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guiding, total weight, and mounts

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#1 spaceydee

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Posted 22 September 2004 - 11:46 PM

Hi, I have a few questions regarding getting stated doing astrophotography. I have an 80mm refractor that I would eventually like to try do astrophotography with. What is the best way to guide the scope, through a guide scope or with an off axis guider? What are the options of using a guidescope when you have such a small apo to begin with, how do you mount a guide scope to it, or perhaps next to it? Next, if I were to have a guidescope in addition to the other scope, how much more weight does this add to the assembly, and thus require a heavier duty mount?

#2 Suk Lee

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Posted 22 September 2004 - 11:55 PM

With an 80mm refractor, a guidescope is the easiest way to guide because the short focal length of the imaging refractor means you won't have differential flexure problems, and it's much easier to find guidestars with a guidescope than an off axis guider.

The traditional way to mount the guidescope is above the imaging scope, like an over and under shotgun. You'll more than double the load on your mount because off the additional rings and hardware to mount the guidescope, and because the guidescope is farther away from the polar axis than the imaging scope.

Whether or not you need a heavier mount depends on how lightly or heavily your mount is already loaded. If you're talking G11 and 80mm refractor you don't have a problem, a GM8 is probably OK too, an EQ3 would probably be overloaded.

An alternative is side-by-side mounting. The mount plate is usually heavy to make it stiff, but both OTAs are close to the polar axis so it's probably a wash in terms of mount loading.

Here's Bluesharks over/under rig, and here's my side-by-side rig.

Cheers,
Suk

#3 Vince Tramazzo

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Posted 23 September 2004 - 12:48 AM

A formula I use. Use half the rated
weight load on the mount. If the mount
is supposed to handle forty pounds I wouldn't
go to much more than twenty. Works for me.

Good veiwing :cool:

#4 spaceydee

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Posted 23 September 2004 - 01:16 AM

My 80mm scope is a short tube, would it be awkward to mount a guidescope ontop of it?? Any suggestions on what would be a good guidescope?

#5 wilash

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Posted 23 September 2004 - 02:26 AM

How about an 80mm f/5 refractor. It will be light and the focal length should be enough to guide your other scope. They are about $120 to $150 dollars. You will need a retical eyepiece - double crosshairs are better than single. And you will need to learn to drift align the mount. Isn't this fun.

#6 Rushwind

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Posted 23 September 2004 - 12:51 PM

I use an ST80 to guide my ED80 (sorry I didn't have it in the over-under setup when you got a chance to see it; I can post a pic, but it looks a lot like Blueshark's rig, with much shorter scopes :)). I think that either an ST80 (f/5) or an AT1010 (f/6) would serve quite well.

Basically, if you can get rings on the imaging scope, you can just run the guidescope rings either directly off of the top of the main rings, or you can add a dovetail in between, which will give the guidescope a longer platform to ride on (I run without a dovetail; just the way it worked out, but you can see the dovetail in Blueshark's rig). In fact, using a dovetail in this way might allow you to run a much longer guidescope (like a 80mm f/11) if you wanted...

Remember to add in the weight of the camera gear as well as the new OTA and rings; I can actually run my ST80 (a real lightweight at ~3#) on my CG-5 mount, if I'm running the K-1000 piggybacked. As Suk suggests, it's because the heavy camera is much farther from the polar axis...

The accessory list for photography starts getting long quickly, but an illuminated dual-crosshair reticle EP is very high on the list.

Jimbo

#7 Rushwind

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Posted 23 September 2004 - 12:51 PM

I use an ST80 to guide my ED80 (sorry I didn't have it in the over-under setup when you got a chance to see it; I can post a pic, but it looks a lot like Blueshark's rig, with much shorter scopes :)). I think that either an ST80 (f/5) or an AT1010 (f/6) would serve quite well.

Basically, if you can get rings on the imaging scope, you can just run the guidescope rings either directly off of the top of the main rings, or you can add a dovetail in between, which will give the guidescope a longer platform to ride on (I run without a dovetail; just the way it worked out, but you can see the dovetail in Blueshark's rig). In fact, using a dovetail in this way might allow you to run a much longer guidescope (like a 80mm f/11) if you wanted...

Remember to add in the weight of the camera gear as well as the new OTA and rings; I can actually run my ST80 (a real lightweight at ~3#) on my CG-5 mount, if I'm running the K-1000 piggybacked. As Suk suggests, it's because the heavy camera is much farther from the polar axis...

