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Inspecting a C8 Prior to Purchase

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#1 Joe Cepleur

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Posted 31 January 2012 - 10:58 PM

Please could anyone offer advice on inspecting a 1970s vintage Celestron C8 orange tube prior to purchase? The current owner wants $500 for it and a 12.5" Newtonian home-built. I suspect it may be possible to buy just the C8 for about half that. I have heard that the mirrors are likely to be good on a Cat despite the age, because they are protected inside the sealed tube. It looks clean in the photographs I have seen (sorry I can't post them just now; my laptop can not resize them). Are there any common defects I should be careful to avoid? It is complete, but I suspect the motor does not work on the polar axis.

I'm thinking of the C8 as a muscular heft-and-go scope, manageable if a few pounds beyond grab-and-go. Is it quick to set up? Am I likely to be happy with it as a general purpose scope? Planets, DSOs -- I'd use it for everything. It would be the biggest scope in my collection and my first cat.

Gotta love those triangular, spring-steel legs!

#2 Grava T

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Posted 31 January 2012 - 11:08 PM

Relatively quick to set up but a little long for cool down. Depending on the conditions I would think a good hour before med-high power eyepieces could be used with great success....but for a general purpose scope, it is awesome. I used mine a lot without the wedge in alt/az mode. Very comfortable to use and one can view seated all night long.

#3 Joe Cepleur

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Posted 31 January 2012 - 11:17 PM

Thanks! I'm not worried about cool-down time, because I'll store it in the barn.

I love that something so old could be so good. I like feeling as though I'm using what I might have used had I pursued astronomy at a younger age, when it first appealed but I was doing other things.

Other opinions?

#4 greju

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Posted 31 January 2012 - 11:45 PM

Make sure the focuser turns smoothly and there is not excessive mirror shift. Also make sure you are buying it for the right reason. Because it is a classic-not for it's optics. :grin:

#5 Brian Risley

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Posted 01 February 2012 - 12:35 AM

Look at secondary reflection in mirror and also look from rear port. Should be nice and clean. Depending on age, it could be a sandcast (holes in forks) or ribbed. Check that the RA lock knob securely locks and that you can easily and smoothly turn it a full 360 with the lock knob released via the RA slow motion knob. It should feel smooth all the way around. (If the RA Lock knob is to the right of the RA Slow Motion, then you have an older style that presses against the gear teeth instead of the version that pushed down on the flat metal side of the gear. I had one of these develop metal fatigue and it does not have the gripping force of the newer style. The newer style uses either a metal or nylon plate under the screw to push against the gear side, the metal can warp and loose grip, the nylon replaced it in the late 70's/80's and is better.)

Check that the Dec lock works and holds tightly. With it locked, the dec slow motion should be checked to make sure it can travel smoothly from one limit to the other. If there is excessive play or a little in/out motion on the dec slow mo knob, that may be adjusted out. If the Dec lock does not hold tightly, it may be adjusted by rotating the locking clamp. There may be a locking nut(s) present as well. These often can be adjusted so the knob remains operating in a normal vertical up/down position.

When plugged in, look at the motors on the bottom. In the little windows, you should see the motors spin.
Check that the RA setting circle can be smoothly turned all the way around. There should be some resistance/roughness as it is designed to move with the base (It is held there by a very large spring clip), and the knobs allow you to adjust it to the correct position, after which it should stay moving with the inner mount parts when turning in RA.
See if there is an indicator on the outside of the mount base that points at the RA circle. If not, then a little pencil mark or piece of tape can be applied and then set the setting circle so that an even hour is at the line. Make sure the RA clamp is locked, and let it run for 10-15 minutes. You should see that the circle has moved that far from the point you set it. That and the motors spinning will tell you if it is tracking ok if you can't do a full star test/setup on a wedge.

