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guiding, total weight, and mounts

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#26 Rushwind

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Posted 25 September 2004 - 01:20 PM

You should be fine. John_B asserts that the ST-4 produces 111x when coupled with the ST80 (I'm assuming that he's calculated it out :)). A 35mm film frame will only produce 40x when imaging through your SCT (50mm lens = 1x, so 2000mm lens = 2000/50 - 40x). You'll probably be fine. Shooting with a focal reducer would help, as would being able to Barlow the ST80 (although 111x is reaching the limits of what an ST80 wants to resolve...), but I'd be interested to see what kind of tracking results you can get with this setup; I don't think that it's as bleak as you worry.

One of the benefits of shooting film is that your "chip" size is so large compared to the chip in the ST-4, that you can get away with a lot less guiding accuracy. If you were shooting CCD, with chips much closer in size to each other, you'd have to worry about guiding focal lengths a lot more. I notice that you're not using an ST-4, but whatever you're using for an autoguider, the chip is going to be similarly tiny compared to a frame of 35mm film.

Since Dee hasn't bought her guidescope yet, it's easy to suggest to her that, if she was going to consider running her 8" SCT as an imaging scope, she should consider a longer refractor for a guidescope now, to give her more flexibility in the future. However, since she's not considering the SCT for imaging at this time, an ST80 is certainly a fine (I daresay "better") choice for her to guide her 480mm imaging refractor. 80mm f/11 guidescopes can be had cheaply further down the road, in any case.

Since you've already bought a guidescope, I say run with it. I bet you'll be pleasantly surprised at how well the little refractor will keep up.

Clear skies,
Jimbo

#27 spaceydee

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Posted 25 September 2004 - 03:08 PM

So where/how do I mount a finder once I put a guidescope ontop of the main refractor? I see pictures of finders mounted to the side of the main scope. Any other options??

#28 Rushwind

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Posted 25 September 2004 - 05:40 PM

My finder brackets kinda go off to the side (a la NCC-1701), so I can actually have a finder mounted on both the imaging scope and the guidescope at the same time if I want (makes it nice to get an idea of where the guidescope is pointing relative to the main without having to remove cameras). In practice, I tend to use a single finder and move it back and forth when needed. Also, the finder stalk is a great place to wrap loose cables; this has saved my ST-4 from crashing into the ground on more than one occasion. stupid setscrews... grr.

All that having been said, I'm seriously considering replacing the ED80's finder with a Telrad. I just need new tape for the base, and I'm set.

Jimbo

#29 raydar

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Posted 26 September 2004 - 01:45 AM

Cheers guys.

I had no idea about the difference in film to ccd size.
Thats good news to me then.

Ta

#30 Blueshark928

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Posted 26 September 2004 - 11:00 AM

this has saved my ST-4 from crashing into the ground on more than one occasion. stupid setscrews... grr.


Jimbo, what size is the focuser of your guidescope? If its 2" consider the Mercury Systems VBs. I just got these yesterday for my 80/9d. Its two separate parts. The Stellarvue specific VB and the triple set screw/brass compression ring 1.25X2" adaptor. The adaptor is also threaded for 2" filters.

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#31 Blueshark928

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Posted 26 September 2004 - 11:12 AM

The 80/9d is much much shorter than the 80mm f/11

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#32 Dennis

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Posted 26 September 2004 - 03:36 PM

I'm on the verge of getting a guiding scope for my 9.25 SCT. I'll be manually guiding initially, and suspect I'll autoguide later on. From these excellent discussions, I realize I need to select carefully. Since I'm using film with my OM-1, I am imaging at about 47x. So, for a guide scope can I get away with an ST80 as long as I guide at higher than 47x with it?

#33 Emanuele

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Posted 26 September 2004 - 03:43 PM

ahhhhh now I'm confused! :(

#34 spaceydee

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Posted 26 September 2004 - 04:00 PM

I'm on the verge of getting a guiding scope for my 9.25 SCT. I'll be manually guiding initially, and suspect I'll autoguide later on. From these excellent discussions, I realize I need to select carefully. Since I'm using film with my OM-1, I am imaging at about 47x. So, for a guide scope can I get away with an ST80 as long as I guide at higher than 47x with it?


I will definitely defer to the experts on this one! It doesn't go along with the "at least 1/2 the focal length rule", and being less than a newbie myself (and having a short F.L. scope anyway) it's easy for me to go with that rule of thumb for now.

#35 Emanuele

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Posted 26 September 2004 - 04:13 PM

ok somebody explain this to me:

I have an LX90 (2000mm FL) and a Megrez (480mm FL).

Basically even if I use a Focal Reducer on the LX90 bringing it to a Focal Lenght of 1260mm, I CANNOT use the Megrez because the focal lenght of the latter is too short?

#36 Suk Lee

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Posted 26 September 2004 - 04:30 PM

Basically even if I use a Focal Reducer on the LX90 bringing it to a Focal Lenght of 1260mm, I CANNOT use the Megrez because the focal lenght of the latter is too short?