The accessory list for photography starts getting long quickly, but an illuminated dual-crosshair reticle EP is very high on the list.

Jimbo

#8 Blueshark928

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Posted 23 September 2004 - 01:05 PM

but it looks a lot like Blueshark's rig, with much shorter scopes


My scopes are getting shorter now. But the tradeoff is that they are much heavier. I'll have to take a new photo when the 102 apo arrives. The 80/9d is almost a 1/2 foot shorter than the 80mm f/11. I think the 102 is shorter than the 120 also.

BTW - running a dovetail between the scopes also makes the setup stiffer and less prone to flexure. You can also see that i have the rings as far apart as i could get them. This also results in less twisting flex thru the dec axis. So far I have not seen any flexure errors in my images.

#9 Suk Lee

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Posted 23 September 2004 - 01:10 PM

Definitely let's see pix of your new rig when it gets here!

Suk

#10 spaceydee

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Posted 23 September 2004 - 02:29 PM

Are these dovetail brackets slotted? I'm wondering how you can get a set of guidescope rings to attach to the top of another scope through a dovetail mount? With my scope being relatively short, what are the odds that there will be a dovetail bracket that will work? Sorry for the ignorant questions, but I'm new to all of this. By the way, the length of the ST80 is about the same as the length of my scope. How odd would it be to mount the same size scope atop the main one?

#11 Blueshark928

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Posted 23 September 2004 - 02:50 PM

Most dovetails (good ones anyway) will have mutiple drillings and/or slots. If you ever see a Losmandy universal plate it looks like metal swiss cheese.

#12 Suk Lee

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Posted 23 September 2004 - 02:51 PM

Usually the guidescope rings attach to a top bar on the rings for the main OTA.

For example, my Tak Epsilon rings have a bar on top that's drilled to accept another set of rings.

Instead, I mounted a Losmandy "D" plate upside-down on the top bar, then use Losmandy accessories to mount guide hardware to the plate:

http://home.att.net/...pment/e160.html

Cheers,
Suk

#13 Rushwind

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Posted 23 September 2004 - 03:58 PM

John,

Looking forward to seeing the new "mini" setup! :)

I ran without a dovetail because the Orion 8" dovetails only have end holes (the two center holes are threaded 1/4-20), so if I threw a dovetail in there, I wouldn't have anything to attach the guide rings to. :( For the short ST80 OTA, it seems a sturdy enough solution.

I'd love to pick up a side-by-side rig, though; does anyone know where these are commercially available? (to fit a CG-5 dovetail)

Dee,

It is usually suggested that the guidescope have a longer focal length than the imaging scope. I run a shorter guider than imager, however (although at a much-increased magnification, which I believe to be the important part). Two scopes of the same (or nearly the same) length will work fine (no pointing and giggling, even :grin:).

Jimbo

#14 spaceydee

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Posted 23 September 2004 - 04:26 PM

If the requirement is a longer focal length then that means a larger scope ontop! I thought I read somewhere on one of these forums that the focal length of the guide scope should be at least half that of the main scope.

#15 Suk Lee

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Posted 23 September 2004 - 05:01 PM

the focal length of the guide scope should be at least half that of the main scope.


Yup, if you're visually guiding or using the now discontinued SBIG ST-4 autoguider. The newer STV gets away with a 100mm guidescope for up to around 1500 mm of main imaging scope...

Suk

#16 Guest_**DONOTDELETE**_*

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Posted 23 September 2004 - 08:53 PM

the focal length of the guide scope should be at least half that of the main scope.



Im imaging at 1250mm and guiding at 400mm with an ST4. The images turn out okay, but i can see if i had a guidescope of 700mm or more the tracking would be much better. One thing i will miss about the 400mm f/5 guidescope is the ease of finding guidestars.

#17 Rushwind

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Posted 23 September 2004 - 11:11 PM

The point is to have the autoguider detect movement in the guidestar before your imaging camera detects it. So, as long as the resolution of the guiding setup is greater than the resolution of the imaging setup, you're golden. An easy way to make sure that this happens is to try to remember a rule of thumb, sush as comparing the focal length of the two sets of optics. In practice, as long as you're tracking smaller movements in the guider than in the image, you're alright.

The ST-4 has a *tiny* chip (2.6mm x 2.6mm) compared to the size of a 35mm film frame (24mm x 36mm). This makes it pretty easy to find a guidescope that's going to work with your 480mm imaging scope. You can go longer, shorter, or the same length. It's all about the magnification.