As mentioned, run the focus both directions and note if there are rough spots. This could mean that it needs to be lubed, but sometimes just repeated full travels will help spread the grease. (If it needs to be lubed, you can do this, there are lots of instructions on taking the corrector and primary out and relubing the primary shaft and focus threaded shaft.) If it grinds, I would be concerned.
Expect 20-40 full turns, depending on the style of the focuser threaded shaft. (The older ones have short stubby knobs above a large bearing holder, while the newer ones had a variety of thin knobs (<1") above a smaller bearing holder, sometimes the bearing holder had a large bottom rim to fit the older larger bearing hole.)

Check rough collimation, looking from the front of the scope about 8-10' away, you should see all the optics form concentric circles, if you don't then something may be way off.

If it has an orange plastic cover over the secondary, pop it off to see the serial number plate/collimation screws (little screwdriver will do it) If it has a serial number plate on the secondary, this normally should be straight level when viewing the scope with the tube level and the forks straight on to you. If it is way out it is possible someone removed the corrector and didn't return it right or the secondary mount was loose at some point. (C was putting them out level, but can't guarantee that 100%)
If it is a metal secondary holder that is smooth with a central screw and 3 outer collimation screws, you can't check this easily.
Don't even think of trying a flashlight test!
There may be dust present inside as well as outside the corrector, as long as it is not excessive.
Any dust or marks external of the corrector may be cleaned up following the proper cleaning methods. (Search about cleaning.)
Do see if you can do a star test, allow the scope to cool for an hour, as thermal issues will not allow a good test.
Check the images just inside/outside of focus, looking for the bullseye pattern. This should be noticeable and when centered, should be concentric. There may be differences in how it looks inside/outside of focus, but you don't want to see it lopsided and flip orientation when going through focus, as that is astigmatism I believe. (For full optical tests, see Suiter or Piekiel's books.)

If the power cord is not present, one can be obtained. If the plug is on the side of the base, then a Cinch Jones 2 pin receptacle is needed (Mouser Electronics). If it plugs in on the bottom of the base by the visible motors, then you can use an HP style plug. We can help you locate a cable/parts if needed. If you don't have power, you can't test the motors.

Hope this helps,
Brian

#6 gnowellsct

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Posted 01 February 2012 - 01:09 AM

For an old c8 and a 12.5" it's a steal just get both and sort it out later.

The inner mechanism is easy to disassemble and you can clean and lube that baffle with some synethetic grease like slick 50 bearing grease or mobile 1 bearing grease.

http://www.astromart...?article_id=594

for disassembly of the c8

#7 Napersky

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Posted 01 February 2012 - 08:56 AM

While I can't help you with your question I concur with GravT Michael that it is a great scope. I love my C8 and it is really a kind of mini superscope.

I believe that the American SCT: Celestron, Meade was perhaps the greatest contribution to Amateur astronomy by
American engineering and manufacturing (past) as I don't count the Tascos, Unitrons, as domestics.

The huge aperature, the control of the fork mount, tracking, what I can do with the SCT I can't do with my Refractors...at least I don't think I can.

Mark

#8 tim53

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Posted 01 February 2012 - 10:32 AM

For an old c8 and a 12.5" it's a steal just get both and sort it out later.


I'm glad you said that! That's what I'd do.

-Tim.

#9 Joe Cepleur

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Posted 01 February 2012 - 11:19 AM

Thanks, Everyone;

I will check every last element of your detailed suggestions. Such thorough lists! It's always wonderful to have so much help available. Amazing how the Internet has changed an arcane hobby into an international community. My grandfather was a farm boy who became a great aircraft mechanic. He spanned the Wright Brothers to the Space Shuttle. Our times are no different!

There is quite a range of opinions about this old C8, mostly favorable, such as:

for a general purpose scope, it is awesome.


But, there are also some cautions. Greju suggests:

Make sure... there is not excessive mirror shift.


I've never really understood "mirror shift," because I've never had a SCT of my own. If I follow, the scope focuses by moving the primary mirror, so any imperfection in the mechanicals means that the image may pan somewhat when focusing. At high magnifications, bad image shift could pan my object out of view. Is that correct?

Also make sure you are buying it for the right reason. Because it is a classic-not for it's optics.