No, that's not quite correct.

The basic theory here is that you want to be able to see guiding errors in your guide scope before the image of stars in your main scope gets blurred. For a typical CCD pixel size (or effective film pixel size for that matter) of 9 microns at 1200mm, you're sampling at 1.5 arc-sec/pixel. You don't want your scope to wander more than about a pixel, or around 1.5 arc-sec. Your Megrez, at sufficient magnification (I'm too lazy to do the calculations right now) with a reticle eyepiece ought to be able to resolve this amount of wander.

What you need to make sure is that there's no differential flexure between your guide scope and your imaging OTA, particularly in the case of an SCT where the mirror can move. If you can lock down the mirror it will help a LOT.

Suk

#37 Emanuele

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Posted 26 September 2004 - 06:04 PM

Thanks Suk for the explanation. Maybe using a Barlow will help, no?
Yeah, the LX90 does not have a mirror lock, and I don`t want to mess around with screws and modifications.

Is there a book that explains all this stuff? :)

#38 Suk Lee

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Posted 26 September 2004 - 07:52 PM

Robert Reeves "Widefield Astrophotography" has a very good discussion on guidescopes, guiding, and tolerances for accurate imaging.

http://www.willbell....idefieldast.htm

Cheers,
Suk

#39 Emanuele

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Posted 26 September 2004 - 08:13 PM

Bookmarked that page, I'm going to order it soon! Thanks Suk

#40 Dennis

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Posted 27 September 2004 - 09:59 AM

As I research this further, it seems that I'm better off with an off-axis guider than a guidescope for my SCT. Is this generally the way to go?

#41 Guest_**DONOTDELETE**_*

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Posted 27 September 2004 - 11:01 AM

I think it depends on your imaging setup. Imaging with film or large CCD chips such as DSLRs opens up more possibilities with guiding equipment.

After reading Suk's last post I figured i'd do the math on my setup. I'm sampling at 1.1 arc-sec/pixel with a f/5 1250mm scope (2.2 if i bin 2x2). I'm autoguiding with a ST4 and f/5 80mm off-axis guider. This gives me 7.66 arc-sec/pixel. Now lets say I choose 52 Cygni as my guidestar and its about ~3 arc-seconds in width. If it drifts into the next pixel for the ST4 to catch it, it could have drifted as much as 4 arc-seconds. From my experience so far this has caused a slight trailing sampling at 2.2 arc-sec/pixel (binned 2x2). Sampling at 1.1 arc-sec/pixel is just totally unusable at this point with my setup.

Once i get my new guidescope (80mm f/11.4) i will be at 3.36 arc-sec/pixel with the ST4 running. This would not allow so much room for a typical guidestar to move around and hopefully i will be able to image in high resolution. :) Am I right in thinking this will solve my autoguiding problem?

#42 Blueshark928

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Posted 28 September 2004 - 04:14 PM

John

Write a PM to Dave C. and ask if it's working for him. He has the St-4/80mm f/11 guidescope on his 8" Newt. The only difference is hes using a G8 and not a G11 and a Digital Rebel.

#43 Guest_**DONOTDELETE**_*

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Posted 28 September 2004 - 07:40 PM

Okay, thanks. :) I may wait until i try the f/11 guidescope. I have a feeling it will solve most of my problems. My ST4/ST80 has no problem guiding the hx916 with a 180mm camera lens because the big difference in magnification. So for that and piggyback film images my current setup works.

#44 Blueshark928

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Posted 28 September 2004 - 09:15 PM

Im sure the 80mm f/11 will fit the bill.

You might want to get two of these. I have had them on all my scopes so far. The hold is vise like. Your st-4 shouldnt fall out.

http://www.astromart...ified_id=300509

I remember sending Joe an email awhile back to see if he made these for the 80mm scopes. I guess i wasnt the only one who asked.

#45 Guest_**DONOTDELETE**_*

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Posted 28 September 2004 - 09:40 PM

I'll have to get one of those. The original one on the the ST80 can't hold much...

What kind of rings would you recommend for the 80mm f/11? Don't they make some that you can adjust the guidescope without moving the imaging scope?

#46 Blueshark928

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Posted 28 September 2004 - 10:48 PM

Share it with the 80mm f/11 too. I think the focusers are the same.

I got the rings and mount bar from Orin. They are 3 point adjustable. Plus they are not expensive. The ones from Losmandy cost a small fortune.

#47 Guest_**DONOTDELETE**_*

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Posted 29 September 2004 - 11:05 AM

Just ordered them so they'll arrive next week. :)

Every accessory from Losmandy costs a small fortune...

#48 Mike Sandy

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Posted 07 October 2004 - 12:44 AM

FWIIW.....I guide with a fairly short focal length guider, without too much in the way of problems. MaxIm claims they can guide to an accuracy of about 1/10 the resolution of a pixel on the guide camera. I think my experience supports that claim. You might want to consider that as you select guide scopes.

Regards.....


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