It began with the worst winter in all my life, a leftover 2" focuser and some serious disdain for our 80mm Synta ED-refractor. 80mm of aperture and f/7.5 - I was not impressed. I had been daydreaming about a smaller and more compact reflector to complement our 203/812 Schmidt Newtonian ever since I completed our travel Dobsonian, but the ED 80 finally got me going. Some quick market research produced no viable solution. Too expensive or too limited. And very little in the way of "compact". So I bought a used 110/440 mirror to build a wide-field telescope around it. But the more I thought about it, the less this plan appealed to me so I finally scrapped the four inch and went for a 150/750 reflector instead. IMHO such a telescope had a variety of advantages:
The commercial offers for 150mm telescopes were not to my liking, so I decided to start yet another ATM project (at that time, our 12-inch still awaited completion). Furthermore, we had booked a five day holiday on Tenerife for the April new moon of 2006, and I wanted the new scope to come along on the trip. Which, at the beginning of February, left me with limited time to design and build the telescope. Let alone obtain the required parts. But I started anyway.
With the limited time at hand I chose to go for a commercial mirror. Grinding a six inch yourself will take its time and finally be more expensive than buying one. I went for an offer for a good quality GSO mirror and a 54mm diagonal. The 2" GSO focuser was left over from our 12-inch reconstruction project and the star pointer came along with the new Synta 90/1250 Maksutov guide scope we got ourselves from Ebay.
The new scope should be usable wih both my SC3-modified webcam and our Canon DSLR. Since the Canon requires 55mm from the T2-thread to the chip, the DSLR dictated the layout. The focal plane is about 135mm outside the upper tube, resulting in both a very compact OTA (61.5cm) and a large secondary of 54mm. The secondary creates a fully illuminated field of about 12mm, illumination at the edge of the 1.6 degree field for the 350D still is 91%. See the Newt file and Andy's Excel sheet for details.
Basic design
From the moment I first thought of such a telescope, I envisioned it as a three-section design. A central section should be responsible for attachment to the mount, while the outer sections should hold the mirror, the secondary and the focuser. The three parts should come together via some truss framework. For transport, the sections should be disconnected and placed into one another. That was the easy part. But, then, an equatorially mounted scope would require a means of rotating the tube. Rotating tube rings? Heavy and complicated. Then, the outer sections would have to be of smaller size than the central section, as they should be transported inside each other. But that would imply (mini-size) truss pole connectors angled in two dimensions. I am not a master lathe and router operator, I could not make 16 of the connectors with the required exactness. These two problems seemed too hard for my mechanical abilities to solve, so I dropped the project again.
Until, towards the end of January 2006, I had the follwoing idea: Forget the rotating tube rings. It is enough to have two dovetail plates attached to the central section at right angles. And, if you use the right kind of strut you can restrict yourself to four struts per section connection, with the cross-section shape of the strut providing the necessary stiffness. And while you are at it, take three dovetail plates instead of two and make them an integral part of the central section. So I sat down to scribble on a piece of scrap paper. The drawing mostly served to show how completely useless I am as a sketch artist. But bad as it was, I could use this sketch to convey my idea to some people at the Munich observatory mirror making workshop (who should know better) and they had no major objections (thanks, Christoph). I went straight into construction and procurement.
Chosen materials
The telescope has been made from a mix of materials. Plywood, aluminum, carbon fibre tube and aluminum tube. Screws are iron, titanium and aluminum. The selection of materials was based on availability and my limited craftsmanship, with preference to lighter materials where possible. I spent quite a bit of money on lightweight screws, but that was mostly for fun (they save around 50-80g of weight). While I did most of the scope myself, I had some parts made in a workshop by professionals. These were the dovetail bars and the primary holder parts.
The upper tube
The upper tube is a conventional two-ring design connected by four pieces of aluminum tube that also hold the secondary spider. Stability is increased by a 1mm plywood sheet glued inside the rings. The focuser is attached to the tube by means of two angle plates and secured by four M4 countersunk screws. The star pointer finder is mounted via an angle plate and a short piece of 10mm aluminum tube and can be fitted into any of the numerous holes in the ring. Two foam rubber stray light baffles have been epoxied into slit stubs of carbon fibre tube and are plugged into ring holes opposite the focuser.