I image at 600mm and guide at 400mm. The ST-4 tracking seems to work great. I also have no problems manually guiding with this combination.

I still think that either an ST80 (400mm) or AT1010 (480mm) will work quite well as a guidescope for your 480mm imager. Are you concerned about later wanting to use your 2000mm 8" as an imaging scope?

Jimbo

#18 spaceydee

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Posted 24 September 2004 - 12:20 AM

Hi Jimbo,
I guess you were asking me that question?? I don't think I'm going to be taking my 8" OTA off its mount unless I want to totally destroy that setup. I think it is a bit too early for me to worry about whether I would want to use my SCT to image... I hear it is a bear to do, so I think it will be quite a while (if ever) that I think about it. I am really just trying to get the right idea on what to get before I start spending money on it, and then regret things and have to either return them or be stuck with stuff!

#19 Rushwind

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Posted 24 September 2004 - 12:24 AM

Pick up an ST80. They're super cheap (you can get them for $100-$150 on the 'mart), good performers in their own right, and you'll be happy with one as a guide scope for your APO. It'll do you right.

I agree with the SCT bear. I was just trying to figure out what the hangup was regarding using a shorttube refractor to guide your shorttube refractor. :)

Jimbo

#20 spaceydee

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Posted 24 September 2004 - 12:31 AM

No hangup, unless you count thinking how funny it's gonna look!

#21 wilash

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Posted 24 September 2004 - 01:33 AM

Just attach the 8i, fork mount and all, to a G11. That way you can keep the GOTO and still do a little imaging when you want.

#22 Rushwind

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Posted 24 September 2004 - 07:35 PM

Would it look funnier than an 80mm f/11 riding atop the SV80S? :lol:

Jimbo

#23 spaceydee

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Posted 24 September 2004 - 09:09 PM

I expect not :)

#24 raydar

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Posted 25 September 2004 - 08:25 AM

The point is to have the autoguider detect movement in the guidestar before your imaging camera detects it. So, as long as the resolution of the guiding setup is greater than the resolution of the imaging setup, you're golden. An easy way to make sure that this happens is to try to remember a rule of thumb, sush as comparing the focal length of the two sets of optics. In practice, as long as you're tracking smaller movements in the guider than in the image, you're alright.

The ST-4 has a *tiny* chip (2.6mm x 2.6mm) compared to the size of a 35mm film frame (24mm x 36mm). This makes it pretty easy to find a guidescope that's going to work with your 480mm imaging scope. You can go longer, shorter, or the same length. It's all about the magnification.

I image at 600mm and guide at 400mm. The ST-4 tracking seems to work great. I also have no problems manually guiding with this combination.

I still think that either an ST80 (400mm) or AT1010 (480mm) will work quite well as a guidescope for your 480mm imager. Are you concerned about later wanting to use your 2000mm 8" as an imaging scope?

Jimbo


Hmm, I never knew this. This is why I love these forums.

I have a short tube refractor (400mm) on an 8"SCT which is 2000mm.

From reading your advice, this means that my refractor is not gonna be big enough.

I haven't tried tracking with this setup yet. I am waiting for the next new moon.

Can I get around this problem by using barlow lenses to enlarge the guide star?

Below is my crude set up for now. But i'm looking at a losmandy in the very near future. I managed to get a meade Pictor AutoGuider from Ebay (which i'm currently waiting for). I'm am going to play with the Meade autoguider first before upgrading.

Anyway, back to the topic, lol

What I was going to do was use the imaging device in the refractor for manual guiding. I am hoping that this makes manual guiding a little more comfortable.

So could I just put a barlow in the refractor to theoretically increase focal length?

Also, I use a 6.3 focal reducer in the SCT. This device obviously reduces the focal ratio, but does this bring down the focal length of the SCT too?

Thanks

Attached Thumbnails

  • 205534-guidescope.jpg


#25 Guest_**DONOTDELETE**_*

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Posted 25 September 2004 - 09:25 AM

It looks like you're using film on your SCT... you might be okay.

Jimbo is right, it is all about the magnification. With my HX916 & XT10 at 113x magnification and the ST4 & ST80 guiding at 111x magnifiction. Any errors the ST4 tries to correct will almost always show up in my image because the magnification is so close. I have tried to barlow the ST80, but have not been successful in getting a bright enough guidestar yet.


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