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Downsizing OTA C-11 to C-8

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#1 Rustler46

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Posted 02 January 2021 - 12:33 AM

Recently my 10-inch Dob has been outfitted with a new Nexus-II digital setting circle. This replaces the original Tangent DSC on my 30-Dollar Telescope, increasing the number of catalog objects from around 250 to many millions in SkySafari. I'm hoping that setup will satisfy my needs for decent aperture, with what I consider my grab and go telescope.

 

Grab-n-Go-0187.jpg

 

Now for my pier-mounted Losmandy G11/G-1go-to mount, I'm shifting back to using a smaller Schmidt-Cassegrain OTA. While I really appreciate the nice views given by the Celestron-11, it is a heavy beast to carry out of storage and get up on the mount. So my old 1985-era Celestron Super C-8 Plus is being brought back into service on the G11/G-1. I'll still be able to mount my AT115EDT piggyback on the C-8. In the past this has given a nice contrast between APO and SCT when viewing different objects.

 

C-8 & AT115EDT-00945.jpg

 

Both the C-8 and the AT115EDT Triplet give great views with my William Optics binoviewer. 


Edited by Rustler46, 02 January 2021 - 09:00 PM.

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#2 MarMax

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Posted 02 January 2021 - 02:22 AM

Before you downsized did you use the AT115EDT piggyback on the C11 with the G11 mount? I'm curious if the G11 handled that pair well and if so what amount of counterweight did you need?



#3 Rustler46

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Posted 02 January 2021 - 03:04 AM

Before you downsized did you use the AT115EDT piggyback on the C11 with the G11 mount? I'm curious if the G11 handled that pair well and if so what amount of counterweight did you need?

Yes, the G11/G-1 handled the task quite well. Sometimes I had a film-era 200mm telephoto lens w/ ZWO ASI290MC camera in the top tube rings for EAA observing.

 

C-11-AT115EDT on G11-G-1.jpg

 

Since the refractor was far from the balance point, it required significant counterweighting. Likely the total load was pushing the mount's 60 lb. limit. This worked fine in visual mode.

  • 21-lb. Losmandy
  • 11-lb. Losmandy
  • 11-lb. generic brand
  • 2 X 5-lb. Losmandy

I enjoyed using that setup. For a change of pace I'm switching to a smaller SCT for a while. That C-8 has been with me for 35 years. But don't get me wrong - that C-11 is very capable. Here's what it did on Mars last year.

 

Mars-004659-RS-2.jpg

 

 

 

Russ


Edited by Rustler46, 02 January 2021 - 03:09 AM.

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#4 speedster

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Posted 02 January 2021 - 03:53 AM

You have me curious - what's the cup on your counterweight shaft?



#5 Rustler46

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Posted 02 January 2021 - 04:22 AM

You have me curious - what's the cup on your counterweight shaft?

What I think you are referring to is a Kroger Cottage Cheese tub. Ok, here's why. I replaced the stock clutch knobs with ones having three levers per knob, providing extra leverage. This made it much easier to adjust RA and Dec clutches. There was one unintended consequence - that is the declination motor cable would catch on one of the levers on the Dec knob during some slews. My solution was the high-tech cottage cheese tub covering the levers. It is held in a suitable position on the counterweight shaft by a hose clamp. Now the offending motor cable & clutch levers never come in contact. When I need access to the Dec clutch knob for adjustment, the hose clamp and cottage cheese tub slid down out of the way. Otherwise the tub guides the motor cable up and over the levers for no interference. Here's a closeup showing the levered clutch knobs. 

 

C-11 Levered Clutch Knobs.jpg

 

That's another of my "use whatever junk is available" solutions. For a while I used a drill-press vise hanging by a rope with other available counterweights to balance the load. Finally I broke down and purchased some more real counterweights.

 

C-11 Counterweights.jpg

 

Oh, different junk, but same idea. The black thing is the front pulley from a 1956 Ford 292 V8 engine. The electrical gizmo is a "Joule-thief" red light to mark the end of the counterweight shaft. This reduces chance of running into something in the dark.


Edited by Rustler46, 02 January 2021 - 09:03 PM.