The spider
The spider is a standard excentric 4-vane construction, the vanes being made from .8mm aluminum sheet. The vanes are connected by a rectangular piece of 30x15x2mm aluminum profile housing the four M3 secondary adjustment screws and the M4 secondary holder attachment screw. The secondary holder was made from a 35mm beechwood rod, hollowed out using a 30mm Forstner bit, with an M4 nut glued into it and a small piece of aluminum sheet glued onto the top. The vanes are attached to slit M4 brass screws and tightened via flat M4 nuts. The total weight of the spider construction (without secondary) is 34g.
The central section
An open construction made of two 9mm birch plywood rings connected by three 10mm carbon fibre tubes and three aluminum GP-compliant dovetail plates. The torsion restistance of this section was improved by laminar attachment of the dovetail plates to the rings with three M3 titanium screws on each front as well glueing the carbon fibre tubes into the rings. Each dovetail plate has two M6 threads placed 130mm apart for direct attachment to a mount or the alternative attachment of a piggyback camera.
Mirror cell
The mirror cell is also a two ring design, here the rings are connected by six short pieces of carbon fibre tube, again with a 1mm plywood sheet glued inside. The primary mirror has been silicon-glued to an aluminum ring that also holds three M4x20 adjustment screws. These screws are threaded through holes in the 3mm aluminum primary holder (which, BTW, had been ordered 4mm strong), which itself is mounted to the cell by six M4 screws. Mirror adjustment is by flat knurled nuts on one side an a combination of 6 and 10mm springs on the other side. The mirror cell features two baffles, one circular foam rubber mat simply stuck inside the cell from above and one ring baffle below the mirror preventing light from passing between mirror and mirror cell.
Tube section connections
The tube sections are connected by four 20x10x1.5mm aluminum struts each. These connections are symmetric so that every one of the eight struts can be used in any position. The struts are screwed into 25x15x2mm U-profiles by M3 screws, the U-profiles themselves are screwed to the rings by M4 screws (countersunk, except for the attachment of the upper tube, where rounded head screws go into threads glued into the connecting aluminum tubes). My metal supplier did not have U-profiles with 20mm inside dimension, so small bits of .8mm aluminum sheet had to be epoxied into the profiles and then filed to fit. The M3 screws directly go into threads cut into the U-profiles (though I am afraid to say that two of these threads already lost the war, the soft AlSi 0,5 aluminum gives in too easily. I may have to replace the screws by longer ones with extra nuts). The section connections are very stiff, even though they do not use a triangular truss design. For one thing, the struts are oblong with the wide side solidly fixed along potential torsion forces. Then, as the outer sections have a smaller diameter than the middle section, the struts are angled inward which produces a form of trapezium for the struts opposite each other. Whatever, it seems to work.
The 150/750 Newtonian was not conceived as a standalone telescope but instead should be part of an air travel compatible photographic setup. So in addition to the OTA itself we had to obtain, build and combine a few other parts.
EQ-3 and MTS-3
The whole telescope assembly was supposed to be carried by our Synta EQ-3 mount equipped with the standard 6V dual axis drive. As the standard drive control is more or less useless for photography, we bought a suitable cable for our Boxdörfer MTS-3 control unit. The MTS-3 has, among other features, DEC backlash compensation and an ST-4 compatible autoguider input. And, what is by far more important, the MTS-3 works. Reliably so.
Twin mounting platform
We needed a compact, lightweight and stiff platform able to carry both the 150/750 OTA and the guide scope. We already own a commercial product for this purpose (the TS-Doppelschiene), but this is bulky and bloody heavy (1374g). Also, it can only hold the guide scope in a fixed position. So, taking into account the different CoGs for the main OTA for visual use and different cameras, we designed a twin mounting platform tailored specifically to our telescopes. Several sets of slits have been routed into this platform. Two slits, spaced 150mm apart, are for mounting the assembly on the EQ-3 while allowing lateral balancing, four other slits are for the attachment of the main OTA and longitudinal balancing. The guide scope is attached by two screws, one fixed and one capable of moving in a +/- 7.5 degree arc in declination. All attachments are done via M6 knurled screws. The platform is a sandwich construction featuring a mostly hollow core of 15mm beech plywood and two layers of 2mm plywood sheet glued around it. The sandwich without screws was weighed at 406g.
