar should simply move forward or back at the same speed. When you release the button, it should STOP, which in reality means the drive reverts back to normal RA tracking speed. This is also what I see on the LXD75 and every other tracking mount I have used.

As far as the correction showing.. if it did, then I am wasting my time! You have to be able to see those errors, and correct them, without any noticeable effect being recorded. This is why you guide at a higher magnification.

Like I said before, think of the guidebox as one giant film grain. As long as the star stays in the grain, you will never see the error. But of course, the size of that allowable error is based on magnification ratio. If you have a high enough ratio then what I just said is true. If you do not then you must only use a portion of the guiding box. At the 11X ratio, I would stay within 1/4 of a guiding box, not the whole thing.

I think I need to create a simulator that shows a magnified piece of film so you can see how this works in detail.

CF

#36 ClownFish

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Posted 29 August 2006 - 06:02 PM

Rob... hold up!!! Do you have a FORK mount with that SCT?
If so.. do NOT buy that Orion guidescope! It's WAY too long for a fork mounted SCT!

CF

#37 rwiederrich

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Posted 29 August 2006 - 06:19 PM

The best way to guide is to have a system setup that allows you to both STOP the RA drive and to speed it up x 2.

Stopping the RA drive effectively BACKS up the star, as the Earth moves untracked.
SPEEDING up the mount x2 makes the star move FORWARD at a very slow speed.

You can see this on my simulator too. Set the guidespeed to #1 (guiding speed) and watch both the screen and the motor drive's gears.

The scope should NOT jump at any time. The guidestar should simply move forward or back at the same speed. When you release the button, it should STOP, which in reality means the drive reverts back to normal RA tracking speed. This is also what I see on the LXD75 and every other tracking mount I have used.

As far as the correction showing.. if it did, then I am wasting my time! You have to be able to see those errors, and correct them, without any noticeable effect being recorded. This is why you guide at a higher magnification.

Like I said before, think of the guidebox as one giant film grain. As long as the star stays in the grain, you will never see the error. But of course, the size of that allowable error is based on magnification ratio. If you have a high enough ratio then what I just said is true. If you do not then you must only use a portion of the guiding box. At the 11X ratio, I would stay within 1/4 of a guiding box, not the whole thing.

I think I need to create a simulator that shows a magnified piece of film so you can see how this works in detail.

CF



No, I get you, I am working out my drift issues.
I don't have a drive like anything you currently use, or have ever used. So I have to retranslate what you say to me, so I can input the data into my own system, so it works for me, and my system.

Currently I see a regular mag 4 star in my reticle box as roughly half the box size. So if I watch it, and guide it to remin in the box, them from what you say, my camera arrangement will never see the error or the correction?

So If I go to using a 2x Barlow, making my mag 456x, then that same star will take up the whole of the box. Now if that star begins to drift outside the box, I can simply keep the whole star tucked in the confines of the box.

Theoretically my ratio is roughly 22x, meaning I could let the star drift nearly half way out of the box, before any apparent detection is done by my camera scope????
In your assessment is this close to being true????

Rob :smirk:

#38 ClownFish

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Posted 29 August 2006 - 06:27 PM

The star shouldn't change in size.. It's not an extended object like a planet.

What will happen if you go to a higher magnification is that any error in tracking will become more obvious. So for example you have a 1 arc sec worth of error. In a low magnification you will never even see it. In a high magnification it will be very easy to spot. The guidebox doesn't change, only the DISTANCE that a star will drift does. In a low mag setup, 1 arc second may take up 1/16th of a box, while in a higher magnification it will cover the whole box.

I do not know why your star mag 4 star takes up 1/2 a box in size, unless it's not in focus.

Ok.. it's 1:27 AM here.. time for bed! Yawn.... Night!

CF

#39 928rob

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Posted 29 August 2006 - 07:26 PM

CF,

OK... order has been cancelled. I have much more to learn. I will check with Losmandy regarding the mounting as well as the counterweight setup.

What do you think would be the best focal length guidescope for use on the CPC800?

Rob

#40 ClownFish

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Posted 29 August 2006 - 11:59 PM

Losmandy?????
I thought you had a CPC800 XLT on a form mount?

Rob.. we already discussed the need to have a long focal length fore MANUAL guiding. If you are autoguiding you can use 1/2 the focal length.

Since your SCT has such a long focal length, you need to either:

1. If mounted on a FORK mount, use an Off-Axis Guider.
2. If mounted on a FORK mount, use a short tube 80mm guidescope and autoguide
3. If mounted on a FORK mount, use a another SCT or MAK with a longer focal length.
4. If mounted on an GERMAN EQUATORIAL MOUNT (GEM) use the long refractor guidescope like the Orion.

