Sporadic Dobstronomer

My experience is that using an IR blocking filter, the gain of a night vision device is about 35% of without it -but this is on Earth. That should give you an estimate of the worst that can happen if you are not troubled by light pollution. The actual results will also be affected by things such as the actual IR range of the material used in the intensifier and also the proportion of IR in the light you are interested in. Also IR might pass through the shroud, if your telescope has one, and ruin everything! So my real answer is you might have to borrow one and try it unless someone else already has. Sorry! 😬

The difference between a sphere and a paraboloid is given by r^4 / 8.R^3 where r is the radius of the mirror and R is the radius of curvature for 130/900 this works out as 0.7 wave so it does matter which shape the mirror is

Not unreasonable! Can you tell me what the blue things at the back are?

I don't see the problem. Surely the laces don't damage your night vision as long as they are red?

If that means you are going to start selling them then that might solve next year's problem anyway!

I think this is very interesting and important. Can you tell us in laborious detail how you did it, or do I ask too much?

Part fiVe Optical quality The mirror has a full 16" of usable diameter. Including the bevel it is a bit larger. It is a green glass sandwich mirror comprising a rear plate 17mm thick, a gap of 7mm containing some glass buttons and a front plate 12mm thick at the edge. Given the 5.7mm depth of the curve, this implies not much more than 6mm glass thickness in the centre. The certificate on the box said the mirror was 1/ 14.7 wave P-V. There was a snag that the serial number on the mirror was one count different from the certificate. I Emailed Tong of Hubble Optics about that and he said it was a typo. I could not tell if that was a fact or an opinion. What I could do was a test. With the drive working, I did a knife edge test: I folded some foil in half to make the edge and taped it to the eyepiece holder. I then pointed the thing at Regulus, adjusted pointing and "focus", and looked for any bumps and zones. What I saw was: nothing. Not a sausage. Zilch. Flat as a pancake. Nothing unless you count blobs of warm air. The mirror has no defects at all that I could detect with that test. I was quite pleased about that! Fast forward: in September or October 2014 I did a star test. A 16" telescope can put more than 75% of the light into a circle 0.5 arcseconds across but seeing that needs conditions that are very rare in my garden. Somehow I managed to get good collimation then looked at a high up faint star vaguely in the direction of the Perseus double cluster. At 900x (8mm Delos, 4x Powermate), I saw an Airy disc with a scintillating first ring. Frustratingly I can't remember much about the second ring but I'm sure it was either invisible or nothing to complain about. So the signs are that the mirror has a good figure! People do ask how quickly these mirrors cool and how the figure is affected during cooling but I still don't really know. I have no reason to think there is any big problem. Shortly after doing that star test I obtained a CatsEye collimation set. It emerged that the central ring was about 4mm off centre -in a mirror with a 2mm diffraction limited field. That, and the fact that laser collimators don't work very well, along with the poor seeing in my garden, means that it is still too early to say what the telescope can really do on planets. I have seen other people comment on the internet about Hubble-Optics' inability to place the ring exactly in the middle. I wonder what they were smoking when they decided to produce good mirrors and then do that to them... Mechanical properties The telescope is light for what it is but seems stable enough when pointing high. When pointing low, it used to wobble rather a lot due to having the whole weight entirely on the sticking-out bits of the altitude bearings. I have improved this by adding two struts. I notice the 18" and 20" versions come with similar struts but all versions of this telescope should have them. Each strut is in two pieces overlapping and with two screws to join them. This two part construction was because I did not have any pieces of aluminium that were long enough but it turned out to be a good thing because they can be adjusted slightly and also they can weave their way around the truss tubes. My stiffening struts are the horizontal silver parts in this picture. Fortunately the ends of the altitude bearings already had suitable threaded holes. There is a review that can be accessed via Hubble-Optics' site or here: http://www.astromart.com/articles/article.asp?article_id=856. It points out that the telescope benefits from added friction in azimuth (but that is not relevant if you have a Goto fitted). My personal experience was that before the electrics arrived I found it easy to push it around and did not have any problems that I remember from the very light movement in azimuth. By the way, that review shows an 18-point cell but mine is only 6-point. The altitude bearings can be folded to make them smaller for transportation. This creates a snag because it is quite hard to get the pieces aligned without a bump which interferes with the smoothness of motion. In manual push mode, I don't remember noticing the bumps but once the Goto is fitted, they cause eccentric behaviour in tracking and especially when slow-slewing. Having said that, it can be lived with once you understand what is going on. The relative precision of motorised slewing shows up any small defects in the bearings and quickly made me fussy about them. It is worth remembering that without the Goto system the telescope's mechanics would not have needed much tinkering to make it satisfactory.

