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GSO 254mm f/8 Ritchey Cretien: First impressions


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after I got bitten by the bug of long focus deep sky imaging using my ex-C9.25, I was after an instrument with an open tube and a flat field. My first approach was an Orion Optics OMC250 which unfortunately is rather a high resolution planetary machine where the field was not good enough for a DSLR chip. So I now jumped into cold water to purchase a 10" Ritchey Cretien - mine was bought at Teleskop-Service in Germany and it came with a 3" monorail focuser.

Last Friday the sought after scope finally arrived. The 735mm long, 15.7kg heavy metal tube (I did not go for the carbon fibre version) came in a double shell cardboard box and styrofoam. I found out later that this package was good enough to even keep the alignment steady - the scope had been aligned and tested at the dealer before - with documentation. Well spent 30 Euros to avoid a lemon ...

Here some details.

The primary is of fused silica glass and overcoated with a dielectric layer of 99% reflectivity. The view on to the primary reveals the RC-typical large secondary and a multitude of baffles in the tube to suppress stray light. What an effort !





The tube can be closed at the front using a black plastic lid.


The back is - as the front ring - made of solid metal (I reckon Aluminium) and it has a nicely textured black paint coat. The 3" focuser is fixed with a large thread that allows rotation of the focuser as well. Primary and secondary are adjustable. The primary cell has 3 built in fans that can be run with the same 12V cable that powers an EQ6 - handy. A battery pack (8x1.5V AA) is included as well.


The baffle bears an antireflective thread.


The scope has a Losmandy-style dovetail on both sides. I modified them plates with some bores to put securing bolts on the mount-sided plate and a dovetail attachment for my guide scope on the other one. When I removed the plates I learnt they are made to high standards with milled out pockets to save weight.



FIrst light

On Saturday (2011 Sep 10) I could test the scope for a couple of hours in "technical conditions" - full moon and murky cloud layers. It was mounted on a Skywatcher NEQ6 pro in my observatory.

The instrument came into focus easily using a 2" diagonal and 2" eyepieces. The focal plane is placed a generous 200mm outside of the tube to have space for accessories as filter wheels or offset guiders. The alignment was spot on, and even the "killer test" with an LVW 3.5mm (571x !) did not show any aberration other than inflicted by the atmosphere as the seeing was not that good and the atmospheric dispersion of Altair was clearly visible. The intra- and extrafocal disks are nearly identical.

Albireo was bright and colourful with the LVW 13mm (154x), here the large aperture really delivered much light. Epsilon Lyra was easiliy split into its four components using a Speers-Waler 5-8mm (400x - 250x) and the double cluster in Perseus demonstrated the high degree of field correction as the stars remained pinpoints to the very edge of my noname 70-degree FOV 26mm eyepiece (77x).

After roasting my eyes on the full moon, I connected my Canon EOS 40D. Here I had to use a 2" extender tube of my collection to get into focus. The focuser needed a bit more tension to cope with the heavy camera. The micrometric screw did not always work, so I decided to sort this out the next day.

Here some photographs made in a quite rough night with imperfect tracking. The potential of the telescope is recognizable:

The moon - one in monochrome as it was pink - a modded DSLR is not first choice for this object.



M13 - single shot 2.5 min dark subtracted, no flatfielding:


h+x, 14 times 2 min with dark subtraction, no flatfielding:


The issue of day blindness

Reducing the data I recognized some dust related donuts that the lunar image did not show but the deep sky shots do:


As the donuts were invisible at the short exposures done for the lunar shots, the light that caused these shadows must have gone another path.

The view through the RC from the detector side reveals a slight daylight blindness issue:

When the focuser is completely retracted, the observer sees sky next to the secondary. However, this is in front of the focal plane.


In the focal plane it looks like this:


If you go to the field edge, a crescent of sky light contamination becomes visible again:


I tested this wih an old film SLR - shutter and back open:


On axis everything is fine:


In the field you get light intake:

Edge of 35mm film format:


Half of this distance:


Top edge of 35mm film format - 12mm above the optical axis:


A slight daylight blindness shows up especially if the sky is lit up as it was with the full moon during the test night. The manufacturer could deliver an add-on stop that could be clamped onto the secondary for photography and removed for visual joy to keep the obscuration low.

The focuser - and a little plastic problem

The 3" focuser is quite a beast. It can be moved smoothly, using a system derived from the Crayford design. However, instead of having bearings at the drawtube opposing the axle, the whole tube is attached on one side on a precision linear ball bearing stage.




The micrometric slow motion did not work when I put the friction screw down to hold heavy equiptment. Here the recipe to sort this out:

The four silver M4 hex bolts are removed and the axle bearing is taken off.


