dark star

I looked at the Schott UK website. It appears that in order to see the prices, for example for BK7, you have to register. It is not possible to register unless you are a company as there is a mandatory field asking for a tax number. Do they sell small quantities of glass to individuals? On the form to contact them it asks for a company name and email address?

I have completed an enquiry form to Galvoptics asking about BK7 blanks. I should have thought of this myself, as I have bought pitch from them a few times. I will post again when I receive a reply.

I would like to have a go at making a refractor. Or possibly a Schupman telescope. For this I would need some lens blanks, probably for a first attempt 4 inch blanks. For the Schupman I would need 2 BK7 blanks or other similar glass. For the refractor one Crown BK7 glass blank and one flint glass blank.. I have a book about making a refractor and another one about making a Schupman telescope. I have a couple of optical flats that would help with testing. In the longer tem I would also like to try making an 8 inch Houghton telescope. For this I would need 2 eight inch BK7 blanks. On the Cloudy Nights website I have read that BK7 is the most common form of optical glass. However, I have not been able to find anywhere to buy lens blanks in the UK, by looking on the Internet. You can buy glass blanks from China on Ebay and to other sites, but not much information is given about the type of glass or how good it is. You can also buy from Ebay, from a place called Silo Surplus in America. Again, there is often little or no inofmation about the quality of the glass blanks. Also, for all these blanks from China and America I would have to pay import taxes. Does anyone know where I can buy BK7 blanks in the UK? Or possibly from the E.U. if I could be sure of the quality of the blanks. I already have a 9 inch blank for making a Maksutov corrector. However, the tolerances for this are very tight, so I would prefer to have a go at making some lenses to get experience before attempting to make Maksutov. I have made one 14 inch mirror for a Dobsonian and am making a larger mirror for a bigger Dobsonian at the moment. But I would also like to have a go at making something different. David

I got my 20 inch borosilicate mirror blank a few years ago from Spectra Glass in Scotland. They still sell blanks. I sent them an email but had to also ring them to get a response. They mainly sell glass to businesses but they were happy to sell me a blank, once I rang and explained what I needed. It is 40 mm thick. I am still working on the mirror! I hope to finish it this year.

Thanks for the reply. I will start saving up! I don;t really understand how importing things from Europe works now. The TS website says that the price includes 19% V.A.T. I presume this is German V.A.T. which would be taken off the price and I would then have to pay 20% UK V.A.T. to the UK Government? Are there any extra charges like import duties?

I love my 12 inch f/4 Orion Optics UK Dobsonian, which I bought second hand. I agree that the metal bases are great. I have managed (just) to fit the base and telescope in the back of my small SUV, the part with the secondary mirror goes on one of the passenger seats. This means that I can sometimes take the telescope with me when me and my family go on holiday. I can carry the telescope tube a reasonable distance, it is a bit heavy and awkward but it is possible. I think the mirror is very good. But I am not an expert, I have been obseving for a few years but the observing sessions I can manage each year are not that many I mainly observe galaxies and and for these it has been great. I want to try it out properly on the planets at some stage. I like it is much that I would want to keep it even if I eventually get a 16 inch Dobsonian, but I don't have enough room. David

Does anyone know where it is possible to buy APM 16x70 ed binoculars in the UK? Astrograph have shown them out of stock for a long time. Have they stopped being available because of Brexit? Even APM in Germany only very occasionally have them showing in stock and very quickly they go out of stock. I would prefer to buy in the UK. As if there is a problem it is easier to get them replaced or fixed. David

What pins did you use and how did you work out the position of the pins? I would like to try an Everest pin mask on my 20 inch. I have always had problems seeing the shadows with a Coude mask. David

I am working on a 20 inch f/4.1 mirror. I have the same problem of a turned down edge after spin polishing on a fixed post machine. Using a 15 inch pitch lap. I tried varying the offset of the lap but it did not seem to make much difference. I read that the offset should be around 15%. My outside ROC Ronchi test looked quite similar to yours. I am currently working on the center zones as they are flatter than the middle and outer zones, using a 10 inch pitch lap and a centre through center stroke, not going over the edge. But I need to fix the TDE. I am now working by hand with the turntable rotating at around 6 rpm. I will be interested in what stroke you use to get rid of the TDE. David

I should have thought of that,! If I put the tool on to the mirror and pour a bit of plaster in to the hole that should fix it. I use two penny peices attached to the tool with small blobs of pitch for grinding.