@bond19 Thanks!

Part Four Fitting the electrics This is when things started to get hard. The arrangement I ended up with does seem to work well enough, but the equipment and instructions provided had not been thought through properly so I had to do a lot of modifying. Someone without above average access to tools simply could not have done it. What follows might be clearer if you look at the arrangement of the motors and electronics on Hubble-Optics' site at http://www.hubble-optics.com/GoTo.html Some modifications were essential and some arguably not. But all of them were needed for getting the light but convenient telescope I wanted. The azimuth drive has a brass knurled wheel which rolls along part of the lazy Susan in the base while the altitude drive has a steel cable which runs along the rim of one of the bearings and round a driven bobbin. The kit contained motors with gearboxes installed, electronics by Sidereal Technology, and various brackets, clamps, screws and other bits to hold the motors and the drive cable in place. The kit included new Teflon pads with a groove for the altitude drive cable. The holes in the pads were not adequately countersunk which I fixed easily. Drive cable snags: To install the drive cable, you have to drill and tap an M4 hole in one of the altitude bearings. What? As in: WHAT?? Most people would not have an M4 tap and wouldn't know how to use one anyway. Fortunately I do through my work. The instructions were not clear about how tight to set the altitude drive cable. I think that is because it does not work whatever you do. The moment you release the clamp for manual slewing the cable will go "pyoing!" and gleefully leap off its bobbin. Solution: I installed some elastic so the unclamped cable is tight enough to stay on the bobbin but loose enough that the telescope will move. It works well although the clamp must always be released soberly and not just given a casual flick. That quickly became a habit. This picture shows the advanced, twenty first century piece-of-elastic. Also seen are a motor shield and a sticking-out bearing that goes with the jack, which are described below. Vulnerable motor: The altitude motor is stuck out where it will eventually have an encounter with a foot which will be bad for both of them. Solution: I made a shield to protect it (and my foot). It has are two sheets either side of the motor separated by chains of spacing pillars. The inner sheet is metal for strength, the outer one is plastic for shock absorption and toe friendlinessTM. Electrical details: The battery is not included Solution: I obtained a 7Ah lead-acid battery and a charger from Rapid Electronics. There is no on/off switch or battery charging point. Also if you look at the recommended arrangement, it uses Velcro to hold the network box such that (based on my experience with Velcro) it will eventually fall off. It also sticks out precariously. Solution: I screwed on a metal plate to extend the battery holder and then made my own small electrical panel with an on/off switch, LED, and charging socket. I also added brackets for mounting the 12V-19V converter. The SkyFi network box is tied to a piece of wood which sits on the plate and is prevented from sliding with Velcro. It can be pulled off when the batteries need changing. There are no instructions for how to run the cables, and if you copy the picture on their website the cables tend to fall to where they are not wanted or otherwise snag on things. This situation is made worse by the modifications I think are needed. Solution: After a few goes, I routed the cables differently (having to supply my own cable ties and tie-bases) and also installed a plywood deck to give some of the cables something to rest on. The plywood is held on by sticky foam pads. Some of the cables run under the rocker although they are kept well clear of the ground. Mechanical snags The bush for the plate holding the azimuth motor did not fit into its hole (the whole motor assembly moves for engagement and disengagement and has a bush to pivot around). Solution: I enlarged the hole with a half-round needle file. The altitude motor and the wheelbarrow handles cannot be fitted at the same time as they share the same part of space. If you are reading this Tong, that was naughty. You should have not sold me the two together without warning me of this problem. Solution: I had to give up on the barrow supplied and make my own using only the wheels and big screws from the original arrangement. The azimuth spoke arms (which hold the azimuth encoder in the middle) are shown in the instructions as installed immediately under the lazy Susan. If you do that they foul against the drive knurled-wheel which is thicker than the lazy Susan and so sticks out below it. In a later drawing the instructions incidentally show a nut used as a spacer (which could work if nut were drilled out) so they must have noticed this issue at some point but did not correct the drawing or instructions. Solution: To lower it, I installed some washers. They turned out to be too wide and did some damage before I realised what I had done (brass, being harder than aluminium, will chew it away when it jams upon collision with a steel washer). With smaller washers it does work. (The motor is attached to the moving part of the rocker which is fixed to the outside of the lazy Susan. The knurled wheel rolls along the stationary inside of the lazy Susan.) One thing that worked really well: the spoke arms are made of modified steel rulers which makes it very easy to get the encoder centred: you fiddle with it until all three show the same measurement. I liked that! Coping with weight Before the electrics were installed, I sometimes used to carry the base/rocker and mirror box in one piece although I didn't like doing it. With the extra weight of the motors and battery it is too heavy for me so I have to carry it in separate pieces. If you do separate the mirror box from the base so you can carry it, it is very difficult later on to reinstall the altitude drive cable -especially when grovelling on the ground in the dark. Solution: I made a jack arrangement so I could have the bearing on that side suspended about 2mm above its normal position so the cable could be moved to the ideal place before lowering it. The jack lifts a ball race mounted on a screw (with washers) that sticks out of the side of the mirror box. I used an old bearing rather than just a shaft so it would roll so the jack can lift the mirror box without any sideways force. The jack sits on a fixed piece of aluminium which doubles up as a mounting for the barrow. It is made from hinges and assorted bits of what I could find. It works but has a very limited lifting range. Used with the jack is a removable piece of plastic which locates one end of the cable on the altitude bearing centreline during installation. It means I can fiddle with the other end of the cable and not have to worry about what the first end is doing. The result is I can adjust everything until the cable is just above the grooves in the Teflon pads and then lower the jack. This makes installing the cable fairly quick and prevents damage to the Formica bearing or the cable itself (the original cable had to be replaced due to accumulated damage but the new cable should last). The jack worked well for a time but with the latest version of the barrow it no longer sits in the best place. That will eventually be fixed. It is not critical while the telescope remains at home because I can leave it assembled and and wheel the whole thing outside in one piece. However, before I next transport it by car a working jack will be needed. What it is like: After all that, you may be interested in what the drive system is like. There are some minor foibles that I won't detail in this already-long post, but generally it is smooth and quiet. It sings gently when it is slewing fast and makes an occasional "twang" sound -possibly from the drive cable, possibly from the mirror cell. There is none of the raucous whirring and screeching you get with some Goto mounts. Result! You can see the pattern: the telescope is eccentric but somehow likeable. And I do forgive a lot for its being so light.