Removing the silver and black knobs, the micrometric motion becomes visible. The thin axle of the black button drives three balls that roll off outside on a bearing surface. I had several Taiwanese focusers in my workshop who all needed propping up a bit as the pressure of the balls is not high enough. Hence the micrometric drive stops working when the friction is increased for heavy equipment.

Let's sort it out then:


After removing the main drive shaft, the silver nut has to be tighened a bit using a 13mm spanner. Just put the silver button back on to have something to hold on while you are doing this job.

After that, the micrometric stage will feel a bit stiffer but it will work even with tough pressure towards the linear slide !


Talking about tough pressure - when I increased the load to make the focuser cope with my heavy camera, there was a sharp crack audible. The housing of the shaft broke !


This part is made of PLASTIC ! Urgent call to the manufacturer: PLEASE make this of metal ! This part is bearing a high load and making it out of plastic cries out for breakage.


I could fix it with some epoxy glue:


The focuser works fine now, but in this point with the plastic part there is significant potential for improvement.


The GSO RC is a powerful instrument of high quality. The optics is up to high standard and the whole instrument makes a good impression to me. If I had to do the decision again, I would do exactly the same purchase without further hesitation.

The only bad points are the insufficient secondary baffling and the focuser issue. Here the manufacturer could improve things with little effort.

Best wishes and clear skies,


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thanks for your nice replies. I strongly recommend at least an EQ6 for this scope. However, for the HEQ5 pro there is the 6" and 8" model available - maybe a thought for owners of those scopes.

If you want 10" visually and for the planets, the (shorter and lighter) Orion Optics OMC250 has to go now. Just pm me if you are interested.

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  • 2 weeks later...

Thanks for your very thorough review Juergen. I was about to buy the 2" SCT version of this focuser to help with the weight of my camera and to make accurate focusing whilst imaging a bit easier. It looked like a good value alternative to Baader's Steeltrack model, but you have pointed out a weakness, so maybe I should spend the extra and get the Baader.


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  • 5 months later...

Hi there. So as some of you might have seen, I have gotten a GSO RC8" lately. I'm encountering some reflection problems (see the thread I posted on this, with links to the pictures). Could this secondary baffling be the problem ? Is there any solution to this ?

Note : I get the problem especially with a CCDT67 reducer. The camera setup is then mounted to the focuser without the M90 rings between the focuser and RC.

I get the problem with a TS 2" flattener as well (is it actually useful ? I wonder), WITH the M90 rings mounted, but much less. The halos will show up on the very edge of the shoots.

Please can anybody tell me what I'm supposed to do now ?

I've repainted the inside of the M90 and 2" adapter rings with black mat spray, and it didn't help.

Please help ! Please ? This is just driving me nuts...

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  • 3 months later...
  • 3 weeks later...

Hi all !

FYI, I've finally solved the reflection problem for good on my GSO 8" RC. And as a matter of fact, it had to go all the way up back to GSO. After several testings I did with a friend of mine, painting the inside of the focuser, flokking the adapters and so on to no avail, they aknowledged that there was a design flaw with the primary mirror baffle tube, and they redesigned it on our request.

The existing RCs can benefit from this new design, because the baffle tube actually has a thread at its end. So you just need to screw the extension they made to the existing baffle tube and here you go, no need to take the thing apart, just find some small hands that can reach the baffle tube through the spider.

I did some testings using a CCDT67 reducer, a configuration that was completely unusable beforehand (I used to have these big hallows all over the place, shooting with a DSLR) and well, I must say it solved my problem.

From there on, I have no clue how one can get the extension. I dealt this problem with a reseller here in France, and since we were the ones to hit the nail on that one, we got our extensions fo free. Maybe just see with whoever sold you the RC. GSO knows about all this, so I guess any reseller should be able to get hold of this item.

Clear skies to all of you !

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  • 6 months later...

Thanks so much this provided a clue to my flat fielding issues using a full frame 36x24mm chip. As above I put a 35mm empty film slide at the focus plane and looked inside I am getting heaps of stray light around the edges. The flats have a bright middle then a dark ring followed by a much brigher then falling off darker in the corners. When I try to calibrate my images they end up with colour casts all over them as the light spill is additive therefore not able to be calibrated out but the normal vignetting etc is multiplicative and can be canceled out. Any idea how much the inner primary baffle tube needed to be extended to solve the baffling issue?

I have a GSO 10"RC. I have found the Teleskop-express TS2.5Flat flattener is perfect for this telescope.

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  • 4 years later...

Appologies for commenting to an old post, but I wonder if anybody has gained any further opinions of the GSO 10" RC please, or know of any other reviews?. Sadly, all the picture links in this review are now dead. I am considering buying this scope on either an iOptron iEQ45 or a Skywatcher AZ-EQ6GT. Many thanks.




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