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.
Just one minor comment wrt) laser collimators.
Their collimation is not all that critical, what is critical however, is that when the laser is pointed at the optical centre of the primary, the return beam lands at the centre of the field stop by the time it reaches the focal plane. A subtle distinction but one worth making. <-- The caveat here is that you may end up with some tilt to the plane of best focus, which may (or may not) be an issue with large format CCD's.
That is not to say that laser collimators should be left in a condition where the exit beam is anything other than orthogonal wrt) its locating flange.
thanks Clive. Of course you are right about the laser collimation - thanks for the clarifying comment. However, the process of getting best possible resolution from this scope has been one of eliminating everything that could conceivably work against that goal - the laser could be collimated, so it was, even though it seemed unlikely that image plane tilt would affect a target with as small an angular extent as a planet.
Regards Ray
hi Peter. I have no experience with SCTs, but suggest that reflective tape would possibly help a metal tube one as follows:
1. the tube would not cool as much as a painted one, so the mirror could reach the internal air temp quicker,
2. the tube/glass would all stay warmer, so dew problems would be reduced (probably by a fair bit).
if you wanted to try it out, wrap one in alfoil
regards Ray
hi Peter. be interested to hear what you find if you do manage a side-by-side.
Thought about it a bit more and the corrector plate of an SCT will try to radiatively cool to the night sky temp (it has high emissivity at thermal IR wavelengths), so you will also need a dew shield to minimise the amount of night sky visible to that element, regardless of what the tube does. The dew shield should also be foil wrapped so that it does not pour too much cold air onto the corrector.
Regards Ray
Note in addition to original post:
We had a few nights when the temperature remained near 30C and the double sided tape holding the primary in place softened enough to allow the mirror to slip slightly sideways and into contact with the backup edge supports. Bad astigmatism was the result, because I had not bothered to align the OTA to get the edge supports in the right orientation re vertical (not normally an issue). Will now remove the tape and use silicone adhesive instead.
added: yep, the acid cure silicone worked well. the scope now shows no sign of astigmatism and the mirror is well and truly locked in place - so well that I have removed the secondary edge supports altogether. Regards Ray
hi Peter. be interested to hear what you find if you do manage a side-by-side.
Thought about it a bit more and the corrector plate of an SCT will try to radiatively cool to the night sky temp (it has high emissivity at thermal IR wavelengths), so you will also need a dew shield to minimise the amount of night sky visible to that element, regardless of what the tube does. The dew shield should also be foil wrapped so that it does not pour too much cold air onto the corrector.
Regards Ray
Interesting.
I have made a dew shield out of camping bed-roll material and it is nice and long. It is effectively made out of insulation, so would that do?
Update on mirror mounting - the silicone was only a temporary fix. The GSO wiffletrees are some form of plastic (I had assumed they were anodised aluminium, since there is metal showing in some holes in the triangles). The silcone slowly de-glued itself from the plastic over the past couple of weeks, so now the mirror is once again mounted on double sided adhesive tape - this time, the 1mm thick white ultra-sticky "permanent" stuff made by Scotch. see how this goes. Sorry for a new post, but I can no longer edit the original. Regards Ray
The inside of my 10" f/4.72 dob is lined with a felt like material. I use my scope mainly for visual use. I also have a three speed computer fan and a cover over my mirror for cooling before I observe. I experimented with foil about 5 years ago.....had it on my finder dew shield and my large dew shield was covered in a reflective substance.
Do you think reflective tape or foil covering the entire tube would help for visual observing to further stabilize thermal gremlins inside of the tube?
Hi Markus.
I'm not sure what effect foil would have on your scope. My guess would be that, if the felt material is thick enough to insulate against the tube temperature, it might not matter what the tube surface is like. If it's not thick enough for thermal insulation, foil would help. It's pretty easy to see tube currents in an out of focus star image, so that might be a starting point. I used low thermal mass thermocouples to measure temperatures around the tube when I was trying to work out what to do and I taped on a reflective emergency blanket to test out the effects of foil before I covered the OTA in the reasonably premanent sticky stuff. regards Ray
The alternative double sided adhesive tape between the wiffletrees and the mirror also allowed the mirror to creep sideways and it fairly quickly resulted in the mirror making contact with the surrounding ring - lots of astigmatism. Tried a mounting consisting of screws to attach to the wiffletrees and silicon adhesive between the screws and the mirror. It worked well mechanically, but noticeably damaged the images, presumably through stress from the hardened adhesive. Going back to the original (functional) arrangement with the high strength double sided tape (the clear stuff) and 4 edge supports. I will just have to live with arranging the alignment of the scope so that edge supports are at about +- 45 deg and replace the adhesive tape every 12 months or so. At least it works well optically.
regards ray
Last edited by Shiraz; 21-05-2012 at 09:21 AM.
Reason: further update
