UKAI is moving to Facebook

Author Topic: GSO Ritchey Cretien 254mm f/8: First impressions  (Read 5556 times)

0 Members and 1 Guest are viewing this topic.

Offline Astrogeordie

  • The Regulars
  • Poster God
  • ***
  • Posts: 887
GSO Ritchey Cretien 254mm f/8: First impressions
« on: 17:01:55, 12 September, 2011 »

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:

Stray light issues

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 as light from the sky can enter the focal plane directly past the secondary mirror:

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 small light leakage 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.
Clear skies,


My scopes: 30 year old Tasco 60/700 on 10K mount and a few others ...

If you have no friends, just buy a lathe and things will change.

Offline Mad Man Moon

  • Full Member
  • ***
  • Posts: 335
Re: GSO Ritchey Cretien 254mm f/8: First impressions
« Reply #1 on: 18:23:04, 12 September, 2011 »
Good report Jurgen, let's hope the manufacturer visit's this forum, hence -  hang fire in ordering me one I'm not interested until they have sorted the focuser issue.
9.25" carbon fibre schmitt. Cassegrain,127mm Meade triplet,William Optics ZS66, 80mm Moonfish,EQ6 Pro.with SynScan,HEQ5 Pro.with Synscan,Webcams:Philips 690K,740K,ToUcam Pro2,DMK21 Mono,Greyscale Connectix Quickcam,Mintron 12V1-EX,Canon 300D,Canon 1000D.
The Argo :Autotrail Cheyenne motorhome

Offline Astrogeordie

  • The Regulars
  • Poster God
  • ***
  • Posts: 887
Re: GSO Ritchey Cretien 254mm f/8: First impressions
« Reply #2 on: 18:33:36, 12 September, 2011 »
Hi Keith,

it sounds a bit like a repair thread, but I should stress the point that despite of the baffle and focuser issues ...


I am literally over the (full) moon with it !

Clear skies,


My scopes: 30 year old Tasco 60/700 on 10K mount and a few others ...

If you have no friends, just buy a lathe and things will change.

Offline chris.bailey

  • Administrator
  • Galactic Poster
  • *****
  • Posts: 10012
  • If at last you do succeed, don't try again
    • Rosemount Observatory
Re: GSO Ritchey Cretien 254mm f/8: First impressions
« Reply #3 on: 19:17:17, 12 September, 2011 »
Thats a lovely looking scope. Shame that they spoiled it a little for a few pence here and there. I assume the focuser could be easily upgraded?
LX200|ZS70|FSQ85|FLT110|Altair DF250RC|EQ6 Pro(Rowan Belt Mod)|ParamountMX
ATIK383L+/EFW2/OAG|Lodestar|Baader 36mm LRGBHaSIIOIII
Starlight Express SXVRH16/ONAG/FW|Lodestar X2|Baader 2" Filters
Starlight Express SXVRH814/ONAG/FW|LodestarX2|Baader 2" Filters
Lunt LS60PTBF1200|DMK41|Quark Chromo
Samyang 135mm f1.8

Offline Astrogeordie

  • The Regulars
  • Poster God
  • ***
  • Posts: 887
Re: GSO Ritchey Cretien 254mm f/8: First impressions
« Reply #4 on: 21:29:31, 12 September, 2011 »
Hi Chris,

I think there are other focusers available, but I will stay with this one for now. I glued the plate, and if it fails I will mill the pocket out and insert a metal piece.

Clear skies,


My scopes: 30 year old Tasco 60/700 on 10K mount and a few others ...

If you have no friends, just buy a lathe and things will change.

Offline astrograph

  • New Member
  • *
  • Posts: 39
    • Astrograph
Re: GSO Ritchey Cretien 254mm f/8: First impressions
« Reply #5 on: 12:02:43, 07 March, 2013 »
Hi Juergan

Its refreshing you assessed the stray light from the focal plane. So many people just look up the tube which is not how baffles are designed to work.

The focuser issue is, I'm afraid a cost issue. The Chinese will cut corners unless instructed not to. Some well known brands of SC even have plastic secondaries!! The GSO can be upgraded to a Starlight focuser, which if your imaging, will not move under load.

One thing you might be interested in is this optical test on an 8" GSO. It highlights how you may be able to extract quite a bit more performance from your scope than is available as supplied.

Happy tweaking!


ukbuysellRemote Imaging from AustraliaSharpSkyblank APTUKAI on Facebook
Powered by SMF 2.0.15 | SMF © 2006, Simple Machines LLC
DarkBreak by DzinerStudio. Theme modified by The UKAI Team

Page created in 0.221 seconds with 36 queries.
TinyPortal © 2005-2012