Part iii First light I was slightly lucky that the clear skies lasted just long enough for me to try the thing out using my old Meade Series 4000 Super Wide Angle 24.5mm (presumably an Erfle) with about 67° apparent field, giving about 75x. What I remember is: -Jupiter so bright that it was almost unpleasant (I don't get that reaction now but it is a shock the first time you see it with such a big telescope) -The Orion nebula with genuine Wow!TM :D -The moon looking more vibrant than I expected. That last one was a real surprise. At 80x with the same eyepiece, my 8" Celestron is well below its maximum power so I did not expect to notice much difference in the view of the moon but the UL16's is somehow vivid. Experimenting with higher powers did not initially produce good results but that was certainly some combination of seeing, collimation, eyepieces, and lack of experience not suited to such a large but fast telescope. (I'll have more to say about the optical quality in a later post to this thread. Mostly positive.) It also emerged that if I point it near the zenith and I stand my 5'9" self on my toes, I can reach the eyepiece but can't stay still enough to see anything so I do need something to stand on. At that stage I found out what the foam grips are for: as you move the telescope, metal truss tubes can make your fingers very cold very quickly without them. I had a total of three 1m lengths of foam which is more than are really needed. Now that the Goto is working they don't get used much but with a pushed Dob something to insulate you from the tubes (if they are metal) is important. The next stage was fitting the electronics, and that turned out to be a lot of trouble...