This is a wonderful comment to find among all the enthusiasm for old C8s. I am fascinated with old scopes. I love the idea that I can develop my skills on gear that is more primitive than the current standard, believing the extra challenge teaches better skills. I also like the feeling of catching up on life, of doing decades later what I might have done when I was younger, yet while using the same equipment. I'm pretty sure this is the scope I saw this scope advertised in comic books and "Boy's Life" magazine when I was a kid. (Yeah; comic books -- inexpensive advertising when test marketing back in the day.) Having a piece of history appeals to me. Those are good reasons to buy a classic.

Moving on to questionable motives: How happy, or disappointed, am I likely to be with my allegedly practical reasons for the purchase?

A new C5 would cost about $500; a new C8, $1,000. Suppose I could buy this vintage C8 for about $250 with the wedge and glorious spring-steel tripod, maybe $300. It would have much more aperture than the new C5 and same as the new C8, yet inferior coatings and less reflective mirrors, and perhaps crankier mechanicals. Is this the low-cost ticket into the world of large aperture, reasonably easily transportable scopes? I confess to be seeking a low-cost workhorse as I continue to learn the skies. I don't mind a few quirks and a little less brightness, so long as it frees me to see far beyond my 60mm refractor and 4" reflector. I do worry about the lessened contrast from the large central obstruction when viewing planets, but not too much, so long as it will show me distant galaxies and star clusters. Would an old C8 fit the bill, or do I need a reality check and more time to save my nickles for a newer scope?

Yeah, I admit -- I'm hoping an old scope in good condition will help me to see the skies well beyond my budget for newer gear. Am I delusional?

And, assuming the scope is in good condition, at what price would I become an overpaying Romantic fool?

#10 Geo.

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Posted 01 February 2012 - 12:49 PM

Right on image shift. Panning is a god way to express it. There's about 1/1000" clearance between the baffle tube and the primary carrier. This is less than your average auto engine rod bearing. But if the grease isn't distributed evenly the clearance is enough to let the carrier pivot or "pan" on the baffle tube during focusing. Bad shift will pan the object right out of the FOV!

Only a few orange Super C8s were StarBright, so the coatings of this C8 will be either standard Al SiO2 on the mirrors and perhaps antirefective on the corrector ("Special Coatings"). Starbright is noticably brighter than the standard coatings. So the light thruput of an orange C8 is about the same as a C6 XLT.

If you are disappointed the current market of an orange C8 kit is about $500. You've got nothing to lose.

BTW, if there is a gray haze on the inside of the corrector, don't worry it cleans right off.

#11 tim53

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Posted 01 February 2012 - 01:20 PM

An old orange tube C-8 is no slouch. I think the chances that the optics might be excellent are better with an early one, who's optics might have been hand-figured, versus a new one cranked out in China on some machine. The new ones are probably more consistent, but probably not as good as an outstanding old one.

Question is, are you getting one of the better ones? And the answer is, if you're getting a C-8 AND a 12.5" Newt for $500, it doesn't matter! You can recover your investment in the unlikely event you've got a dog. Maybe keep the Newt and sell the C-8.

I think an old C-8 in good condition should be worth between about $300-500, depending also on accessories.

I got lucky last October. I found one on the Las Vegas craigslist that was on my way to Utah, for $250. Be sure to scroll down on this thread to where I imaged Jupiter with it. It turns out it's probably the best SCT I've ever owned. I had a ~1980 orange tube that was almost as good optically, but I sold that to a friend a couple years ago.

1975 Orange Tube C-8 thread

#12 wfj

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Posted 01 February 2012 - 02:52 PM

Lately I've been using my 80's "Orange Tube" C8 ("Special Coatings") as a table top alt-az. Plop it down on a picnic table or wood stool and use it with a short stool.

After collimating it with a barlow and a 5mm, and finding a weak zone at the 30% radius and undetectable SA via secondary breakout - it does quite well on Mars(406x) and Jupiter (6mm 338x) when well cooled.

