Astrobits

A few thoughts. Having made a number of mirrors I can confirm that keeping things wet will totally eliminate any dust to the extent that you do not need any eye or face protection. Nothing flies up from steady wet working. Don't try to make a slurry of the coarser grits in a bottle, it will just settle far too quickly and become more of a hindrance than a help. I only made slurrys with the two finest ( aluminium oxide ) grits that I used. The coarser grits ( silicon carbide) were applied to the work from a shaker jar, a coffee jar with holes drilled in the lid, or alternatively just using a spoon to ladle from the bulk container and sprinkled over the tool. I don't know how thick your tiles are but you better plan on making a second tile tool when the first one wears out. If the tiles are about 6mm thick they are unlikely to last to the finest grit. You will then need another to finish off. This second one can be made to the curve that was generated by the first but will still need bedding in. I used 10mm thick glass tiles and they will last for one mirror 8" mirror. When placing your tiles don't use a regular pattern, that will almost certainly lead to a mirror with zones, Also make sure that there are no very narrow spaces between the tiles where grit can become difficult to remove. I usually filled the spaces between the tiles with candle wax ( melted in a pan and spread around where necessary with a hot air gun ) which could be melted out at the end of a grit size ( using the hot air gun ) bringing any embedded grit with it, then fresh wax was poured on sealing in any wayward bits of grit. Just make sure that the level of the wax is below the level of the tiles if you do this. As regards plaster then any plaster will do, just make sure that your casting is allowed to thoroughly dry ( it takes days. However, beware-- all my faster drying attempts using heating in an oven resulted in the tool cracking) and then waterproof with varnish. Wet plaster is not as strong as dry plaster. When casting a plaster tool you can use surprisingly flimsy appearing material for the circular dam wall. I have used polythene bags ( the thicker ones like the large dog food bags) cut to size and held with sticky tape around the mirror blank. The advantage of using a flexible material is that when you pour the plaster into the mould the shape naturally becomes round whereas a rigid dam will not. For cleaning the tool and mirror during grinding I do not think that sponges are the best choice. Just dunk them in a bucket of water and use your hand to brush the water over the mirror surface ( a finely ground mirror surface feels wonderful😊). More care needs to be taken with the tool as there will be sharp edges. Here a nail brush is most suitable to get between the tiles. For the work area then copious water flow would be best. For examining the ground surface a 10X loupe is sufficient even at the finest grit sizes. The higher the power, the smaller the area in view and the longer it will take to examine the entire surface to find that one rogue pit. More important than magnification is method of illumination. I made a light box with a glass top. A piece of black card with holes in it ( approx 1" square chequerboard style) was on top and the mirror placed on that. It is easy to focus on the pits at the (out of focus ) boundary between dark and light. I am sure that you will have many questions once you get going. Have fun. Nigel

I note that your location is Fishguard. Is the deposit on your equipment salt spray deposit? If so you will have to dissolve it off with water with little or no rubbing in the first instance. You can follow that with Isopropyl alcohol/Baader fluid and finally distilled water. Nigel

It is almost certainly true 7x 50. A popular ruse by cheap manufacturers to make the bins seem very powerful is to quote the mag as area not linear. All 7x (linear) 50 bins give 49 x area mag. Often used to see ads in the magazines years ago for 50x mag bins. Nigel

Are you sure that it's the telescope and not your eyes? Eyes change with time and Opticians cannot always fully correct for astigmatism. They can only correct for the major axis and there could be some minor astigmatism at a different angle remaining. I know because I have this problem and I can detect changes in my astigmatism over time-scales of less than 1 year. Just look at the Moon with and without any glasses you use. Is there a another, fainter, image nearby? Do any glasses you use eliminate all secondary images or is there still a faint secondary image remaining, probably in a slightly different position? Whatever, if you can detect a secondary image then you have uncorrected astigmatism. I would not have thought that at 10 years old your telescope would be deteriorating unless it has had a knock severe enough to put the lenses out of alignment. Nigel

I too have a mirror ( 8 3/4" dia ) with very similar writing on it. I believe David Hinds used a 4 figure code which is present on your's and mine. Nigel