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#6 Traveler

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Posted 02 January 2021 - 04:32 AM

Recently my 10-inch Dob has been outfitted with a new Nexus-II digital setting circle. This replaces the original Tangent DSC on my 30-Dollar Telescope, increasing the number of catalog objects from around 250 to many millions in SkySafari. I'm hoping that setup will satisfy my needs for decent aperture, with what I consider my grab and go telescope.

 

attachicon.gifGrab-n-Go-0187.jpg

 

Now for my pier-mounted Losmandy G11/G-1go-to mount, I'm shifting back to using a smaller Schmidt-Cassegrain OTA. While I really appreciate the nice views given by the Celestron-11, it is a heavy beast to carry out of storage and get up on the mount. So my old 1985-era Celestron Super C-8 Plus is being brought back into service on the G11/G-1. I'll still be able to mount my AT115EDT piggyback on the C-8. In the past this has given a nice contrast between APO and SCT when viewing different objects.

 

attachicon.gifC-8 & AT115EDT-00945.jpg

 

Both the C-8 and the AT115EDT Triplet give great views with my William Optics binoviewer. 

 

 

 

Why not put the greenhouse around the pier? cool.gif

Then you won't have to carry the C11 anymore...A C11 is imo a very nice instrument which is considereble more capable then a C8...as you know...


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#7 junomike

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Posted 02 January 2021 - 08:38 AM

I'd keep the C11 for awhile (even in storage) just in case you start to miss it!


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#8 JMP

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Posted 02 January 2021 - 11:59 AM

Hi Russ;

I live in Eugene and purchased a few odd parts from HOC back in the day. Got a good laugh from the trailer hitch counterweight.

Jeff Phillips
Eugene, OR

#9 Rustler46

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Posted 02 January 2021 - 09:32 PM

Why not put the greenhouse around the pier? cool.gif

Then you won't have to carry the C11 anymore...A C11 is imo a very nice instrument which is considereble more capable then a C8...as you know...

That would work great if I could figure out how to make the greenhouse roof slide off for access to the sky. For the last 18 years I have been wanting to make another observatory shelter like what I called Klamath Observatory, seen below. This housed a Celestron-8  used for photoelectric photometry of variable stars.

 

Klamath Observatory-1.jpg

This shelter with slide-off roof was simple and effective for a permanently mounted telescope. Now 35 years later the same 4-inch water pipe pier sits in my garden. And the 800+ lbs. of concrete is still attached to the pier as it was poured in the ground in the above photo. I've drug that around to several different residences since then. But now it is permanently in my garden observng site.

 

Klamath Observatory-2.jpg

Klamath Observatory seen from the west. The slide-off roof was light enough to completely remove and set on the ground on the south side. If I was to make another one like this, it would have another set of piers on south side that would hold the roof as it slid to the south.

 

I intend to keep the C-11 - it's not going anywhere other than in my garage for a while. For now I'm switching over to the C-8 on the G11 mount. Larger aperture duties will be carried by my 10-inch reflector. At one time when I had the financial resources to buy one, I lusted for a C-14. Then I loaded my mechanic's tool box with 45 lbs. of tools to simulate the minimum weight of a C-14. One attempt to heft that up to shoulder height was enough to tell me I didn't want to do that again. The C-11 at 28 lbs. seemed more in line with what I was willing to do. Now the C-11 OTA has grown to around 35 lbs. with accessories. As I have aged, the effort to heft the OTA onto the mount was getting to be a chore. It had even entered my ind that I could drop it. So I had thought of selling it:

Fortunately for me one of the options was keeping the C-11 and getting an APO refractor, the AT115EDT. Thanks to Greg (gknowllsct) I've learned how to mount that optical tube with minimum effort. And the APO refractor serves piggyback duties atop either of my SCTs.

 

I have a lot of different telescopes to choose from. Now I'll be giving my old C-8 a turn at the sky.

 

Best Regards,

Russ


Edited by Rustler46, 02 January 2021 - 09:52 PM.

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#10 Rustler46

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Posted 02 January 2021 - 09:45 PM

I'd keep the C11 for awhile (even in storage) just in case you start to miss it!