CF

#41 928rob

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Posted 30 August 2006 - 11:24 AM

CF,

When I spoke to Orion regarding canelling my order, the tech guy mentioned that Losmandy had some nice dovetail systems made especially for the Celestron 8's, as well as a good counterweight setup.

I will probably look into (like you said) a short-tube guidescope.

Rob

#42 BWDenver

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Posted 08 October 2006 - 05:13 PM

CF Hypothetical,

If I’m using a camera and a 400mm lens, could I mount the camera setup on the counterweight shaft, at the same distance as the OTA center of view and not run into field rotation problems if I guide with the C8 OTA?

Bryan

#43 ClownFish

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Posted 08 October 2006 - 06:17 PM

If there is not too much difference between the guide-star Declination and the target declination. This holds true regardless of where you mount the camera. With only a 400mm lens, you should do quite well to simply be in the same area of the sky as the guide-star.

It does not matter how far from the tube you mount the camera. It can be mounted anywhere on the platform, from anywhere on the scope to anywhere on the counterweight shaft.

The one thing to watch out for is FLEX. Be sure everything is very tight. If there is any flex, it will show up quite easily. Watch out for mirror flop in the C8 too. If you see a sudden, unexpected movement of the guide-star, it probably means the C8 mirror shifted, so you should stop the exposure and start again,.

CF

#44 rwiederrich

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Posted 09 October 2006 - 01:55 PM

It does not matter how far from the tube you mount the camera. It can be mounted anywhere on the platform, from anywhere on the scope to anywhere on the counterweight shaft.



After reading this I was wondering if you meant *anywhere*, so long as it is on the DEC axis.

If one mounts thier guide/imaging scope below or above the DEC axis you will surely see field rotation along with a nightmare of balancing issues.... :smirk:

Rob

#45 Suk Lee

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Posted 09 October 2006 - 03:20 PM

It does not matter how far from the tube you mount the camera. It can be mounted anywhere on the platform, from anywhere on the scope to anywhere on the counterweight shaft.


If you mount the guiding OTA side by side with the imaging OTA, you'll get field rotation on long exposures due to the offset from the RA axis.

The greater the offset, the greater the rotation/shorter the time until it shows up. The Losmandy side-by-sides are OK for around 60 minutes before stuff starts to show up...

Cheers,
Suk

#46 ClownFish

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Posted 09 October 2006 - 03:58 PM

Yes, you can mount the CAMERA on the counterweight shaft. That was the question.

CF

#47 rwiederrich

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Posted 10 October 2006 - 01:15 PM

Yes, you can mount the CAMERA on the counterweight shaft. That was the question.

CF


CF, I don't think the issue is that you can, but if you do it had better be exactly center the DEC axis or you will surely have field rotation issues, along with balancing ones.

As mentioned, if you mount a camera anywhere on the counterbalance shaft, you will in effect be putting the camera above or below the center of axis. It needs to be mounted exactly on the end of the shaft, as I have done, to prevent any field rotation, and by doing it this way you will have the camera exactly on the center of the axis.

Placing the camera anywhere else on the scope, except in a piggyback fasion, will yield field rotation in longer exposure photographs.

I know you know all of this, but, this is what I think was being mentioned by Lee.

Rob(but what do I know) :smirk:

#48 rwiederrich

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Posted 10 October 2006 - 01:27 PM

CF Hypothetical,

If I’m using a camera and a 400mm lens, could I mount the camera setup on the counterweight shaft, at the same distance as the OTA center of view and not run into field rotation problems if I guide with the C8 OTA?

Bryan


Bryan,

You can mount the camera on the countershaft, but you must remember to mount it at the end, cause if you do not, even though it is on the shaft it will be in essence off axis, and will show field rotation in longer exposures.

This is what I did.

Rob

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#49 Suk Lee

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Posted 10 October 2006 - 04:15 PM

Placing the camera anywhere else on the scope, except in a piggyback fasion, will yield field rotation in longer exposure photographs.

I know you know all of this, but, this is what I think was being mentioned by Lee.


Yeah, that's what I meant, but I didn't say it that clearly...

Cheers,
Suk

#50 raydar

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Posted 11 October 2006 - 02:03 AM

Okay, I always thought that a camera riding piggyback could be mounted anywhere and could even be pointing at a different part of the sky. In my mind everything is moving at the same rate so how does field rotation happen (if the polar alignment is good)?

So this setup in the link below is not really all that reliable?
http://www.naturespe...bleexposing.jpg

And (question for Suk)if that config in the link is wrong, what is the part of the takahashi rings that protrudes outwards that I have my wide camera on for? I thought it was for piggybacking.

Cheers

Ray


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