While it is quite easy to test primary mirrors with a variety of tests, it is not so easy with secondaries. During manufacture, secondaries are tested against a reference flat when the interference fringes can be seen. Once the secondaries have been coated and in use for a while there are two problems that arise. Firstly, they will have some material ( dust/dirt ) that has settled on the surface and has stuck so that the usual gentle cleaning does not remove it. It does not need to be large enough to be visible, a few wavelengths in size is enough to prevent a glass reference surface getting properly parallel to get a good set of interference fringes. This can be overcome by using a liquid reference surface. Secondly the secondary is very reflective while the reference surface is not. The difference causes the fringes to be very faint because of the imbalance in the reflectance of the two surfaces. Using a liquid reference surface where the mirror is under the liquid, usually water, this can be partially overcome by adding some water soluble dye which attenuates the reflectance of the mirror. It needs rather a lot of dye as the layer of water over the mirror needs to be very thin in order to get stable fringes. I have tried this and it works to a limited extent with the fringes a bit easier to see. Nigel

I have made Acrylic sheet. I worked in Perspex research. The sheet is cast against ordinary glass sheets and will therefore AT BEST take up the surface morphology of the glass. If you test ordinary glass sheet you will find that it is not optically flat but covered with hills and valleys of at least a few wavelengths high and low. These undulations will be present on BOTH sides of an Acrylic sheet. As mentioned in above posts the bulk of the material may, or may not, be homogeneous. All these defects will degrade any image in addition to your spherical primary. Chriske mentions that float glass is used in some SCT's but that will have been selected for homogeneity and optically polished both sides to produce the complex curve SCT's require. Maks require more simple curves but of very short radius and the thickness of the corrector is important. Go check the formula for a Mak. Nigel

For doing occultations, IOTA ( International Occultation Timing Association) require analogue cameras coupled to a VTI ( Video Time Inserter) and recorder. Nigel

I don't see the lists when I view as guest on my tablet but they do appear when I sign in. There's even more SGL stuff if viewed as desktop though as noted by John above. Nigel

I don't think I saw a mention of alignment shift with focus/backlash previously in this post. Sorry if I missed it. If that is the cause it still implies that there must be a limit to such movement while the telescope is in one position. You don't mention if you have an observatory but views from the house would indicate not. Pity, as you could leave the scope pointing in one direction for a day or two and then take some shots without moving the scope to another sky position. Any slow slop should have reached it's limit by then. Shots of the brick wall might not be representative due to the mirror being out of it's normal operating position. I was thinking along the same lines as Dave with the extra mass of the RASA causing extra friction in some components or slippage in a clutch ( would not have thought so with the claimed load capacity but you never know). Nigel

While I agree that the movement in one individual exposure is only arc seconds, your post on the 18th: Quote: "To illustrate the issue better, these are four 300 second exposures guided. Guiding was helpful in this case, but up to a point after which it couldn't keep up with the drift. I would have to stop guiding and exposing and re-centre the image every five subs otherwise it would slowly drift quite a ways". end quote. clearly states that the movement continued for at least 5 consecutive subs at which time the total movement was sufficient to require re-centering the target. This amount of movement is much greater than a few arc seconds and should have caused an out of collimation situation if only one of the optical components was moving. As this does not seem to be the case then the whole of the optical system must remain in their correct positions. The fact that the guiding set-up works at some points indicates that there is nothing moving within the RASA when the guiding is on.at that time. I cannot see this trail going anywhere but back to the mount and drive system. Nigel

If whatever it is that is moving is doing so consistently over a considerable period of time without stopping then logically whatever bit is moving will fall over or out!! I assume that that hasn't happened. Also if something in the optical train is moving to shift the star positions then the collimation will be out at some point and should show up in your longer star trails as a changing of the width of the tracks in at least one corner. I see no evidence of this. I think you should look again at the drive train of the mount. Nigel

Looking at your images, it strikes me that where there are long trails they are far too regular to be caused by some sloppy component. Surely, a sloppy component would slop from one position to another fairly quickly and then stop. A sloppy component would have a limit to it's movement so the movement should become less with time in one telescope position. Do repeat subs show the same amount of movement over the same time periods? Nigel

The cell that failed for me didn't have any horizontal screw holes and it failed diagonally through the sheet from a surface screw hole. A very difficult way to break MDF. I expect that Aquarium Silicone will be O.K. I have just looked at my supply, it is Ever Build Everflex 225 and it particularly mentions glass to glass applications. Other products are available? Nigel