I confident it will be back under the stars before long. That size of SCT is a "sweet spot" of sorts. It does require a significant increase in mount to properly carry. For now the C-8 will be sweet too. Here it is in original configuration.

 

Celestron-8.jpg

Celestron Super C-8 Plus from around 1985

 

It is easy to remove the OTA from the forks for mounting on the Losmandy G11.

Celestron-8 Solar Setup.jpg

Set up for the 2017 Solar eclipse

 

Kind Regards,

Russ


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#11 Rustler46

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Posted 02 January 2021 - 10:17 PM

Hi Russ;

I live in Eugene and purchased a few odd parts from HOC back in the day. Got a good laugh from the trailer hitch counterweight.

Jeff Phillips
Eugene, OR

That was one of the highlights of my retirement years, working for Larry Hardin of Hardin Optical Company (HOC). As for the counterweight, gravity only cares about mass, not aesthetics. I kind of like the "steam-punk" aspect of junk weights. The Ford V-8 pulley has been returned to its former glory on my home made 8-inch reflector.

 

8-inch RFT.jpg



#12 Rustler46

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Posted 05 January 2021 - 01:30 AM

Here's an update on the transition of C-11 to C-8. Both optical tube assemblies (OTAs) will remain available for use. Here are the OTAs and mounts that I have available:

Having 4 of the larger OTAs utilizing the Losmandy G-11 mount, shows the wisdom of devoting a considerable fraction of financial resources into a capable mount. So now I'm equipping the Losmandy mount with the C-8 and AT115EDT refractor on top. The C-11 will be in semi-retirement for the time being.

 

Here's a photo showing that combination.

C-8 & AT115EDT-00945.jpg

 

This photo shows the Dollond spyglass atop the C-8 in test mode.

C-8 with Dollond-00486.jpg

 

The above photo shows the orange Celestron D-style dovetail plates, top and bottom.

 

I have become concerned about the strength of these dovetails in supporting the loads imposed. Each is connected to the Celestron-8's end castings by 3 very small allen-head screws, one on the corrector casting and two on the rear casting. Now these screws have a very high tensile strength, and are not in danger of breaking. But the dovetail plates themselves, along with the threads in the casting holes are sources of concern.

 

Each dovetail is an aluminum extrusion, that contacts the end castings only at the outer edges. It is not shaped underneath to match the curvature of the optical tube, as shown in the next image.

 

Screen Shot 2021-01-04 at 9.25.41 PM.png

 

This shows the 4mm gap below the dovetail on the corrector end. At this location it is held by a single allen-screw in the short slot visible. Now my concern about this attachment is not that it might break - it all seems pretty solid. But one dovetail on top supports in excess of 15 lbs. of refractor, tube rings and dovetail clamps, all on top of the C-8 OTA. But the dovetail directly connected to the mount carries all that weight plus another 15-20 lbs. of C-8 OTA, finder, DewBuster, Dewshield, 2 dovetails .... Well, all this adds up 30-40 lbs. (14-18 kg.) being supported by three small screws, holding an extrusion that is not in good contact with the OTA front/rear castings. This was a concern to me. 

 

So I decided to buy a pair of Parallax C-8 tube rings. Astronomics had one in stock for $220. When I told the telephone sales person of my concerns (I forget his name), he suggested a less expensive alternative - that is two Losmandy C-8 dovetails for $190. This is shown below.

 

Screen Shot 2021-01-04 at 9.29.02 PM.png

 

He explained that the two curved dovetail blocks give a much more secure and stable connection than the orange Celestron dovetails. Supplied with the Losmandy product are high quality stainless screws needed to connect to the OTA. Further he said with the parallax tube rings, securing the orange dovetails to the flat boss above and below would not be a very secure or stable connection. He recommended against that in favor for the less expensive Losmandy product. I appreciate the knowledgeable staff at Astronomics. 

 

Now they only had one Losmandy dovetail in stock. But that will replace the lower dovetail carrying the most weight. Later if I decide to replace the upper one, I'm still money ahead over the less suitable Parallax rings solution. So with the dovetail and a lens brush to get over $100, shipping was free. The tracking number shows they send the parts via 2-day FedEX. Kudos to our Cloudy Nights sponsor - Astronomics.

