[Shiraz (Ray)]

Hi.
FWIW, below are details on mods I needed to do in order to use a GSO 12inch Dob for EQ planetary use. Hope it might be of some use to anyone contemplating such a project. Regards Ray


Modifying a Dob for planetary use.


Why bother?
I reached the limits of my 8 inch scope and wanted to move up in aperture. For budgetary reasons, the need to use an EQ6 class mount placed a limit on how big a scope could be used – the next step up in mount class is almost an order of magnitude more expensive. Options on this mount were an 11(or maybe 14?) SCT or a Newt up to about 12inches. Mike Salway had good results with a 12 inch Dob on an EQ6, so I decided to try that route.


I bought a 12 inch f5 GSO Dob and was amazed at the optics quality. Based on images in Harold Suiter's book on star testing (a must-read by the way) the mirror is corrected to about 1/7 on the wavefront with a little turned edge and some slight roughness – not quite perfect, but excellent for commercial quality gear and really good to use.


Mods needed


tube rings – had some alloy bar rolled at the local metal shop – panel beat them into final shape with trusty hammer and anvil. Cut hinges and locks from a spare set of 8 inch rings – these are underpowered a bit, but have been reliable (see image). A pair of bungy cords is also used – just in case.

primary mirror
The centre dot was 6mm out of place. Removed it and cleaned the remaining adhesive from the centre of the mirror (gingerly) with a cotton bud and an absolute minimum possible amount of turps - worked OK. Replaced central spot with one cut from adhesive label.
The mirror clamps were too tight and had slightly damaged the mirror surface – hidden so not a practical issue, but the clamps needed loosening.
The mirror was only attached with adhesive tape under three points of the 9 point cell – not enough to fully support the mirror and sideways slip was sufficient (a few mm) for the mirror to come to rest on the rubber clamps, with obvious astigmatism the result. I removed the mirror and re-attached it with high-load double sided adhesive tape under all 9 support points. Also made up 4 edge supports from brass (see image) and set these up to provide additional side support for the mirror at low angles - should the mirror shift sideways by more than about 0.5 mm, it contacts 2 edge supports at 90degree spacing (provided the OTA is aligned correctly). This belt and braces approach works OK and astigmatism is now not a major issue.
The mirror support springs are way too soft – everyone says this but GSO keeps on with the standard sloppy springs – they must have bought a 10 ton job lot of the darn things. I couldn't find better compression springs, so improvised with springy plastic cups (see image) cut from Zork wine bottle seals – they fit over the springs and add just the right amount of extra stiffness.
With the modifications, the primary mirror remains pretty well in collimation, requiring only minor tweaking.


secondary mirror - the end of one of the spider vanes where it attached to the centre block was bent at a different angle to the others, throwing the whole assembly out of alignment – I shimmed it up and the better alignment improved collimation stability. The GSO mirror carrier is flexible plastic and tends to drift in collimation alignment, so an alternative was made up using plumbing fittings, an “Ikea” nut and a large steel washer (see image). This is much more stable than the original and a major improvement in collimation. The new mounting also allows the secondary to cool much quicker than in the original shroud design – a further useful advantage.


thermal issues – the main problem was with the painted steel OTA tube. Measurement showed that it could reach temperatures many degrees below ambient due to radiative cooling from the cold sky – this resulted in fierce tube currents, even when the mirror had stabilised. I coated the tube with highly reflective tape of the type used in the air conditioning business and the tube temperature problem went away – a major improvement for a few bucks.
I also removed the fan from the main mirror and replaced it with a two speed fan on rubber band support – this allows the fan to continue to run at low speed during imaging. At high speed, the fan is able to keep the mirror quite close to ambient under most conditions, but when imaging is required in rapidly dropping temperatures, the scope is pre-conditioned in an air conditioned room before use. A 50watt thermoelectric cooler has also been built to provide additional mirror cooling if need be (up to 7C below ambient is possible). The TE cooler fits over the end of the scope with a foam shroud (see image). The cold air stream from the TE cooler distorts the mirror due to thermal gradients and the cooler must be removed and the fan run with ambient air for about 10 minutes before imaging to allow the mirror to stabilise.


focuser – the standard focuser is OK for visual use, but manual adjustment is not an option for long focal length imaging. I fitted an available Moonlite DC motor focuser which is just strong enough to support the Barlow chain required to get the focal length up to 6m+. The hard surface on the focuser tube has failed under the large sideways loads, but it still works OK. It would be better to use a stepper motor system, since the steps in the DC system are about 40microns, which makes it slightly touchy to find focus. The Moonlite has a large baseplate and is quite stable on the OTA, with minimal evidence of OTA flexure (which was surprising, based on earlier experience with a bendy 8 inch tube).


EQ6 – this amazing mount carries the 20+kg load without protest, provided the wind is not too strong. I have had to adjust the backlash in both axes (including motor gears), but the tracking accuracy out of the box was quite good enough for the task. Three standard counterweights on an extension bar balances the load.


Other – messed around with a variety of Barlows, but found that Televue devices were way better than anything else. I use a 3x with extensions to get out to about 5x magnification. Settled on Astronomik filters after playing with no-name alternatives (odd colour balance). I use a TIS618 camera – this has higher QE than anything else I have tried and has been a major upgrade, allowing high frame rates to be used in poor seeing. Collimation is with a laser (which also needed to be collimated itself), followed by fine primary alignment on a nearby star or one of the moons of Jupiter or Saturn. Barlowed laser collimation is used if imaging at dusk.


Overall, this project has turned out well, allowing passable images of the most interesting planets to be routinely produced in conditions of good seeing. The modified Dob approach is a relatively low cost way to get into this fascinating field.