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RCs a scope for masochists?

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#1 glend

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Posted 03 October 2015 - 06:50 PM

Before the flaming starts let me state that I own one of these scopes (a GSOmade Astrotech/Orion now/ and many other brands), mine is the GSO 8".  Most of us (that own them) know that these scopes are considered to be "Projects",, your buying the basic RC optics package and they all need work to get the best out of them, and that works includes the need to buy stuff to replace the stuff they came with (focuser for example) or stuff they didn't come with but should have (like a collimation ring), and specialised tools (like the TAK Collimation Scope) to be able to get them tuned to a point where they can be used for astrophotography.  I have been fighting with mine for months now, bought all the stuff and tools, and got it collimated and star tested which required further changes to collimation. Recent imaging sessions taught me a few things, one: imaging at f8 is very different to f5 on my wonderful imaging newt.  And while the greater focal length reach is nice, it brings with it a few other things to sort out like guide setup etc.

 

I am at the point where I wonder if it has been worth the effort just to get a little more focal length (1600mm on the RC verses 1250mm for the Newt) and that there may have been better choices to achieve that.  

So who else has run the gauntlet of GSO RC setup and what have you decided about living with these scopes long term?  Being a practical person I am leaning to something that is less of a challenge.



#2 rpineau

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Posted 03 October 2015 - 07:27 PM

I have an AT12RC .. and like you ran into the usual problems and bought all the toys to solve them (FT focuser, TAK collimator ...) . My solution .. sale it (it's still for sale) and buy a proper RC (mine will come from CFF Telescope Europe, should be here soon).

I'm mostly selling mine for 2 reasons : the weight (my back is not what it use to be) and the proper separation of the focuser and the primary.

Regards, Rodolphe



#3 rainycityastro

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Posted 04 October 2015 - 03:05 AM

The GSO RC can be a frustrating scope to collimate precisely. They have a couple of obvious flaws that might have been fixed in the most recent versions: 1. The primary and focuser tilt together when adjusting the primary mirror collimation screws. If there is any non-orthogonality between primary and focuser tube,  you will collimate wrong. 2) Under the weight of heavy cameras, the collimation can shift at different parts of the sky.

 

Both the above issues can be easily diagnosed and fixed in the daytime by using a hotech collimator and a tilt adjustment/tightening ring that astronomics sells.

 

There is a 3rd problem that RCs suffer from: spacing issues between primary and secondary. Improper spacing can manifest as spherical aberration. The best way to diagnose this is with a Ronchi eyepiece and test on a bright star just inside of focus and verify that the Ronchi lines are straight.  Even better is to do a null Ronchi test on an optical table set up for autocollimation. This step will magnify errors by a factor of 2 and help diagnose issues nicely during the daytime. 

This step is usually not required because significant despace (> 4 mm?) before it starts becoming obvious. But I've heard of cases where tweaking this helped tremendously. 

 

I think these RCs are ideal scopes for tinkerers and DIY sort of people. You could probably luck out and not have to do anything to your scope and get consistent sub 1.8" FWHM stars edge-edge. But in general you have to ENJOY tinkering with and tweaking your RC to get the best performance.

 

I had lots of fun with my 8" RC which didnt have any issues but eventually upgraded to a 12" CDK derived astrograph simply because I was seeking more aperture. This new scope like all cassegrain type scopes also requires careful collimation for best performance. 



#4 Richard Whalen

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Posted 04 October 2015 - 11:06 AM

I have a Mallincam VRC10-T, have none of the issues mentioned. After the first collimation it has stayed dead on since. Focuser works fine, but I did replace it with a FLI electronic focuser attached to a custom OAG & flip mirror system for my DSLR. Anything else I stick the original back on. Optics are excellent for such an inexpensive 10" RC, just a matter of getting everything square and centered during the first collimation.