@hrgreen I have no specific information on how it behaves while cooling as seeing is the limit most of the time. Also collimation, which should be improved as I have recently bought a CatsEye set. I am not aware of any problem related to cooling but with English weather, collimation, and my inexperience with temperamental fast Dobs it is still too early to give a definitive answer. I did not bother to talk to HO about the assembly problems as they are a long way away and I did not see any purpose. It is possible they might sort things out. They might even read this thread. The mirror does seem to be good at least some of the time! I shall be adding some details to this thread about what I have seen.

Part 2 Initial assembly I remember that assembling the telescope took longer than I expected. You have to glue the secondary in place, so it takes at least 24 hours, and there is quite a lot to assemble all of which you are doing for the first time. I actually let the secondary sit for two days as it was standing in a cold place. There was a near disaster the first time I attached the top ring: I lost control and it went crashing to a concrete floor bending one of the truss tubes on the way down. Fortunately this happened before the secondary was installed and the bend was in the flat bit near the end so I was able to straighten it by placing it on the floor and bashing it with a hammer. The focuser tightness then needed adjusting but overall I seem to have got away with it. A lesson from this is that installing the top ring is a fiddly process that can go wrong because having located it on one pair of tubes, you cannot assume it will not jump off. (Fast forward to October 2014 and I have installed a barrel bolt experimentally as a quick way to lock one pair of tubes with one hand to secure it for a few seconds while I fiddle with the others. I'm hoping the risk of dropping the thing will now go away forever.) In any case, it helps to have elastic bands or cable ties to keep the tubes together when putting the top ring on. In one of the Formica strips there was some minor damage (which I just put up with) and some unglueing. It does not seem to have cause much harm. I have had some trouble with it but glueing with Copydex (rubber solution) rather than the recommended contact adhesive seems to have worked. I did do some other minor damage when assembling it. The whole telescope is a bit delicate. Once assembled, it is strong enough if you are sensible with it but I would not like to have unruly people getting too close. However, I guess delicacy is inevitable with such a light structure, and being light is very much the point. A bug was that the wing nuts used as lock nuts on the feet don't stay locked. This can have a nasty effect because they tend to wind inwards so the other end stands proud above the lazy Susan where it can foul on things as the telescope rotates. The solution was to fill the space with washers so there is a hard limit on how far a foot can screw in. Another bug: the collimation springs were not strong enough. This seems to be a plague in Dobs by other makers too. Weird. I seem to remember that an effect was losing pointing accuracy when looking high up as the mirror started to drift (this was before I figured out what was happening, let alone trying for good collimation). Another effect was that if the mirror box was in the car, bits would fall off during the journey. I fixed this by getting some new springs from RS Components. Bug three: the secondary mirror could not be placed exactly in line with the focuser (this only became clear in 2014 after the CatsEye collimation kit arrived). I used it for a long time with it shoved as far as it would go and eventually used a needle file to lengthen two of the slots enough for it to fit. This problem probably happened because the top ring is not a perfect dodecahedron. Bug four: the finder is not focused on the cross hair (and cannot be adjusted at that end). My guess is that I happen to have a bad one. I did not realise exactly what was wrong early enough for it to be worth getting a new one sent over. Eventually I intend to glue a cheap lens to the back of it so that I can see the cross hairs. It is not that big a problem though or I would have already done something about it. Bug five: the finder would not stay still in its mounting but tended to slide down. That meant that the lens bell would catch under the mounting ring and then I could not align the finder with the telescope until I had worked out what was happening. Solution: a ring of cardboard that stops it slipping down too far. Hubble Optics do not supply a mirror cover. The reason probably is that with their sling arrangement, a good cover is hard to make. I made two covers. The upper cover is a large sheet of polycarbonate that I happened to have, with some nylon screws to act as locators. It is strong and turns the mirror box into a useful table when you are fiddling with things. It is off-white coloured which is a good thing because a concave mirror must not be kept under a transparent cover if you don't want your house to catch fire some sunny day. The lower cover is a large sheet of cardboard tediously bent at the edge to be close fitting and is supposed to keep dust and cooking vapours out. It does not seem to work. I suspect that the cardboard itself is shedding dust. Eventually I intend to replace it with something yet to be designed. And then I shall think carefully about cleaning the mirror...