I'm don't know if the Evo Stick adhesive will do the job. Ordinary mirrors are silvered on the back and this is then overpainted so the Evo Stick is intended to stick to paint. You're welcome to try it but I have used the greenhouse silicone for over 20 years without problems and don't see any reason to try something different the next time I stick a mirror to MDF. I found priming the MDF was essential as without it the failure was always the silicone to MDF bond. In the 90's I sent an 8" F/6 Newtonian tube via couriers who proceeded to drop it with such force that the 19mm MDF in the cell broke sending the mirror down the tube, with the remains of the cell still siliconed to it, to hit the secondary That proved to me that the procedure that I have adopted was good ( apart from the use of couriers that is ). I have come across the felt option mentioned in Ed's post and I found it difficult to separate the mirror from the cell when needed as the felt blocks up the saw teeth and, being wool, doesn't surrender to wire very easily. It is also very important to use only three blobs of silicone and not too large, 1" dia for an 8-10" mirror is sufficient. . Use 4 or more, or go too large in area and you are immediately in astigmatism territory. My 16" mirror has been on three blobs of less than 2" dia for at least the last 10-12 years. Nigel

NO. It will not give you two important properties. 1. space between the MDF and mirror to remove the mirror easily next time. 2. prevention of astigmatism by contacting the mirror at only 3 points ( small areas, about 1" dia ) only. If you have a raw MDF surface then prime it with a thin coating of epoxy and roughen it. Apply three blobs of Silicone ( I use the clear type that mentions use for glass applications ) , enough to give a blob about 1"dia and 3mm thick and place three spacers about 3mm thick on the cell before presenting the mirror to the cell and pressing down to the touch the spacers. Wait 24 hours before removing the spacers to let the Silicone cure. Job done. Nigel

Kodak's Technical Pan film no 2415. A fine grain film that responded to "Hypering" or Hypersensitising. There were several ways that were used. Simple baking in an oven ( water tends to desensitise film ), baking in nitrogen or even in Hydrogen ( v. dangerous but was the best result ). Nigel

Yes, He used any suitable supplier, mainly David Hinds, but I still believe that he started making his own at some point. Therefore the early telescopes would be purchased mirror sets but the later ones will be a mixture of purchased and Dark Star made mirrors. Nigel

Yes, I believe that Dark Star did start making their own mirrors later in the life of the business. Unless DS was for David Sinden. Might be an idea to check the figure on the mirror? Nigel

Here's a tutorial on a GSO focuser strip-down and re-assembly to give you some idea as what to expect. http://polluxchung.com/gso_crayford_fix/ Nigel

Here's a diagram that I obtained from a Chinese observatory when I was making my own units. The shaft goes through the three ball bearings and the balls ride in that groove. You will probably have to dismantle the whole focusser unit to get the knob back together again including removing the shaft from the fine focus knob. Nigel

Yes, Gina, I was sure that you were aware of creep. My post was directed to those who might be reading your thread and, being unaware of creep, planning an all, or mostly, plastic system via 3D printing. Nigel

Just a word of caution to anyone planning to use plastics for load bearing components. It is unfortunate that plastics not only flex but creep under load. That is, under load, they deform over time from the original shape and hence the location of the attached components move with the deformation. This could spoil any highly accurately aligned setup. Different plastics have different creep characteristics. Polyethylene (LDPE ) has high creep while polypropylene (PP) is much lower. For example, you should not make bottle crates from LDPE as when stacked the bottom ones will distort relatively quickly while PP is o.k. for this application. Adding fibres ( glass, carbon etc. ) reduces creep significantly but doesn't reduce it to the level of metals. Another feature of plastics is that they can absorb oils making them more flexible, so make sure that you keep oils away from plastic components where it could cause a problem. Hope this helps. Nigel

I had my ally tubes powder coated with a matte black finish. Not perfectly matte black but not shiny so less reflection than a shiny finish. Nigel

I made these several years ago. Found a Chinese web site that had the diagrams that guided me. It didn't mention the tapered barrel, I had to find that out for myself and determine the best taper angle. The tapered barrel and shafts I made with Stainless steel having found out that brass was too soft. Here's the web site, suggest using Google Translate: http://bbs.astron.ac.cn/thread-54988-2-3.html Nigel