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dark star

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About dark star

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  1. 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
  2. I went out in my car to a fairly dark site last night, Margaret Roding in Essex. It has been mentioned a few times on here as a good location. The last time I went it was in winter and everything dewed/ frosted up. I now have I dew heater and a hair dryer. I just needed the hair dryer a little bit on my Telrad, but otherwise it was fine. I got there when it was still light and got in a good couple of hours before the moon came up. After many years of trying I finally saw M51 the spiral galaxy. Using my 14 inch dobsonian which I made myself. So I am very happy. I could see maybe a hint of the spiral arms. i found it using a Telrad, I looked up where is should be in relation to Alkaid in Ursa Major, using a telrad chart from the internet-also in Stellarium, before I set off. I tried to find the Leo triplet but didn't succeed, but I didn't prepare how to find them as well, so probably just missed them. Hopefully next time. Still, I am very happy to have seen M51, it is totally invisible from my garden on the outskirts of London. It has taken me so long because it is rarely clear skies when I have a day off work the next day and can go somewhere dark, it seems to be cloudy for months at a time at the weekends!
  3. Hi, I have a 14 inch truss dobsonian, which I made myself. Does anyone have any suggestions for dew prevention/ heaters, for the primary mirror, secondary mirror, 80mm finder scope and telrad? I went to a dark site last December and everything dewed up! There are a lot of different dew prevention kits on sale, so I find it confusing. Something to attach to the back of the primary mirror to gently heat it seems a good idea. I use a shroud, but there are some holes cut in to the mirror box to reduce weight, I don't know if this will make any difference? Also, suggestions for a battery to power everything. I have bought a 12v hair dryer which I would prefer to run from a battery rather than the car. David
  4. I made it out to MR a couple of weeks ago. It was great, apart from everything dewed up on my 14 inch truss dobsonian! Secondary mirror, eyepieces and Telerad, also I think the primary a bit. I have bought a 12v hairdryer and am going to make a shroud for the next trip. It was really nice to see a lot more stars in Leo than I normally do from my back garden!
  5. I didn't go anywhere in the end as I got a migraine headache and had to go to bed. Hopefully next weekend! Thanks for all the replies. The car park at Dengie nature reserve, is that at the end of East End road, where there is a locked gate and the track to the church after the gate?
  6. I am planning to go to the Dengue Peninsula in Essex to stargaze tonight as it looks like it should be pretty dark. Does anyone know a good place to park up and observe there, preferably with little or no through traffic? I have finally got a car so can at last get to dark sites! Or I might try the site in Margaret Roding. Although I tried to find it once in daylight and couldn't. Could anyone give me the coordinates of the Margaret Roding site in a form that can be entered in a sat nav? I have already discovered that post codes don't work very well in the countryside!
  7. I am planning to go to the Dengue Peninsula in Essex to stargaze tonight as it looks like it should be pretty dark. Does anyone know a good place to park up and observe there, preferably with little or no through traffic. I have finally got a car so can at last get to dark sites!
  8. Hi, could you give some more information on the erector transfer assembly? This would go in front of the secondary mirror? I have a 10 inch f/4 mirror with a hole in it and am planning to have a go at making a secondary for a classical cassegrain when I have finished the 20 inch mirror for a dobsonian I am currently making (which won't be for a while!). David
  9. I will hopefully start grinding my 20 inch mirror blank this weekend, this has been delayed by the need to make a very heavy duty grinding table and the fact that I am still finishing my 14 inch truss dobsonian (nearly there at last!). I therefore need to start thinking about the mirror cell. I was planning on making the triangles in the cell from 1/4 inch thick aluminium (will this be strong enough?). However, I have just seen on line the 30 inch TROK truss dobsonian. Here the triangles for the cell were made of 3/4 inch thick birch ply. I have lots of this, and it would save me quite a bit of money buying 1/4 inch thick alumium, which is not cheap. Also, I would not need to buy a metal cutting bandsaw now (although I will almost certainly end up buying one in the long run anyway). On the TROK site they stated that they used the cell for a few years without problems. Is there any way in which aluminium is superior to birch plywood for this application? David
  10. Thanks for the reply about the spit blocks and spider mounting, it will come in very useful for ideas as to how to improve my 14 inch dobsonian. Another question, where did you get the wood? I have somewhere I can buy birch ply but not ash etc. David
  11. A couple of questions, what wood did you use for the split blocks? I really like your secondary mirror holder and adjustment. I have nearly finished a 14 inch dobsonian but the secondary holder and spider is a botched job and I will need to redo it! Will you have a way of adjusting the spider from the sides to get the secondary accuratley centered/ adjust collimation? David
  12. Actually, I tried to make the focus length a few centimeters too long in my calculations, as you can't make the truss tubes longer but can make them shorter. So despite my mistake with forgetting the truss tubes are at an angle I may have still made them too long. I will try moving the truss tubes down in to the mirror box around 3 centimeters and see what happens (they are resting on the sides of the mirror box at the moment). David
  13. So I should hold something like greaseproof paper in front of the focuser with no eyepiece in and see where the moon comes to focus, assuming it does come to focus I will know how much I need to increase the focal length? Thanks a lot, that's a great idea! It is a pity about the weather forecast, but telescope making (and mirror making) is teaching me patience! David
  14. I think I have figured out why my calculations of the length of the truss tubes are off, I made a very simple error. I forgot that the truss tubes do not go straight up to the secondary cage but are at angle! This means that they should have been longer than I thought. Hopefully someone can confirm that if with the focuser all the way out the telescope just focuses on a leaf on a near by tree I need to increase the focal length. If I make another truss dobsonian I will take the time to make a scale drawing and hopefully avoid silly mistakes like this. It is not a major disaster as the way I have connected the trusses to the mirror box is by bolting them to some long peices of angle (possibly steel) which are attached to the mirror box with screws. I can just move the angles further out of the mirror box and so increase the focal length. The angles are pretty strong so I don't think it will be a problem. I will post some pictures in a while once I have improved the telescope a bit. At the moment it more or less works but looks a bit of a mess! As this is my first attempt at making a telescope I am happy that it works at all! But it needs improving. David
  15. I have nearly finished a truss dobsonian for my 14 inch mirror. I am having a lot of problems getting it to focus on the moon. I am planning to check the measurements from the focuser to the secondary mirror and the secondary mirror to the primary mirror. Obviously I have miscalculated somewhere. I will check the focal length of the mirror again but am pretty sure I have this correctly. i meant to leave the truss tubes around 2 inches two long to begin with. When I rack the focuser all the way out I can just get focus on some leaves on a nearby tree. Does this mean I need to increase the focal length to get focus at infinity or reduce it? I am sure this is obvious to those with more experience! I could not focus on the moon gradually moving the focuser all the way in, I also tried with a barlow and a high power eyepiece. I was definitely pointing at the moon as if I looked through the focuser without any eyepiece I could clearly see it reflected in the primary mirror. David
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