I made the grinding tool. Unfortunatley I made a silly mistake, one I have not made before. I need to make a hole for a socket in the middle of the grinding tool to hold a metal shaft from the fixed post machine. I forgot that the hole should not go al the way through the tool! So now I have a hole in the middle at the front of the grinding tool which as far as I can see makes it unusable. I will have to make another tool. To look at the bright side, this time when making the tool I weighed the plaster and water. There was some left over when I poured the dental plaster, so next time I will know to use a bit less. I should have enough dental plaster left to make another tool.

I am going to make a 15 inch diameter grinding tool for fine grinding my 20 inch mirror. I am spin grinding on a fixed post machine. I have picked up some scratches when figuring so I am going back to fine grinding. I poured the pitch for the pitch lap on to the original grinding tool so I cannot use this. I have some Dentstone KD dental plaster. I wanted to double check that I have understood the mixing instructions correctly so I don't make a stupid mistake. The instructions say water to plaster ratio ( by weight) 30%. Plaster to Water Ratio (by weight) 3.33:1 This is probably obvious, but I am afraid I don't have a very logical brain when it comes to anything with figures, however simple! I assume this means that I need just over 3 times the weight of plaster compared to water, and not the other way round? Also, I have looked at on on line calculator for mixing cement , and it appears that I should use around 12 kg = 26.45 pounds of dental plaster. Does this seem about right? I am planning for the tool to be around one and a half inches thick. It appears that I should use 3.6 kg = 7.94 pounds of water? In the past I have just guessed at the amount of dental plaster needed which has often worked. But once I made a tool so heavy it was not usable! So I want to stop guessing. David

I guess it is like a mold. It is just something to make the dental plaster or cement make a circular shape, and to stop it spreading all over the surface of the mirror. The garden buckets are a good idea. But I need the grinding tool to be 15 inches in diameter. As a 2/3 sized tool works well for spin polishing. So they are a bit too big. I guess some kind of plastic sheet would work fine. I have some but it is not quite thick enough to work really well. The circumfrence of the tool is around 50 inches, 1300 mm, so I need quite a big sheet or strip.

On the Cloudy Nights website people have mentioned using aluminium flashing for the circular form to pour the cement or dental plaster in to, to make a tool for grinding a mirror. Or a base for a pitch lap. Does anyone know what aluminium flashing is called in the UK? It is thin enough to bend easily but thick enough to stand up without flopping around. I have found something called Alcan alumiumn flashing on- line from a roofing merchant. But I am not at all sure that it will be the right thickness, and it is sold in 8 meter rolls. It looks like it might be too floppy. I pour the dental plaster on to the mirror, which is borosilicate (like pyrex). I have done this a few times and not had any problems. I protect the mirror with aluminium foil. The dental plaster eventually gets rather hot, but I take it off the mirror before it gets too hot, as it gets hard quickly. I have used cardboard and also plastic sheet for this, but although they worked they were not ideal. I need to make a 15 inch tool for fine grinding my 20 inch mirror. I picked up some scratches when using the pitch lap to figure the mirror. So I am going back to fine grinding. I poured the pitch lap on to the tool I used for grinding so I have to make a new one. In future I will make a new pitch lap base instead. I spin grind and spin polish on a fixed post mirror making machine. I have had good success with this. But figuring the mirror is proving a challenge. I have made a 14 inch f/4.8 mirror without too many problems. But a 20 inch f/4.1 mirror is an order of magnitude more difficult! However, I am learning a lot about mirror making and different figuring strokes, David

Hi, does anyone have a recommendation for a dew band for a 6 inch mak corrector? I have already wrapped the Mak in reflective material and added a camping mat to extend the dew sheild. It is an Intes MK66. This would be for when the temperature is at our below zero, as it was recently. I am hoping not to need a dew band for the corrector when it is not as cold. I have a Kendrick dew heater controller. I was concerned to read a couple of posts about dew bands catching on fire! However, this appears to happen mostly after a couple of years of frequent use. I only use my Mak occasionally for planets. I use my 12 inch dobsonian a lot more, as I am mainly interested in deep sky. I may try to figure out how to add fuses for the dew heater bands, as some people have done this. David

Thanks for thre reply. I am now debating which dew band for 6 inch telescopes to buy. Or possibly making my own. But given that I have limited amounts of free time I will probably buy.

