dark star

I have a 14 inch truss dobsonian which I made myself. I am trying to use Nexus DSC to locate objects, mainly galaxies and nebula. At the moment the best pointing accuracy I have got is within the general area, but a few telrad or eyepeice jumps away using my widest eyepeice. I assume this is because the mount is not square/orthoganal. Using the local sync function to sync on a star near where I am searching improves things but not enough to make it usable. I am trying to build up a model of the mount in the DSC so it can correct for the mount not being orthoganal. This is called Mpoint. There is a problem with this. It will only let me add 4 calibrations points. I point at four stars in succession and press enter after beginning Mpoint start and the number increases up to 4. But then if I try to enter another star the number does not change. If I try to do Mpoint analysis it says that there are not enough calibration stars or I need to reduce the number of Mpoint parameters.I have the encoder instalation kit for skywatcher dobsonians. I have now managed to increase the number of calibration stars to 6, but this is still not enough to get Mpoint working. Mostly, however many times I press OK when trying to add a calibration star the number does not increase, very occasionally it works. The altitude encoder is a magnetic tape. I have entered the encoder steps for altitude as 1308000 according to the manaul for 12-14 inch skywatcher dobsonians. However, to fit the magnetic tape for the altitude encoder I used a small metal altitude bearing for a GSO dobsonian, which was spare from another project. I fitted this to the altitude bearing. This is according to the manual for GSO dobsonians. I will try entering the encoder steps from the manual for GSO dobsonians for altitude which is 716000. Also, for some reason I entered the az encoder steps as 10000, I will change this to 8192. I will see if this improves things. When I fitted the metal altitude bearing I tried to get it centered on the axis of rotation as accurately as I could using laser pointer. Is anyone using nexus DSC with a dobsonian mount that is not very square? Does anyone know what the problem might be? Also, what parameters do I need for Mpoint for a dobsonian and which ones should I remove? There are a lot of parameters! I have tested the encoders they work OK. Over time I am going to try and test how out of square the dobsonian is and try and rebuild it to make it work better with the DSC. David

I have bought a Nexus DSC and encoder kit from 365 astronomy. I bought the encoder kit for skywatcher dobsonians as I thought I could adapt this for use on my home made dobsonain. The azimuth encoder fits on the groundboard fine. However, the design of the altitude encoder has changed and now has a magnetic strip. I can see no way of fitting this to my dob. I have a full size altitude bearing-a half circle. The simplest solution seems to be just to buy another encoder and fit it mysefl, I can make brackets etc. I appear to need '10000 steps quadrature encoders ttl'-I presume an optical encoder? I would like to buy from the UK to avoid probable long delivery times from Australia. Can anyone advise me where to buy an encoder in the UK, preferably on that I can just plug the cabe for the nexus DSC in to? I have tried using google but so much information comes up that it is very confusing! I am hoping that someone has bought such an encoder and can let me know the best place in the UK to get one.

I have bought a Nexus DSC and encoder kit from 365 astronomy. I bought the encoder kit for skywatcher dobsonians as I thought I could adapt this for use on my home made dobsonain. The azimuth encoder fits on the groundboard fine. However, the design of the altitude encoder has changed and now has a magnetic strip. I can see no way of fitting this to my dob. I have a full size altitude bearing-a half circle. The simplest solution seems to be just to buy another encoder and fit it mysefl, I can make brackets etc. I appear to need '10000 steps quadrature encoders ttl'-I presume an optical encoder? I would like to buy from the UK to avoid probable long delivery times from Australia. Can anyone advise me where to buy an encoder in the UK, preferably on that I can just plug the cabe for the nexus DSC in to? I have tried using google but so much information comes up that it is very confusing! I am hoping that someone has bought such an encoder and can let me know the best place in the UK to get one.

Thanks for the information. I have completed the form to ask about membership of the Castle Point Astronomy Club. Hopefully it will be possible to go the club's dark sites at weekends, as I work full time this is usually the only time I can travel. As usuall the weather forecast is constanly changing. For today at the moment it shows both clear skies for Thetford Froest and the Dengie Peninsula. If the forecast stays the same I will go the the Dengie Peninsula as I have been there before at night and know it is OK.

I have been to Thetford Forest looking at birds a couple of times in the last few weeks and thought it mught be quite a good place for stargazing, For example St Helen's Picnic site car park, where there arn't too many trees to block the view. Has anybody been stargazing in Thetord Forest? The good thing is if I arrive 2-3 hours before it gets dark I can have a look for some birds and it won't be a totally wasted trip if it turns out to be cloudy. I have been to Margaret Roding in Essex a few times, but Thetford Forest looks like it may be a bit darker. It is around one and a half hours drive from where I live on the London/Essex border. Another good thing is that it appears from the weather forecasts that when it is cloudy in Essex the skies are sometimes clear in Thetford Forest, as it is quite a distance away. I want to also try the Dengie peninsula in Essex, the car park for St Peter's Church in Bradwell on Sea, which is really out of the way. But the only time I managed to get there it was cloudy, even though the forecast said clear skies. I did see some bats though!

