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Old 29-08-2013, 02:25 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,798
Modifying a GSO 200f4 for high res imaging

Hi
Has been suggested that I post a summary of mods to a GSO 200f4 to get it to the point where it can produce reasonably hi res images - so here goes .

I wanted a galaxy imaging system (ie highest possible resolution) and after some study, concluded that a 200mm scope (or larger) would be seeing-limited most of the time at my seaside location, so there was no need for anything much bigger to get the best possible detail in most conditions - sensitivity could be better with a big scope, but not detail.

After a fair bit of modelling, decided on an interim system based on a combination of an f4 200mm Newtonian with coma corrector and a high performance CCD with small pixels to yield 1.17 arcsec pixel scale and a chip size suitable for 1.25 inch filters. This combination is small and light enough to be comfortably carried by an EQ6. The intention is to upgrade to a 10 or 12 inch scope on an EQ8 in the near future to take advantage of the rare “sub 2 arcsec” nights, although the resolution of the existing system has proved to be well matched to my typical seeing conditions. The scope is a 5 year old GSO which has seen a lot of use, but which has always been difficult/impossible to set up properly – indicating mechanical instability. The following mods were carried out to stabilise it and accommodate the chosen RCC1 coma corrector.

1. Imaging train
The RCC1 provides 91.5mm of back focus and this generous space was one of the main reasons for choosing it – I wanted to be able to fit an AO if it seemed necessary. The back focus is currently filled by a 5 position Atik wheel and an OAG housing being used as a spacer (the rest of the OAG will be fitted soon, but a finder/guider is doing well enough for now). The main camera is a Starlight Express H694. A major feature of the imaging train is a weight of ~1kg total (no scope), which allows the GSO focuser to be used and minimises flexure all round.

2. Focuser
The standard GSO focuser is a pretty good design, but fairly flimsy. A small amount of side force applied to the camera easily induces visible flexure and movement. I tried the Moonlite focuser from my planetary scope – it was better, but it has the DC motor (I wanted a stepper for the DSO scope) and there were still some alignment problems. So I decided to see if radical mechanical mods could tidy up the GSO focuser before I looked at more expensive options – it worked out remarkably well and the GSO focuser is now working nicely with light loads. Mods were:
- Cut down the top and reduced the length of the draw tube to lower the focuser profile and help with back focus
- Drill/tapped screws to clamp the RCC1 into the draw tube in two places and cut a window in the housing to clear the screw heads – double clamping fixed issues with RCC1 alignment variability due to the RCC1 rocking slightly in the draw tube under the original single clamp point. This can be an insidious fault in focusers of all persuasions, since it moves the coma corrector axis away from the sweet spot and tilts the focal plane – makes a real mess of stars, with varying shapes across the field of view.
- Fixed twisting of the drive shaft carrier plate in the housing by drill/tapping extra screws to tension the plate at the top corners rather than at the single central point. In use, the normal tensioner screw is used to tighten the drive shaft enough to hold the image train weight, the extra screws are set finger tight and then the tensioner is backed off – this mod really helps to stabilise the focuser to the extent that it does not seem to move at all under normal loading and slewing.
- Polished the flat on the draw tube to remove roughness in the drive
- Fitted one of Dave Trewren’s fantastic SharpSky stepper focusers which focuses nicely and allows automatic compensation for temperature – necessary with a steel f4 scope.

3. Secondary
A 70mm secondary was fitted using a holder made from plumbing fittings and an “Ikea” nut – the secondary was glued to the holder with 3 dabs of silicone. The basic GSO secondary placement/adjustment mechanism is just strong enough for the 70mm mirror if the spider vanes are kept tight.

4. Primary
The primary is a pretty good quality commercial mirror with excellent star tests showing better than wave correction and no major faults – plenty good enough for high quality imaging at ~1 arc sec sampling with the 200mm aperture.
- The mirror edge was darkened with black texta to cover obvious stray light sources (surface damage from clamps and a rough chamfer) – this slightly reduced the light gathering area and looks ugly, but it greatly reduced the glare around bright stars.
- The mirror was glued into the holder with 3 dabs of silicone at 40% radius. Sideways movement was stopped with 6 equally spaced dabs of silicone around the edge – this may need to be refined since there is now a tiny amount of astigmatism. Clamps were discarded.
- Collimation springs were replaced with stiffer ones, but I still use the crazy original locking screws – they work well enough.
The BK7 mirror cools close enough to ambient to stop tube currents within about hour and tracks quite well in changing temps, so no extra cooling was needed. I have not required any dew control yet, but plan to fit a fan with a small heat source if this ever proves to be necessary.

5. OTA tube
The OTA is a standard GSO steel tube with the following modifications:
- Aluminium tape over outer surface to limit radiative cooling under cold sky - significantly reduces dewing and tube currents
- Much of the interior is coated with black felt to reduce internal reflections
- Length reduced 55mm to allow focus with RCC1
- External reinforcing plate fitted under finder and finder/guider to prevent differential flexure
- Internal reinforcing plates cut from an old tube ring and fitted to stop tube flexing near the focuser (reinforcing plates screwed and glued)

6. Alignment
Many of the mods were intended to improve collimation stability. After initial alignment, the system generally requires only an occasional tweak if any part of the imaging train is modified. A laser collimator is used through the RCC1 (screw the laser on the T thread in place of the camera/filter wheel) and with a little bit of care, this provides alignment that is quite good enough for imaging. At the start of each imaging session, the focus is backed off a bit and the out of focus stars are inspected – if anything is misaligned, the out of focus star shapes distort and it is obvious that realignment is required. There is a small amount of astigmatism (probably from the primary mirror edge support), which slightly distorts star shapes if focus is not spot on – needs to be fixed when I have time, but is actually a pretty sensitive focusing aid for now.

Overall, the system is performing as designed. The mods to the scope have resulted in a stable system that is easy to align and that reliably produces quite well resolved and corrected images in seeing down to below 2 arcsec. It's not very pretty because it bears scars from my limited engineering skills and from previous unsuccessful mods - but is great fun to use when the seeing cooperates.

Thanks for reading - hope it was useful. Regards Ray
Attached Thumbnails
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Click for full-size image (focuser.JPG)
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Click for full-size image (tt2secondary.JPG)
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Click for full-size image (reinforcing.JPG)
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Click for full-size image (reo2.JPG)
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Click for full-size image (rig.jpg)
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Last edited by Shiraz; 30-08-2013 at 07:55 AM.
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