 

So that's where I'm at with transitioning to my old Celestron-8 on the Losmandy mount. I keep you'll posted.

 

Best Regards,
Russ


Edited by Rustler46, 05 January 2021 - 02:41 AM.

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#13 Traveler

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Posted 05 January 2021 - 02:08 AM

"The dovetail on top supports in excess of 15 lbs. of refractor, tube rings and dovetail clamps, all on top of the C-8 OTA. But the dovetail directly connected to the mount carries all that weight plus another 15-20 lbs. of C-8 OTA, finder, DewBuster, Dewshield, 2 dovetails .... Well, all this adds up 30-40 lbs. (14-18 kg.) being supported by three small screws, holding an extrusion that is not in good contact with the OTA front/rear castings. This was a concern to me."

 

+1 always a big concern for me as well.

In fact, a C9.25 has also just 3 screws (two tiny ones at one side, and a fatter one on the other side of the tube...brrr...imo a clearly engineering mistake for sure...)

 

Wich holes of the C8 can be used to screw on the two blocks or does one have to drill and tap some screwholes?



#14 Rustler46

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Posted 05 January 2021 - 02:35 AM

"The dovetail on top supports in excess of 15 lbs. of refractor, tube rings and dovetail clamps, all on top of the C-8 OTA. But the dovetail directly connected to the mount carries all that weight plus another 15-20 lbs. of C-8 OTA, finder, DewBuster, Dewshield, 2 dovetails .... Well, all this adds up 30-40 lbs. (14-18 kg.) being supported by three small screws, holding an extrusion that is not in good contact with the OTA front/rear castings. This was a concern to me."

 

+1 always a big concern for me as well.

In fact, a C9.25 has also just 3 screws (two tiny ones at one side, and a fatter one on the other side of the tube...brrr...imo a clearly engineering mistake for sure...)

 

Wich holes of the C8 can be used to screw on the two blocks or does one have to drill and tap some screwholes?

I believe it uses the same holes that the stock Celestron dovetails use. I would expect you use the supplied (Losmandy) screws to attach the blocks securely to the OTA castings. Then the other supplied screws secure the dovetail to the blocks. Of course it is also possible that long screws extend down through the dovetail and the blocks to the casting threaded holes. I'll let you know for sure in two days when it arrives. Also a quick call or email to Astronomics would no doubt have an answer for that.

 

Edit: The Losmandy.com website has this to say

Celestron dovetail plate for 8", bolts to existing holes.

Wt. 3.7 lbs.

Length 14"

HS 12.325/12.70,

Ht. top of tube to top of dovetail bar 1.20"

Thickness .75"

 

As I recall my C-11 has two screws on each end, front and rear. It has an even heavier load with the OTA (plus accessories) at around 35 lbs. So I'll likely replace its lower dovetail on the C-11 as well.

 

Russ


Edited by Rustler46, 05 January 2021 - 04:08 PM.

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#15 speedster

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Posted 05 January 2021 - 02:58 AM

The cottage cheese tub gives me an idea - get a bigger tub or maybe a gallon milk jug, and a hair drier or heat gun, and melt a cover for the whole mount covering all the bracket, levers, and bolts that could snag a cable.  cables would just slide over the outside of the milk jug shield.



#16 Rustler46

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Posted 05 January 2021 - 03:11 AM

The cottage cheese tub gives me an idea - get a bigger tub or maybe a gallon milk jug, and a hair drier or heat gun, and melt a cover for the whole mount covering all the bracket, levers, and bolts that could snag a cable.  cables would just slide over the outside of the milk jug shield.

I'd go for chocolate milk. 



#17 alphatripleplus

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Posted 06 January 2021 - 02:37 PM

 

 

So I decided to buy a pair of Parallax C-8 tube rings. Astronomics had one in stock for $220. When I told the telephone sales person of my concerns (I forget his name), he suggested a less expensive alternative - that is two Losmandy C-8 dovetails for $190. This is shown below.

 

attachicon.gifScreen Shot 2021-01-04 at 9.29.02 PM.png

 

He explained that the two curved dovetail blocks give a much more secure and stable connection than the orange Celestron dovetails. Supplied with the Losmandy product are high quality stainless screws needed to connect to the OTA. Further he said with the parallax tube rings, securing the orange dovetails to the flat boss above and below would not be a very secure or stable connection. He recommended against that in favor for the less expensive Losmandy product. I appreciate the knowledgeable staff at Astronomics. 