Last night the sky was clear and there was no moon. So I drove with my 14 inch dobsonian to a reasonably dark site in Essex, around 40 minutes drive away. I saw M51 for only the second time. I managed to see the Leo Triplet galaxies for the first time ever. All these galaxies are totally invisible from my light polluted back garden, I live on the edge of London, just inside Essex. It was great to see the galaxies, but they all looked like faint smudges of smoke. According to the Clear Outside website where I was, Margaret Roding, is Bortle 4. I am curious if it should be possible to see more detail in the galaxies? With M51 I could see hints of the spiral arms, but the Leo Galaxies were just vague shapes. Where I live is classed as Bortle 8. I haven't managed to find anything better than Bortle 4, researching on line, in one and a half hours drive from where I live. But I guess this is normal anywhere near London! I am going to try the Dengie peninsula in Essex sometime. It is a lot longer drive and also seems to be classed as Bortle 4, but I think it should be darker than where I was last night. I am still amazed that I managed to see the Leo triplet, after searching for it many times before! I moved the telescope a few times so they that they were not in the eyepiece and managed to find them again, they are not too hard to find from somewhere reasonably dark. I use a Telrad and 80 mm finder scope. I didn't use a shroud on my truss dobsonian, next time I will try with a shroud and see if it makes a noticeable difference. I spent around 3 hours looking at the galaxies, it was freezing cold but I had a great time. While I was driving there I saw a large deer with big antlers crossing the road, so it is a good idea not to drive too fast when in the countryside at night!

I am polishing a 20 inch mirror. I had a wide turned down edge. I am spin polishing on a fixed post machine using a 15 inch pitch lap. After around 15 hours of polishing the TDE has reduced in width and is right at the edge. But after more hours of polishing does not seem to be getting any better. I am using a over hang of the pitch lap varying from 2 inches to 3 inches, most of the time I have been using a 15% of the tool over hang, as seems to be recommended with fixed post machines. I have now picked up some shallow scratches. I think maybe from a wire brush I was using to scratch the pitch lap, some bristles may have come off and got stuck to the lap-although I don't know how as I always scrub the pitch lap with water and a brush before using it on the mirror. Does anyone know where to buy polishing pads for optical work in the UK? I have found some on line, but it is not specified if they are for optical work on glass. Has anyone used them? I would try to polish out the scratches with them. If this does not work I will go back to fine grinding using 320 grit. I will need to make a new coin tool. I could then use the polishing pads to get a quicker polish again before going back to using a pitch lap. Some people have said that polishing pads can lead to TDE, if this is the case I obviously don't want to use them! I am waiting for some grit to be delivered in case I have to go back to fine grinding so am going to try some more polishing with a 3 inch over hang and see what this does to the TDE. It is of course possible that the TDE is so bad that it will never go away. But it is strange as a few months ago I took it to a telescope making class I go to. There is someone there who has been making and testing mirrors for many years. I appeared to have a large over corrected outer zone and under corrected inner zone but no TDE. It seems that I got the TDE when trying to figure. I think I did W strokes that were too short. This was at home-not at the class. I only take the mirror to the class occasionally as the class is after work and I have to take the mirror and a large test stand with me in the car-I am usually too tired to do this. David

Sorry if I rambled a bit (well rather a lot) off topic, I tend to get carried away! I will stick to questions about mirror making and testing here (of which I will have lots!) and start another thread about turntables etc. David

My turntable is wooden, varnished and covered with plastic used for kitchen work tops, how well this will work I don't know. I am determined to make a powered turn table at some point, whether I will try to make an actual grinding machine just yet I am not sure, I will see how hard the turn table is to construct! I am planning to use my 750 watt drill press as the power, it has pulleys for changing speeds, I haven't figured out if I should try to connect one of these pulleys to another pulley to get the correct rpm or try to connect a pulley direct to the spindle (if that is the right word) where the chuck goes. The lowest rpm of the drill press is 210 and I want to get it down to around 40. I did buy a 4 pole electric motor from Machine Mart, but was advised on another forum not to connect it up to the mains until I have a greater understanding of electricity! Which is probably very good advice. As it will probably take me a few months to understand the wiring, DOL switches and No Volt switches (if I have remembered the terminogy correctly) for the motor it will hopefully be quicker to use the drill press which I can just plug in to the mains safely. I hope to eventually develope an understanding of electric circuits etc. so I can safely use the 4 pole motor. I will try to plan the pulleys and construction of a way of holding/ supporting the part of the drill press I want to use under the grinding table this weekend. I need belts and some kind of rod to fit to the underside of the turn table. Also start grinding the mirror flat by hand. If I succeed in getting the powered turntable working I will post some pictures, it will take me a few weeks at least to get to that stage. I will start a new thread if I make any progress with the turntable. David

I am in the process of planning a fixed post grinding/ polishing machine. First I am going to try to make a powered turntable and see how I go from there. I have one pulley so far and need another one or two, I am probably going to use the motor from my drill press. I need to calculate the sizes of the pulleys to get the right rpm. This will all take a while especially the construction! But in any case I first need to grind both sides of the mirror flat, as it has ridges, and this will take a lot of work and quite a long time. I will be doing that by hand. David

I really admire your staying power! I have nearly finished a 14 inch mirror which has taken around a year. The truss dobsonian mount for it is taking a long time to finish and will probably take another few months to get right. I am about to start grinding a 20 inch mirror (I aim to start this weekend having made a very heavy duty grinding table). I hope to show the same determination as you! By the way, I have found it impossible so far to take a photograph of a Ronchi or Foucault test that shows anything useful at all, but will try again at some point. David