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About to give up on the 8" RC

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#1 Phil Hosey

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Posted 14 November 2013 - 01:55 PM

The last issue I cannot resolve seems to be that the heavy weight of my camera and focuser are causing the primary cell to flex and therefore knock collimation out when not pointed at the zenith. So.. back to the drawing board for a long focal length OTA. :bawling:

#2 rflinn68

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Posted 14 November 2013 - 02:02 PM

Sorry to hear this. :( How heavy is that QSI? Isnt this the same scope as my Levenhuk and Madratters Astro Tech? He uses an SBIG 8300 mono and filter wheel with the stock focuser.

This is really disappointing to hear as I have been looking into various monochrome cameras and filter wheels and was planning on my RC being my main imaging rig. :(

#3 Footbag

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Posted 14 November 2013 - 02:05 PM

The last issue I cannot resolve seems to be that the heavy weight of my camera and focuser are causing the primary cell to flex and therefore knock collimation out when not pointed at the zenith. So.. back to the drawing board for a long focal length OTA. :bawling:


This isn't the first time I've heard this. The "flex" of the RC tubes helped convince me to go with the Edge. It's too bad. If the build quality of the Newts or RC's were slightly better, people would be having a lot fewer issues with them.

#4 rflinn68

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Posted 14 November 2013 - 02:08 PM

The last issue I cannot resolve seems to be that the heavy weight of my camera and focuser are causing the primary cell to flex and therefore knock collimation out when not pointed at the zenith. So.. back to the drawing board for a long focal length OTA. :bawling:


This isn't the first time I've heard this. The "flex" of the RC tubes helped convince me to go with the Edge. It's too bad. If the build quality of the Newts or RC's were slightly better, people would be having a lot fewer issues with them.


The tube or the cell? He is saying the cell is flexing. If its the tubes then mine is a carbon fiber and is different. I believe the OPT brand, Levenhuk, and Astro Tech cells are all the same though (GSO).

#5 Footbag

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Posted 14 November 2013 - 02:13 PM

You are right. It's the cell that seems to be causing issues.

#6 Madratter

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Posted 14 November 2013 - 02:34 PM

I haven't tried imaging at 1625mm yet. So my usage has been less critical than Phil's. I'm also resigned to the fact that the collimation is not going to stay perfect. I new that when I bought it. It is, at least for now, staying within roughly 0-4" of perfect. And that generally gives pretty good results that have been limited by seeing, not the collimation.

#7 mpgxsvcd

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Posted 14 November 2013 - 02:42 PM

The last issue I cannot resolve seems to be that the heavy weight of my camera and focuser are causing the primary cell to flex and therefore knock collimation out when not pointed at the zenith. So.. back to the drawing board for a long focal length OTA. :bawling:


How much does your camera weigh? I have a Newtonian with a coma corrector and the same thing happens with it. I was hoping to get the RC one day thinking that it would be better for coma.

Are you using a filter wheel? How much weight does that add? Is it a two inch filter wheel?

#8 mpgxsvcd

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Posted 14 November 2013 - 02:44 PM

Are you going to sell the RC then?

#9 bseltzer

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Posted 14 November 2013 - 02:46 PM

So let's say someone is looking to start working at a focal length greater than is available with most refractors. What would be a good choice? I've seen the Celestron Edge HD scopes suggested, but the 2000mm focal length on the 8" version seems somewhat daunting to me. The Meade LX-ACF series has a more attractive focal ration at F8, but their smallest OTA is a 10", so we're right back to a 2000mm focal length. It sounds as though a focal reducer would be the solution, but don't they all induce some degree of aberration?

FWIW, I've got a CF tubed AT8RC that does show some de-collimation when loaded with a QSI683wsg-5. For the time being, I'm trying to mitigate the flex by going to a beefier 2.5" format focuser. If it's the rear cell itself that flexing, I don't know how successful that'll be. So I'm asking about alternatives.

#10 Madratter

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Posted 14 November 2013 - 02:48 PM

Not sure about Phil, but my camera together with filter wheel probably tips the scales at around 5 pounds.

#11 rflinn68

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Posted 14 November 2013 - 02:50 PM

Well this is concerning to me going forward. I collimated mine when I first got it and havent had to touch it again. I am eventually wanting to get a monochrome camera and filter wheel. Will this become a problem for me?

I dont plan to image at 1625mm because it is f/8 but I have been considering upgrading my mount and buying the new truss tube RC to image at 1730mm with my focal reducer. I wonder how the cells are made on these scopes? It seems like a really good deal at only $500 more than the 12" steel tube.

Guess I should be content with my DSLR a while longer or else a OSC CCD? :(

#12 dcbrown73

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Posted 14 November 2013 - 02:52 PM

Ouch. I was thinking about getting an 8"RC for AP. I hadn't heard about this issue.

#13 rflinn68

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Posted 14 November 2013 - 02:54 PM

For the time being, I'm trying to mitigate the flex by going to a beefier 2.5" format focuser. If it's the rear cell itself that flexing, I don't know how successful that'll be. So I'm asking about alternatives.


Phil has already bought a Moonlight 2.5" focuser :(

#14 mpgxsvcd

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Posted 14 November 2013 - 02:55 PM

Not sure about Phil, but my camera together with filter wheel probably tips the scales at around 5 pounds.

Wow that is heavy. I am dealing with 1.2 pounds for my heavy camera and .85 pounds for my lighter camera. Maybe an RC would work for me?