#5 MartinW

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Posted 04 October 2015 - 01:20 PM

I have a AT10RC and have added a collimation ring.  I added a FT focuser and I have to say once collimated the scope says bang on.  Like Viking 1 said some of the shortcoming of the early versions have been addressed in the current truss design.  The most important change is being able to collimate the focuser seprate from the primary.  A Takahashi collimation scope should be in any advanced photographers tool box.  I'll never understand why people spend a small fortune on equipment and skimp on collimation gear.  So my advice, get a nice laser to center the focuser and secondary.  Now use a Tak scope to tilt your primary and secondary.  Check secondary with laser again as a few iterations are needed for perfect collimation.  A star test may reveal no more collimation is needed.  Martin



#6 andysea

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Posted 05 October 2015 - 12:02 PM

The new truss tube RC's from Astro-tech have the focuser that is decoupled from the mirror cell.

I have owned the AT 6", the 8"CF and the older 10" Truss tube. The best of them was the 6", I still miss that scope.

I eventually bought the Deep Sky Instruments RC10C, mostly because it's better mechanically, has a secondary focuser and supposedly better optics. I'm extremely happy with it.

The Astro-tech RC's are a good value for the money tho and gave me the opportunity to learn how to collimate and image with longer focal length instruments.



#7 andysea

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Posted 05 October 2015 - 12:03 PM

I have an AT12RC .. and like you ran into the usual problems and bought all the toys to solve them (FT focuser, TAK collimator ...) . My solution .. sale it (it's still for sale) and buy a proper RC (mine will come from CFF Telescope Europe, should be here soon).

I'm mostly selling mine for 2 reasons : the weight (my back is not what it use to be) and the proper separation of the focuser and the primary.

Regards, Rodolphe

Please keep us posted on your experience with the CFF.



#8 DeanS

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Posted 07 October 2015 - 11:15 AM

I just got the new version of the 10" truss and have not found it that difficult to collimate thanks to all the threads on this group.  But get the newest version with the focuser seperated from the mirror cell.

 

I use a Glatter laser to center the focuser.

 

Glatter circle holigraph to center the primary.  Have found that removing the baffle is a must.

 

Then use a Tak Collimation scope to fine tune.

 

Finally using the DSI instructions will do fine adjustments using stars. 

 

Not that hard once you do it a few times.

 

Dean



#9 Guest_djhanson_*

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Posted 07 October 2015 - 02:20 PM

Dean, that's been my experience as well (I have a CFF350 classical cass).  The focuser is also separate of the mirror cell and I find the HG laser + Tak scope does a nice job of rough collimation (one does have to be careful of laser/Tak scope registration or slight bobbling movement within the focuser of course).  Last night, I removed the baffle (it easily unscrews) and collimation was easier to detect.  I end with a final star collimation for any final adjustments.  cheers, dj



#10 glend

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Posted 07 October 2015 - 06:40 PM

Gents, I found with my RC that the final star test collimation adjustment (done with my cooled DSLR using Liveview mode ) takes the adjustment fairly far off the bench collimation settings.  I have no doubt that the bench collimation with the Howie Glatter and TAK Scope were exact its just that the weight of the camera on the focuser is causing flex and only through collimation through the camera could I get my star shape perfect.  So in my opinion, regardless of how well you think it is done on the bench, only the complete imaging train tuning can get perfection.



#11 DeanS

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Posted 09 October 2015 - 09:43 AM

Gents, I found with my RC that the final star test collimation adjustment (done with my cooled DSLR using Liveview mode ) takes the adjustment fairly far off the bench collimation settings.  I have no doubt that the bench collimation with the Howie Glatter and TAK Scope were exact its just that the weight of the camera on the focuser is causing flex and only through collimation through the camera could I get my star shape perfect.  So in my opinion, regardless of how well you think it is done on the bench, only the complete imaging train tuning can get perfection.

I have also found this to be the case with mine even though I have a massive Feather Touch focuser.  I checked it last night with the Tak Collimating scope after doing a star adjustments and it shows it off.  My stars are showing it is about perfect so that is what counts.  The Deep Sky Instruments method for tweaking is very easy to learn and produces good results for me. 

 

My guess is that if these where made as good as a RCOS, AGO, or Planewave, then it would not be this way.  Part of the reason the imports are less money is the detail in the design and machine work compared to the high end scopes.  Still a good deal for the money but like everything else, the last small percentage of improvements are usually exponential in costs.




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