 

Now they only had one Losmandy dovetail in stock. But that will replace the lower dovetail carrying the most weight. Later if I decide to replace the upper one, I'm still money ahead over the less suitable Parallax rings solution. So with the dovetail and a lens brush to get over $100, shipping was free. The tracking number shows they send the parts via 2-day FedEX. Kudos to our Cloudy Nights sponsor - Astronomics.

 

So that's where I'm at with transitioning to my old Celestron-8 on the Losmandy mount. I keep you'll posted.

 

Best Regards,
Russ

Thanks for this discussion. I have occasionally toyed with the logistics of eventually adding a mid size refractor (like the 115EDT) onto my C8. I currently have just a smaller one on top.



#18 Rustler46

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Posted 06 January 2021 - 05:06 PM

Thanks for this discussion. I have occasionally toyed with the logistics of eventually adding a mid size refractor (like the 115EDT) onto my C8. I currently have just a smaller one on top.

The two Celestron Dovetails have served my needs quite well, even if they are only connected by 3 screws each. Greg (gnowellsct) mentioned the screws themselves have enough tensile strength to withstand more load than we will likely put on them. But he mentioned different branded dovetails with shaped blocks give a more secure connection, compared to the Celestron one. 

 

My take on that is the more secure connection, with shaped blocks between OTA and dovetail, will greatly minimize any flexing and sideways forces on the screws where they interface with the threaded holes. While the aluminum alloy is quite hard and robust, there are a limited number of threads that are holding each screw. So if one has the financial resources to buy the better designed dovetails, it is of benefit, particularly if astrophotography is in the picture. Differential flexure of guide-scope versus imaging OTA will be less with then more expensive product. Of course and OAG will solve that problem. Still, I'm please with the changes being made to my C-8/G11/G-1 setup.

 

On a bit different subject, I have been quite thrilled with my first modern refractor, the Astro-Tech AT115EDT. Whether alone on the G11 or atop my SCTs, the views with that APO are quite outstanding. After being a reflector and SCT aficionado for so long, I pleased to have "gone over to the dark side". rofl2.gif roflmao.gif I'm a fanboy of all different optical designs.

 

Alphatripleplus, I hope you continue enjoying your telescopes. What small refractor do you have? I'm sure it provides a nice comparison atop your C-8. Even atop the C-11, my little APO triplet stands the comparison quite admirably.

 

Kind Regards,

Russ



#19 Rustler46

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Posted 07 January 2021 - 12:08 AM

I believe it uses the same holes that the stock Celestron dovetails use. I would expect you use the supplied (Losmandy) screws to attach the blocks securely to the OTA castings. Then the other supplied screws secure the dovetail to the blocks. Of course it is also possible that long screws extend down through the dovetail and the blocks to the casting threaded holes. I'll let you know for sure in two days when it arrives. Also a quick call or email to Astronomics would no doubt have an answer for that.

 

Edit: The Losmandy.com website has this to say

Celestron dovetail plate for 8", bolts to existing holes.

Wt. 3.7 lbs.

Length 14"

HS 12.325/12.70,

Ht. top of tube to top of dovetail bar 1.20"

Thickness .75"

 

As I recall my C-11 has two screws on each end, front and rear. It has an even heavier load with the OTA (plus accessories) at around 35 lbs. So I'll likely replace its lower dovetail on the C-11 as well.

 

Russ

The new dovetail arrived today via 2-day FedEx. What a wonderful piece of CNC machining! The insides of the plate have excess aluminum machined away for weight reduction without compromising strength. I appreciated the black anodized finish and inch markings along the edges to have repeatable placement of OTA for balance. Also supplied is a stop bolt to catch the optical tube if the dovetail clamp doesn't hold the dovetail securely. Many of the holes visible in the photo are threaded. The allen-wrench seen seen in the photo is my own.

 

As expected, the curved spacer blocks are first attached to the optical tube using the supplied stainless steel allen-head bolts. Other SS bolts secure the dovetail to the blocks. These screws will replace the black steel bolts supplied by Celestron. 

