2020BC
scholastic sledgehammer
Reged: 08/03/05
Posts: 820
Loc: Australia
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Hi,
I've spotted a department store Celestron 127mm (5") newt that startled me because it claims to have a focal length of 1000mm. The OTA is just 500mm long!
I looked inside the tube and it's definitely a newtonian, however looking up the inside of the focuser tube there's a big permanent lens there. It looks like Celestron have installed some sort of permanent barlow (x2?) and it looks like it can be unscrewed!
Now, I've been thinking about my options for buying a FAST and short focal length guidescope to sit in parallel with my Orion ED80 and/or widefield cameras.
Seems an f/4 newt with 127mm of aperture would have no problem finding a guidestar !! that is if the guidestar isn't shaped too much like a banana. 
QUESTION: If I can remove the x2 barlow from inside the focuser tube will I be able to put a camera there and use this as a 500mm F/4 guidescope ?? Will distortion be too much of a problem - remember that I'm only interested in guiding with it (not imaging) and all I need is enough field of (clear) view to fit a prime-focus 6mm CCD camera chip. 
I'd love to experiment with this but I don't want to waste money on a lemon that's not going to work.
Your thoughts much appreciated. 
Click the link to read scope description
Edited by 2020BC (12/06/06 03:02 AM)
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Ben Ritchie
Lost in Space
Reged: 01/31/05
Posts: 4339
Loc: Bosham, UK
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Quote:
I looked inside the tube and it's definitely a newtonian, however looking up the inside of the focuser tube there's a big permanent lens there. It looks Celestron have put some sort of permanent barlow (3x?) there and it looks like it can be unscrewed.
There are some really, really horrible ebay 'scopes that use this design. I'd hope that Celestron weren't putting their name to total junk, but my initial feeling is to treat it with caution. I have no idea what removing the barlow would do to the optics. The risk is that you can't reach focus without it, or image distortion is too severe to guide with accuracy. You don't want to image with it but the optics can't suck too badly, I presume most guide software needs round stars to work to full (sub-pixel) accuracy.
My gut feeling would be to avoid it unless you can either track down someone else who has got a very similar setup to work ... or the store has a really good returns policy.
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Astro-Physics 130EDT StarFire, 80ED (x2), 305mm f/5 dob, VLT
Astro-Physics 1200GTO, evolved HEQ5/pro
Coronado SM60/BF10, 3-6 Nagler zoom, 8 & 13 Ethos, 28UWAN
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Ben Ritchie
Lost in Space
Reged: 01/31/05
Posts: 4339
Loc: Bosham, UK
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Oh, and at 17lbs it's a heavy guidescope! That's a pound more than my 130mm refractor weighs.
(unless they're talking about the whole package? hard to tell)
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Astro-Physics 130EDT StarFire, 80ED (x2), 305mm f/5 dob, VLT
Astro-Physics 1200GTO, evolved HEQ5/pro
Coronado SM60/BF10, 3-6 Nagler zoom, 8 & 13 Ethos, 28UWAN
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2020BC
scholastic sledgehammer
Reged: 08/03/05
Posts: 820
Loc: Australia
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Thanks, Ben. It's one of those "I'd like to tinker with that and see if it will work" kind of projects... You know how it is. That weight would have to be with tripod and counterweights. I felt the OTA in the shop and it was ok. As a guidescope it would be a lot lighter than using my SN-8.
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LXD-75 SN8; Baader MPCC Coma Corrector
Orion 80ED; WO x0.8 Field Flattener
Mintron 62V1P-Ex; & 13V1C Video & DMK 21AU04.AS
Canon Digital Rebel 350D DSLR (50mm f1.8 & 200mm f2.8L USM II)
Visit My Website
Edited by 2020BC (12/06/06 02:59 AM)
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Ben Ritchie
Lost in Space
Reged: 01/31/05
Posts: 4339
Loc: Bosham, UK
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Yeah, i'm a huge fan of "fiddle with it and see if it works" projects (which I guess is why I own a G11  ).
I think my main worry would be reaching focus without the barlow, I don't know how much that element will move the focus point. If you can try and mod it without breaking it then i'd go give it a try, that way you can sell it on or return it to the store if it doesn't do what you need.
edit: I figured that 17lbs is probably the total weight, but the specs say "TELESCOPE WEIGHT 17 lb (7.71 kg)" 
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Astro-Physics 130EDT StarFire, 80ED (x2), 305mm f/5 dob, VLT
Astro-Physics 1200GTO, evolved HEQ5/pro
Coronado SM60/BF10, 3-6 Nagler zoom, 8 & 13 Ethos, 28UWAN
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 5599
Loc: Islamabad, Pakistan
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Grab a used 80mm refractor like this ONE.. Then remove the Alt/Az mounting brackets. Buy some rings and you're good to go.
CF
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Learn all about POLAR ALIGNMENT with my Drift Method Tutorial and simulator!! Or visit my Foreign Service Blog!
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jgraham
Postmaster
Reged: 12/02/04
Posts: 5367
Loc: Dayton, Ohio
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The problem is the lens isn't just a Barlow, it's also a corrector. The primary mirror is spherical and at f/4 the off-axis coma will be aweful and even on-axis I doubt it would give a sharp focus. Based on reports I've read comparing the long- and short-tube Meade DS-2130's the image through the short-tibe version is softer than the long-tube version.
