GSO Ritchey Cretien 254mm f/8: First impressions

[Astrogeordie]

Hiya,

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.

Verdict:

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.

[Astrogeordie]

Hiya,

as I cannot find the EDIT button to change my text I posted yesterday, here an amendment:

In Germany we have the term "Tagblindheit" for Cassegrain-style telesocpe to describe the problem that light from the sky can go past the secondary mirror into the focal plane.

This is the reason a proper baffling is required for this telescope layout in contrast to refractors (all light passes through the object glass) or Newtonians (light comes from the black wall opposite the focuser - this wall acts as baffle).

I introduced the term "daylight blindness" above as a direct word-by-word translation of the Tagblindheit-term. Probably this is nonsense.

What it is all on about is light leakage from the sky to the focal plane past the secondary mirror.

This may be suppressed by ...

... a larger secondary baffle: More obscuration, no more field vignetting

... a longer tubular primary baffle: No more obscuration but more field vignetting

I hope this clarification works.

[davew]

Hi Juergen,

What a fantastic write up. Thanks.

In your professional view which of the two alternative methods would you prefer to use to defeat the light leak ? A secondary baffle would be the easiest after market solution but I would have thought the manufacturer should make a slightly longer baffle tube.

How much extra vigneting would a correctly sized baffle tube make ? Would a DSLR sized chip " See " the difference ? Also, if using a reducer, would this create too much vignetting ?

Any chances of you making a small card / paper tube extension and trying it ?

I won't even mention your plastic housing !

Dave.

[Astrogeordie]

Hi Dave,

thanks for your reply.

It is difficult to tell - there is a multitude of possiblities how to fix it. All depends on the shape of the stops (flat or flared secondary stop / flat or flared primary baffle tube) and of the observers preference.

For visual use the primary baffle tube can be made longer as some vignetting in the field is not that critical. You just want to keep the tube in the conical shadow created by the secondary mirror after reflected off the primay - that is the reason the tubes are often flared as they follow the primary mirror focal ratio. However, for large detectors the field vignetting may get too large.

Hence, for applications where the central obscuration does not matter too large, the secondary baffle is increased. This can happen with a flare as well as you see the inner flare wall from a shallow angle. By this perspective effect you minimise the central obscuration. However, especially for very large detectors those baffles can be quite hefty as visible here at the Calar Alto Observatory in Spain:

http://www.caha.es/CAHA/Telescopes/2.2m.gif

Advantage is a more uniform field illumination.

If would help to have the exact dimensions of the existing baffles and their distances to calculate the baffle optimisation.

[davew]

Cheers Juergen,

From your description it looks like Visual = leave well alone if it doesn't bother you and for Imaging = home made secondary baffle. This I take it would be best lined up by eye on a trial basis depending on chip size ?

If a reducer was thrown into the mix would that " Hide " the problem ?

All good stuff. Thanks again,

Dave.

[Astrogeordie]

Hi Dave,

I think exactly like you - visual mode as it is plus optional add-on stop for photography where the additional obscuration does not really matter.

To the reducer - as the reducer effectively compresses a larger field of view to a smaller area, the stop has to be designed for a chip being larger by the reciproke of the reduction factor.

For example: Chip is 14.8 x 22.1 mm (Canon 40D), reducer is 0.8x.

Then the stop needs to be corrected for a chip of

(14.8mm x 22.1mm) * 1/0.8 = 185.mm x 27.625 mm

This assumes that the reducer is large enough not to introduce additional vignetting by itself.

To "hide" the problem you can use an area small enough that the glare does not settle in yet. This can be achieved either by using a Barlow lens or by a smaller chip, e.g. a CCD system.

[philming]

Hello !

Please check my answer on the other GSO RC10 topic. We have worked along with a reseller and went straight back to GSO. They aknowledged a flaw in the primary miror baffle design and redesigned it. They actually sent us an extension to be screwed on the existing baffle tube (yes, it's threaded), and my reflection problems, especially when using a CCDT67 reducer are gone as far as I can see.

The shape is that of a simple tube (approx 10-15cm long) with a narrowed, slightly conical front side.

No need to unmount the whole thing, if your hands are small enough to get past the spider, it's a matter of seconds to mount this add-on.

Regards,