 

Losmany C-8 Dovetail-02430.jpg

 

I'll be ordering another of these from Astronomics for use on top of the C-8 for piggybacked equipment. This is presently out of stock, so the Celestron item will serve until it arrives. Also I'm ordering the Losmandy C-11 plate, which is in stock. It's nice to receive a 3% discount from Astronomics as a Cloudy Night's member for these Losmandy products.

 

Russ


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#20 Rustler46

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Posted 07 January 2021 - 04:47 PM

So next task is to insulate the C-8 optical tube assembly (OTA). This has proved quite successful in dealing with thermal issues on the C-11. While the C-8 is much less subject to such issues, it will likely benefit from a similar treatment. There's an excellent (but lengthy) discussion that is of value for those interested:

I entered the discussion on page 10, #227. But I feel enlightened by all that was shared. Here's the C-11 looking like a satellite, ready to be launched into orbit. I just need to add the solar panels.  smile.gif

 

C-11 + Dewshield Insultated-01481.jpg

 

So I located some leftover Reflectix, which will be enough to similarly outfit my old C-8. Here's the method I ended up using for insulating the C-11 OTA.

 

C-11 + Mesh-01489.jpg

 

The mesh fabric under the Reflectix increases the R -value significantly. Great fun on extended cloudy nights.

 

Russ

 


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#21 alphatripleplus

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Posted 08 January 2021 - 08:57 AM

 

 

Alphatripleplus, I hope you continue enjoying your telescopes. What small refractor do you have? I'm sure it provides a nice comparison atop your C-8. Even atop the C-11, my little APO triplet stands the comparison quite admirably.

 

 

I primarily use the C8, but occasionally I mount a small AT72EDII piggyback on top. The piggyback mount can handle the weight of this small scope, but anything like the AT115EDT would need rails.



#22 alphatripleplus

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Posted 08 January 2021 - 08:58 AM

I've also added Refectix to my C8 (double wrapped the tube), and have been pleased with the results so far.


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#23 Rustler46

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Posted 08 January 2021 - 09:24 PM

Last night I installed the new Losmandy dovetail after solving a minor problem. The supplied stainless steel screws are first class. But the single screw holding the front curved block was longer than the threaded hole it connects to. It bottomed out before the block was securely held. So I found a tiny washer that was small enough in outside diameter to fit within the cutout in the block. Also the inside diameter had to be large amount to accept the screw. But searching through my little jar of tiny washers I found one that works. It enabled the SS screw to tighten on the block before bottoming out. 

 

Tonight there is one clear night between many cloudy and rainy ones. So I'm heading out with with my 10-inch Dob reflector with Nexus-II digital setting circles. It's my grab-n-go optic.

 

Russ


Edited by Rustler46, 08 January 2021 - 09:26 PM.


#24 pweiler

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  • Loc: Chicago, IL, USA

Posted 09 January 2021 - 05:09 PM

I confident it will be back under the stars before long. That size of SCT is a "sweet spot" of sorts. It does require a significant increase in mount to properly carry. For now the C-8 will be sweet too. Here it is in original configuration.

Celestron-8.jpg
Celestron Super C-8 Plus from around 1985

It is easy to remove the OTA from the forks for mounting on the Losmandy G11.
Celestron-8 Solar Setup.jpg
Set up for the 2017 Solar eclipse

Kind Regards,
Russ



#25 pweiler

pweiler

    Explorer 1

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  • Posts: 74
  • Joined: 20 Oct 2020
  • Loc: Chicago, IL, USA

Posted 09 January 2021 - 05:10 PM

I confident it will be back under the stars before long. That size of SCT is a "sweet spot" of sorts. It does require a significant increase in mount to properly carry. For now the C-8 will be sweet too. Here it is in original configuration.

Celestron-8.jpg
Celestron Super C-8 Plus from around 1985

It is easy to remove the OTA from the forks for mounting on the Losmandy G11.
Celestron-8 Solar Setup.jpg
Set up for the 2017 Solar eclipse

Kind Regards,
Russ


Is the Celestron Super C8 plus OTA weight 28 pounds?


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