-John
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-John
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Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I & II), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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solshaker
Pooh-Bah
Reged: 06/06/06
Posts: 1305
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i actually own this scope. picked it up for 30 or 35 dollars a while back at a yard sale.
when acclimated and properly collimated, the scope gives good views. cloud bands, festoons, grs, moon and shadow transits on jupiter. cassini division and some cloud bands on saturn. ive plucked many many M objects out with it as well.
after reading up on it i learned of the barlowed focuser. i attempted to take it out as i felt i was missing something in my views after all i had read. after removing, i couldnt come to focus with any of my eps, so i put it back in. i only fooled with it for one night, so others more knowledgeable may be able to make it work, but im still a rookie.
i cant answer the question of a guide scope, but thats my experience with this product and its barlow. i know its not the best, but for my visual needs it works just fine.
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My Gallery
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2020BC
scholastic sledgehammer
Reged: 08/03/05
Posts: 820
Loc: Australia
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Thanks. Was it easy to remove (and put back) the barlowed focuser?
CF - that one's too long and slow for my purpose.
John - "awful" doesn't sound good...
Maybe a 500mm f/4.5 camera lens would be better. It'd probably cost a bucket though.
This One looks very interesting.
Edited by 2020BC (12/06/06 03:38 PM)
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mnaf
professor emeritus
Reged: 01/03/06
Posts: 626
Loc: Bay Area, CA
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I'll throw my 2 cents in :
How about this one?
here
It's short, faster yet, and cheaper.
Mike
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~Mike
Atlas EQ-G
Orion 8", AT66ED
Discovery 80mm refractor
Meade LPI, DSI Pro, DSI Pro II
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2020BC
scholastic sledgehammer
Reged: 08/03/05
Posts: 820
Loc: Australia
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Ha, ha. Looks like we found that simultaneously. Yes, Mike it does look interesting! It gets good reviews, too!
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LXD-75 SN8; Baader MPCC Coma Corrector
Orion 80ED; WO x0.8 Field Flattener
Mintron 62V1P-Ex; & 13V1C Video & DMK 21AU04.AS
Canon Digital Rebel 350D DSLR (50mm f1.8 & 200mm f2.8L USM II)
Visit My Website
Edited by 2020BC (12/06/06 03:45 PM)
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jay52
Carpal Tunnel
Reged: 09/26/04
Posts: 2280
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Too slow for a guidescope? No such thing. The classic design for a guidescope is 80mm and f/11-ish, which is why AP has made one for so many years. There's no such thing as too slow...there will be a plethora of guidestars to choose from, regardless of the "speed." I wouldn't jack with that Celestron scope. As was advised, a simple 80mm refractor will work just fine...and you'll avoid the countless issues with trying to mount such a tube without getting flexure, not to mention if you can actually get it to work in the first place.
Smaller is better when talking about guidescopes, particularly when it comes to figuring out how you'll mount it.
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jay
www.allaboutastro.com
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mnaf
professor emeritus
Reged: 01/03/06
Posts: 626
Loc: Bay Area, CA
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Great minds think alike!
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~Mike
Atlas EQ-G
Orion 8", AT66ED
Discovery 80mm refractor
Meade LPI, DSI Pro, DSI Pro II
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UrbanStarSeeker
sage
Reged: 04/23/06
Posts: 313
Loc: Fountain, CO, USA
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Quote:
Too slow for a guidescope? No such thing.
That depends on what you're using for a guide camera. If you're using a video camera or unmodified webcam, then a fast scope is pretty important.
Allan
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Celestron G8-N (8" f/5 Newtonian)
SV Nighthawk II
Moonlight CR-1 + Motor Focus + PC Interface
Canon Rebel XT/350D (unmodified)
Orion Atlas EQ-G
Doubt is not a pleasant condition, but certainty is absurd. - Voltaire
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mnaf
professor emeritus
Reged: 01/03/06
Posts: 626
Loc: Bay Area, CA
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For what it's worth..
My guidescope is an 80mm, f/15 refractor (which is longer than my 8", f/4.9 newt!). I sometimes have an issue with guidestars but that's because I'm using an LPI as a guidecamera. 
Mike
Edited by mnaf (12/06/06 04:56 PM)
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s58y
Post Laureate
Reged: 12/12/04
Posts: 4850
Loc: Eastern NY
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Quote:
That depends on what you're using for a guide camera. If you're using a video camera or unmodified webcam, then a fast scope is pretty important.
I think it also depends on the mount and the longest guide exposures you can manage. With my rough/jumpy G-11, 1-second guide exposures seem to be best.
With an 80mm f/6 guidescope and the ST-402, I've not yet seen a shortage of guidestars near actual targets, all of which (so far) are close to the Milky way. However, when doing my pseudo drift alignment with the autoguider, I sometimes see shortage of guidestars far from the milky way, unless I lengthen the guide exposures to 2-3 seconds (which actually works better for measuring DEC drift, even with the G-11).
Of course, with the smooth AP mounts, maybe f/11 may be OK for autoguiding.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, SV66 guidescope
AP900, G-11, Barndoor tracker
http://www.pbase.com/s58y
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jay52
Carpal Tunnel
Reged: 09/26/04
Posts: 2280
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Okay, I'll buy that. I was assuming the use of a real autoguider/CCD camera, not a video/webcam.
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jay
www.allaboutastro.com
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solshaker
Pooh-Bah
Reged: 06/06/06
Posts: 1305
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real easy. just screwed in and out like glass in an ep.
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My Gallery
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