I have a 6 inch Intes MK66 Mak. The night before last I was looking at Mars for a few hours. I eventually managed to see a little bit of detail on Mars after putting a dew heater band on to the eyepiece. But this was after a couple of hours and it seemed to take around one and a half hours to get rid of the dew on the eyepiece. I have a Kendrick dew controller. The Mak is kept in an unheated shed, in a flexible cloth case with padding. I have wrapped the Mak in reflective insulation material. There still seemed to be some minor problems with the corrector dewing or icing. The mak already has a dew shield. But I added a longer one using an old camping mat, this seemed to help a lot. The Mak corrector seemed be dew free. But it was hard to tell for sure using a torch in the dark. Plus most of the time the telescope was pointing high up at Mars, which made it harder to see the corrector. Do people think I should get a dew strip to put under the insulation to heat the corrector when it is really cold like it was last week? Just to be on the safe side. Nest time I will put a dew band on the eyepiece as soon as I start observing. David

i saw Mars going behing the moon using 10x50 binoculars. I only went out in to the back garden beause I woke up very early. This was unplanned as I had to go to work afterwards. It was very interesting to see. I am really glad I saw it, as I have never seen this before and don't know if I will see it agsin! Hopefully over the weekend I will get a proper look at Mars with my 6 inch Mak, I have not had a good view of it as yet this year.

This week I stayed at the Dolgoch Hostel in Wales. The skies are Bortle 2, which is the darkest sky I have ever seen. I took my Orion Optics UK 12 inch Dobsonian. I saw the Western Veil Nebula and the Eastern Veil Nebula for the first time. They were visible without a filter, but very faint. With an 0111 filter they were very obvious and the Western Veil especially was impressive. I also looked at M51, M97, M101, M57 and the Blue Snowball Nebula. Also the Andromeda Galaxy. Planetary nebula did not seem that much better than my usual dark site, which is Bortle 4, maybe because they are relatively bright? The Milky Way was amazing. The hostel is in a valley and there are some trees, but there was a pretty good view all around the sky. The hostel has no electricity so there ae no lights to lessen the view of the night sky. There is a viewng spot/parking place around 10 minutes drive from the hostel which is probably even better, as it is higher up, so more could be seen lower down. But my wife would not let me drive on the mountain road at night. although I am sure it would have been fine. The road to the hostel is a bit scary, it is a hIgh mountain road, often without A fence, it goes over a steep hill called the devil's staircase, but my car mamaged the road fine, if you go slowly it is OK. I would recommend the hostel to anyone who wants to experience a really dark sky. I would like to go back there or somewhere in the same area when more galaxies are visible, for example in May.

I could not resist ordering the desk edition from Amazon, as it was reduced. It hasn't been despatched yet. I will report back when I get it. David

My understanding is that a dual axis platform is also driven on the North/South declination axis as well as right ascension. Commercial versions are sold by Tom Osypowski in America and TEPP in Hungary. I have successfully made a 14 inch Dobsonian. But since it was not square enough to use with digital setting circles, I will most likely attempt a simpler EQ platform first. As I think making even a single axis one with sufficient accuracy will be a challenge for me. The platform you are making does not look extremely complicated, and I have lots of experience of cutting circles out of wood with a router.

I wanted to ask if this equatorial platform could be modified to make it a dual axis platform? I would like to make one of these so that I could use my 12 inch dobsonian for EAA and maybe some astrophotography. There is a commercial dual axis equatorial platform that can even have a guide camera added to it. But it is expensive and also very heavy. David

Hi, what is the diameter of the cross roller bearings and of the hole in the centre? I assume you attached it to the angle bracket using one of the circular ring of holes and to the nylon circle with the other circular ring? Where did you get the things 3D printed? I am making a 20 inch dobsonian and the cross roller bearings look like they might be good for the altitude bearings, the design I am planning does not use the usual wooden circular altitude bearings. But I would need 2 and they are rather expensive! I have a mni milling machine and could cut nylon circles, but the cost of the material, for example 12 mm thick nylon sheet, would not be cheap. Hopefully 3D printing is cheaper. David

Hi, where did you get the angle bracket ?

I have recently bought an Orion Optics UK VX12 dobsonian second hand. It is really great. I have finally got my Nexus DSC to work-I have a home made 14 inch truss dobsonian and it would not work properly on this, no doubt because the dobsonian mount was not square. The dobsonian came with Sky Commander DSC, which I also like, it is very simple compared to the Nexus DSC but works well. The problem I have is that the moount has 4 curved teflon pads that the altitude bearings sit on. The telfon pads seem to come off sometimes when I lift the telescope on and off the mount. Has anyone else had this problem and does anyone have any idea how to fix this.? The teflon pads have to be banged back in to place to get them back on the mount so it seems strange that they come off. Tonight I have lost one of the pads in the garden, hopefully I will manage to find it tomorrow when it is light. I have some teflon but do not think it would be easy to cut the curved shape needed to fit it back on to the mount if I needed to make one. David