dark star

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.