For me with SCT's the make/break point is collimating and star-testing on Polaris - if you have such an accommodating seller. If it collimates and you can measure accurately the breakout of the secondary shadow inside/outside focus to a fraction of a turn, it passes my "good enough" test.

I have some excellent 4" refractors and maks, but I still see more detail (seeing & cooling permitting) with SCTs than them. Newts do better same aperture or bigger, but they are heavier/bulkier.

#13 Joe Cepleur

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Posted 01 February 2012 - 06:18 PM

An old orange tube C-8 is no slouch...

1975 Orange Tube C-8 thread


No slouch, but apparently not as easy to set up as I had hoped. Or, was it just a bit of a bear to prepare for astrophotography? For visual work, I typically toss a scope on the ground with a fair approximation to a polar alignment, and then just tweak the dec axis along with the polar axis as the image drifts. If that were your standard for the C8, would it be quick to set up?

Seeing the going prices for C8s, I should grab this puppy while I can. The 12.5" Newt sold with it is a homebuilt, no idea if it's built with an old shaving mirror or a fine Cave, but the mount looks good in pictures. If I could only store the thing and it were good, it might be the featured scope in an observatory someday.

#14 tim53

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Posted 01 February 2012 - 07:24 PM

For visual, you don't need to go to great lengths to polar align. You could set it up on the wedge and point it to 90 degrees declination (the pole) and move it around in azimuth until you see polaris in the finder.

Setting up the way you describe would take a minute or two at most, and would be fine for visual. I really consider the orange tube C-8 a grab and go. I need to take mine off the tripod to get it in my workshop door, but if you have a wide enough doorway it's easy enough to move the whole scope by carrying it.

It's somewhat of a hassle to set up for deep sky astrophotography, but the setup for visual is usually fine for planetary imaging - just tweak the dec and RA controls to keep centered.

The Newt sounds interesting. A big Newt can take up a lot of room, though. But at the price for the pair, I would still encourage you to buy both and sell one to pay off the investment in the other.

I can't remember if I asked: Do you have pictures of the scopes?

-Tim.

#15 davidmcgo

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Posted 01 February 2012 - 10:09 PM

For visual I sometimes use mine in alt az mode with the drive base set on a tall barstool, and me on a low chair and just use the slow motions to track. A right angle finder helps in that mode.

Tim, I can't wait to see how your repaint of old #135 turns out. Glad mine was a smooth finish vice the velvetone.

Dave

#16 Joe Cepleur

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Posted 02 February 2012 - 08:28 AM

Got pix from the seller. Been iced into my road for several days. Never know whether a seller is bluffing or honest about alleged other buyers.

I am hesitant to resell a scope at a profit, because I trust the original source who notified me of the scopes. The seller is selling not just her scopes, but her heart to a good home. It would be good business, but bad karma, to profit from the purchase. She adds that the 12.5" Newt has not been used in one year per inch of aperture.

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#17 Joe Cepleur

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Posted 02 February 2012 - 08:29 AM

Mirror cell...

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#18 Joe Cepleur

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Posted 02 February 2012 - 08:30 AM

Home-made focuser?

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#19 Joe Cepleur

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Posted 02 February 2012 - 08:31 AM

Anyone recognize this mount? Looks as though it would clean up well.

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#20 Joe Cepleur

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Posted 02 February 2012 - 08:31 AM

Another of the mount...

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#21 Joe Cepleur

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Posted 02 February 2012 - 08:32 AM

Did I mention a C8?

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#22 Joe Cepleur

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Posted 02 February 2012 - 08:34 AM

Maybe not a bad purchase for a Romantic Classicist whose astronomic aspirations outclass his budget...

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#23 Joe Cepleur

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Posted 02 February 2012 - 08:35 AM

and who prefers grab-and-go to schlep and heft...

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#24 Joe Cepleur

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Posted 02 February 2012 - 08:36 AM

Eyepiece case available separately.

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#25 Joe Cepleur

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Posted 02 February 2012 - 08:37 AM

Volcano tops! Kelners, okay; of historic interest...

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