Thanks for the nice comments! To answer the questions: -it is a sandwich mirror -The shroud does get in the way and I don't use it I have written stuff that expands on what I did and what it is like. I shall add it to this thread eventually...

The story of my UL16 is long-ish so I'll chop it into a few parts Part the first Background I have had a Celestron Ultima 8 since about 1990. When I bought it, it was the best Schmidt-Cassegrain in its class but it is a bit heavy. Not very long after I bought it, a friend got a 10" Meade which duly did not get used much until he bought an observatory. The upshot is I was well aware that weight and convenience are important when owning a telescope; a too-heavy one won't get looked through. I had also noticed that deep sky objects are hard to find in the light polluted place where I live if you don't have a GoTo system (the Ultima 8 is a little too old to have one). The Decision After several years of rarely looking up, in late 2012 I saw the Hubble-Optics UL16 in Sky and Telescope described as a 16" telescope with good optics which weighs only 60 pounds and can have a GoTo fitted. I immediately wanted one! I took its appearance in S&T as an endorsement, which may have been fallacious, and I thought there was an element of risk but decided to go for it. At that time the 16" f/4.5 was their only offering. It is still the best one for me as the 14" is smaller, the 18" would be a little too wide and heavy to be easily carried through my non-ergonomic house, and the 16" f/5 would be tall enough to cause problems. What I am describing is mostly from memory as I did not keep a diary... The purchase I bought the telescope from Hubble-Optics' site which meant paying through PayPal. The PayPal guarantees do not apply to that kind of transaction so I was aware of a financial risk but I for peace of mind I was able to monitor the state of my order and get information about despatch and parcel tracking reasonably easily using Email. The site is here: http://hubbleoptics.com/UL16.html The options I went for were: UL16 f/4.5 Telescope Goto system (electronics by Sidereal Technology with mechanical parts by HO) Finder Wheelbarrow Shroud Bag for truss tubes Battery holder (a metal tray) 12V-19V converter Foam tube for making grips Artificial star (a LED torch with 5 pinholes of different sizes) So pretty much everything except the wireless hand controller. I also needed to buy: An Apple iPad Sky Safari planetarium software I ended up having to buy a fair number of other things too. The telescope arrived, about a month late, shortly before Christmas 2012 (at the beginning of a period of cloudy skies and just after a long period of clear skies although Luck did grant me a night for first light). Most of it had been carefully packed but there was slight damage to the Formica where the altitude bearings had rubbed. The electrics arrived shortly after the new year. The handles for the wheelbarrow were missing so they were sent in a later parcel. My experience; the very short version: The telescope mechanics and optics without the Goto can be made to work well with only a few bugs to be ironed out. On top of that I have been doing things to make it convenient to use and to extend its capabilities a little. However, turning what arrived into the telescope I want has been a slog not yet ended. I have had a lot to do because the Goto installation design was not fully thought through. It needed tools most people don't have plus modifications I regard as essential. Also the HO wheelbarrow cannot be combined with the motors so I ended up making my own. BUT: the big positive is I now have a usable 16" Goto telescope with a nice mirror that is light enough for one man to wheel around or load into a car so I expect eventually to have a very desirable item. Despite all the snags, I do like it!

I compared it with what appears in Celestia and the features match. A rough calculation suggests a telescope that size puts 50% of the light into a spot 0.5" across. This image shows details down to about 0.2" Amazing, but just about possible!