#15 Footbag

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Posted 14 November 2013 - 02:58 PM

Not sure about Phil, but my camera together with filter wheel probably tips the scales at around 5 pounds.

Wow that is heavy. I am dealing with 1.2 pounds for my heavy camera and .85 pounds for my lighter camera. Maybe an RC would work for me?


From what I remember the last few reports of this issue were with heavy CCD's. 5lbs. That's darn heavy.

#16 rflinn68

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Posted 14 November 2013 - 02:58 PM

What about one of these?

#17 bseltzer

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Posted 14 November 2013 - 03:02 PM

Not sure about Phil, but my camera together with filter wheel probably tips the scales at around 5 pounds.


Ive got the QSI683wsg-5 that I use with a Lodestar guider, and a AT2FF field flattener. I just put the whole works on a commerce certified scale, and it came out to 3.78 pounds. Which camera do you have?

#18 bseltzer

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Posted 14 November 2013 - 03:13 PM

For the time being, I'm trying to mitigate the flex by going to a beefier 2.5" format focuser. If it's the rear cell itself that flexing, I don't know how successful that'll be. So I'm asking about alternatives.


Phil has already bought a Moonlight 2.5" focuser :(


Aarrg!! That's not what I'd hoped to hear. I will say this, though. Having looked at the images produced by some of the folks here using the various clones of the GSO 8" RC, they look pretty good to me. So I guess my question is how big an issue is this flex really?

#19 rflinn68

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Posted 14 November 2013 - 03:18 PM

Well Madratter has took some outstanding images with his heavy setup. I have not even heard of this issue until now and have not had any trouble, but all I've had in mine is a Canon T3 mated to the Astro Physics CCDT67 Telecompressor.

#20 bseltzer

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Posted 14 November 2013 - 03:20 PM

What about one of these?


A focuser collimation ring of some sort is IMHO an absolute necessity for proper collimation of these RC's because it's the only way to separate focuser alignment from alignment of the primary mirror. That said, I don't know if that has any impact on the flex issue or not. I rather doubt it, but I don't know for sure.

#21 Madratter

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Posted 14 November 2013 - 03:22 PM

Well, I overstated it. I looked up the STF-8300m (my camera) and it is 1.8 pounds. I also the FW8-8300. I can't find a weight for it, but it is probably less (though probably not a lot less) than the camera once loaded with filters. So 3.6 pounds or a little less is probably closer to the mark.

#22 Madratter

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Posted 14 November 2013 - 03:26 PM

Wow that is heavy. I am dealing with 1.2 pounds for my heavy camera and .85 pounds for my lighter camera. Maybe an RC would work for me?


I won't say it will be a non-issue. Any weight will flex this design some. But I would anticipate it being a minimal issue with that weight.

The design IS clearly non-optimum. There is a reason they can sell these for so much less than other RCs. I personally find mine quite usable at 1190mm. I don't know how I would feel about it at 1625mm as I haven't tried yet.

I will also say I have seen Phil's images at 1625mm. Obviously I haven't been able to do FWHM on his images, but I thought his images were very acceptable despite this issue.

I knew about this issue before I bought the scope.

#23 Phil Hosey

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Posted 14 November 2013 - 03:49 PM

Hey guys sorry I'm late to respond back. I've read all the responses so far and I want to address a few questions. First of all, Madratter, the images that I have posted so far at 1625mm seem to be a lucky shot, I haven't been able to repeat them in 3 consecutive tries. My guiding error is less than .5 arcsec RMS but yet I'm getting 'pear' shaped stars. With seeing around 3" FWHM my stars should be round.

My camera, which is the QSI 683wsg-8, weighs over 3 pounds, the moonlite 2.5" focuser weighs about as much. The Moonlite focuser actually makes the situation worse because it adds even more weight to the back of the tube. The problem is that the rear thread that everything attaches to is connected to the primary mirror cell. The focuser itself weighs almost as much as the camera. I probably have about 7 pounds of stuff hanging off the back when you include the guide camera, focuser extensions, etc. By the way, my camera connection to the focuser is all threaded so there should be no flex there.

When I get home this evening I'll post some images of the issue I'm having now for everyone to see, but I'm pretty certain that it is being caused by this flexure. If the weather is good tonight I may give it one last go, I have a couple of things to try, but at the very least my aim is to confirm my suspicions. Oh, one more thing about the Moonlite.. last night it was about 25F (-4C) outside when I was doing this and the fine focus knob would not move the camera inward. I'm not sure if it is the weight of the camera or something to do with the cold or both.

#24 Madratter

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Posted 14 November 2013 - 04:00 PM

One thought Phil, and you have probably already been there done that. But this the collimation is a push-pull. Is everything real tight?

I haven't actually changed the collimation of my back cell because it was pretty good to start with. I have no idea how tight they come.

#25 Phil Hosey

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Posted 14 November 2013 - 04:30 PM

One thought Phil, and you have probably already been there done that. But this the collimation is a push-pull. Is everything real tight?

I haven't actually changed the collimation of my back cell because it was pretty good to start with. I have no idea how tight they come.


Yes, in fact that was a concern of mine earlier, so I made double sure it was all locked down tight. You can see what I am dealing with now by looking at the image here.

This has basically been the same issue for the last 3 outings, ever since I got the heavier Moonlite focuser. And just to make it more confusing, here is the CCDI plot:

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