Guide scope
The guide scope is a Synta 90/1250 Maksutov-Cassegrain with primary mirror focusing. A Gerd Neumann guiding excenter can take either a webcam or a Meade Pictor 201XT autoguider.
Energy supply
To further save on luggage weight, the ordinary counterweight of the mount was replaced by an energy supply made up of two 12V7.2Ah batteries in plastic food containers, padded with pieces of a foam sleeping mat. These are connected to a small switchbox that has a 2A fuse and both stereo jack and standard DC jack outlets. I have a twin jack for cigarette lighter plugs that goes into the stereo jack, so I can easily power the autoguider and the MTS-3 simultaneously.
Packing for Tenerife
Packing went without problems. The individual parts of the mount were split between me and my girl and went into our duffle bags. The OTAs and the other photo stuff went into our hand luggage, the Newtonian barely fitting through the neck of the knapsack.
Reassembly of the scope
The 150/750 is not a quick setup scope and was not designed as such. It is supposed to be assembled at the start of your holiday and stay that way until you go home again. With 20 screws and several baffles this process takes about half a beer and is best done in a leisurely environment. Of course, the scope also has to be collimated after assembly.
The inadequacies of the mount
Unfortunately, the EQ-3 did not quite live up to our expectations (which likely had been disproportionate in the first place). Tracking was somewhat erratic due to both the fact that the mount was overloaded with 5.5kg instrument weight and the photographically unusable drive train. DEC backlash changed from time to time so it could not be compensated automatically and the flexible connection between the RA motor and the worm drive prevented exact RA corrections.
The telescope in action
Here you can see the telescope at night outside the Parador hotel. A lot of cable connections ever so willing to put the entire contraption into wild oscillations.
First photographic results
Nevertheless we were able to produce some results at least, mostly with the DSLR in prime focus. Some were obtained guiding by hand and sometimes the autoguider actually worked with the EQ-3.
Assessment
The 150/750 travel Newtonian works. It could be taken on an airplane trip and allowed to take reasonably sharp images of deep sky objects. The cheapo focuser works well enough, and the tube is stiffer than I had expected. The same holds for the twin mounting platform. Guidescope movability is good, even though I sometimes wish for an additional swing arc in RA as well. The power supply works very well. 14.4 Ah of power surely last for a while with scope and guider together guzzling half an Amp max. The not so perfect results of our telescope's maiden voyage were solely due to the flimsy EQ-3 mount which will have to replaced by something in the Vixen GP class with a better tripod.
Possible improvements
Having said that, the new telescope is far from perfect. The U-profiles for truss attachment could be re-machined from a harder alloy with a longer thread. Alternatively, the existing ones could be fitted with longer screws and extra nuts, though this would require carrying a 5.5mm wrench in addition to the 2.5mm hex key that currently is sufficient for all assembly and collimation tasks. Then, at 2546g, the OTA is IMHO still too heavy. A really lightweight 150mm scope should be able to stay below 2000g. The current version still contains a lot of metal that could be replaced by carbon fibre. The commercial 18.5mm mirror could make way for a selfmade one from a 10mm blank (this would save at least 350g alone). Then there is the problem of the focuser. In the 2-inch configuration it weighs 416g, some gain should be possible there (even though not with the commercially available offers, which are all even heavier). Unfortunately, making the OTA lighter this way would have unwanted consequences for the position of the CoG in the various configurations. In the least, the symmetric truss design would have to be given up with shorter struts in front and longer ones towards the mirror cell. Or, more radically, it might be necessary to move from the triple section design to a twin section design with a longer upper tube also holding the dovetail bars and a subdiameter mirror cell that recedes into the front section for travel.
Acknowledgements
I would like to thank the people who helped us build this telescope. Most prominent among those is of course Manfred Mauz (MAM-Astronomiegerätebau) who provided priceless help aiding us in the use of the machinery at the workshop of the Volkssternwarte München. Also, we want to thank everyone from the mirror making and ATM workshop, in particular Stathis, Martin, Harald and Rainer.