Astrobits

It is no more difficult than grinding, if anything it is easier not having to stop so often to replenish the abrasive material. However it takes longer than you first expect. Initially, a polish appears quite quickly but to remove the last traces of pits seems to take much longer as you will also be pressing the lap to the mirror often to maintain the contact and you can then go have a cuppa or whatever🍻. The one drawback that I found with the pitch lap on top is that the polishing slurry doesn't stay in the lap grooves but falls out and is pushed off the edge of the mirror which means that you tend to use more slurry when lap on top. Nigel

Watch the polishing pattern carefully. With the mirror on top you will see a difference between the centre and edge after as little as 5 minutes. I work both mirror and tool on top to even out the polish. If I left the mirror on top until the mirror was fully polished there is a danger of going too deep in the centre and having a hyperbolic figure which is difficult to polish back to parabolic. A hyperbolic figure can occur very easily with an f/8 but your f/4.5 will take more effort and time to go hyperbolic. Nigel

For what it's worth here's my take on solar filter mounting: The telescope is a GSO 80mm f/5 about 25+ years old. It has a fairly long dew shield which is just over 105mm internal diameter. A piece of 4" PVC pipe fits in with a little space to spare. I blacked the internal surface and stuck some Baader Solar film on one end with sticky tape. This is then put into the dew shield film first. This way there is no way the wind can blow the filter off. It is important that the filter holder does not fit too tightly in the dew shield so as to allow the filter to slide in and to be able to remove the filter without blowing the film off the end of the tube. Final pic is one I took last December with this setup. ( Focus is not perfect.) Nigel

Talk to your optician about your problem. My local optician ( part of a well known chain) never mentioned prism when checking my eyes. It wasn't until I asked them to do it did the prescription include a prism correction. I have always known that I had a slight prism error which had been mentioned many years ago during an eyesight test ( back then the optician called it "wedge" ). It has never bothered me in normal life but when using any binocular instrument ( Binoculars, microscopes etc. ) It was much more apparent. Nigel

JPL keep a current list of NEO's here: https://cneos.jpl.nasa.gov/ca/ Nigel

Aluminium is probably too soft for that application. The load on the bearing contact will be very high. It is a narrow line of contact and there is the possibility of the Al deforming under that load. Also Aluminium will slowly rub off on the PTFE and leave fine black dust. The reason we use textured Melamine laminate/PTFE combination when making conventional Dobsonians is that using smooth melamine gives rise to stick-slip motion. I don't know if using smooth steel and PTFE will do the same. Nigel

It is generally recommended that surface analysis is done with several complete interference bands across the surface. If the analysis is done on the image shown in the report then it is not following best practice. There might be a good reason for this and there is no suggestion that the results would be any different if more bands were shown and/or used. The report could also be a bit more informative with regard to the actual measurements. Two sections are headed "Surface/Wavefront" and as Vlaiv points out, one is twice that of the other, so what is being shown/claimed? Nigel

Not necessary. The silicone is flexible enough to absorb slight changes in temperature. Anyway, I'm sure that your night time temps vary throughout the year so which night time temps would you match? Just make sure that your blobs are not too large. For your 5.3" secondary I would aim for one central blob about 1" dia OR three about 1/2" dia. Nigel

Firstly, silicone will not fail at a ( grease free ) glass boundary. If it is going to fail it will be at the wood boundary or if you have prepared that correctly then within the wood itself. I cannot vouch for all silicones, I use ones that specify that they are suitable for glass applications ( i.e. greenhouses ) I once sent a 200mm Newtonian telescope ( solid tube design ) by courier. They dropped it so heavily that the mirror, which was bonded to the MDF backing plate with silicone, and part of the MDF went down the tube to hit and break the secondary. The failure was within the 19mm thick MDF board and not at any of the (3 x approx 25mm dia.) adhesive joints that were holding the mirror to it. To prepare the MDF I smear some epoxy over the bonding areas and when set just rough it up with a bit of sandpaper. I would suggest that you do the same with the ply that you intend to use. Secondly, DO NOT USE MORE THAN THREE points of adhesion or you WILL get astigmatism. Also make sure that the silicone is about 3mm thick between the mirror and support. This will further prevent astigmatism by giving the mirror a softer "cushion". Personally, I use only one central silicone blob on my secondaries, about 20mm on a 70mm M/A mirror. My current 16" scope has lasted 20 years so far without any adhesive problem. By allowing about 3mm space between mirror and backing it will be easy to remove the mirror when the time comes for re-coating it. I had another time when a customer failed to heed the three blobs x 3mm thick rule and that resulted in astigmatism ( in 19mm thick glass ) and a very difficult time to remove the mirror causing some damage to the backing plate. Nigel

Nova N CASS 2021 discovered 18th March. Current V mag in mid 7's. Co-ordinates: R.A. 23 24 47.73 Decl. +61 11 14.8 It is coincident with a known eclipsing variable star, W UMA-type CzeV3217 Nigel

Surely, it is simple trigonometry to determine the size of the top cage needed. Just use the max field of view you and the distance from mirror to the top of the cage. Nigel

As a mirror maker I would always recommend the fingertip method. Why? Fingertips are very sensitive and will detect any hard material allowing you to stop from dragging it along the surface with potential scratching as a result. The hard bit can then be dealt with before continuing. It is amazing just how hard some bits floating about in the atmosphere are. Once they settle on the mirror they can stick quite firmly to the surface and can corrode the Aluminium coating producing Aluminium Oxide which is scratchy enough to be used as a mirror grinding material. I know, I use Aluminium Oxide for final stages of grinding mirrors. Using any man made material disconnects you from sensing hard, scratchy bits and can result in longer scratches as a result. Furthermore, man made materials will never be guaranteed 100% free from contaminants. A very high percentage, yes, but never 100%. If you must use man made materials use the lightest pressure possible such as dragging a sheet across under it's own weight as mentioned in previous posts here. It is best to use a tight fitting cover for the mirror whenever it is not in use to minimise the frequency of cleaning. Nigel

The asteroid was at mag 16.9. You are unlikely to see it, especially through cloud. I had solid cloud.☹️ Nigel

Looking at the cloud forecast there is 0% chance of seeing it☹️ Nigel

I use both this one : https://www.asteroidoccultation.com/ and this one: http://www.poyntsource.com/New/Global.htm happy-kat link takes you to the specific page in the first link for this event. The second web site tends to follow the first by a day or so. Nigel

Although this should be in the observations section I thought that it warranted a more prominent post here. Moderators move if you think necessary. As title, Regulus will be occulted by asteroid 2589 Daniel on March 9th at approx 20h 21m . The narrow shadow will pass through southern England from Minehead to Worthing. The image below shows the centre line with the predicted limit in blue with the 1 sigma error lines also shown. The asteroid is quire small at only approx 9.5Km wide so the occultation will last less than 1 second. At least this one can be done without optical aid. Good luck and hope the weather plays ball. Nigel

Ahh! O.K. It's just that your image seemed to show a pivot point in the centre of the disc and is explained as line of sight effect by AlexK. Thanks for the clarifications. Nigel

Your Boxmount is a most interesting design, Chriske. I am currently planning to make an equatorial platform and will investigate this option for my 16" scope. However, with the rotation axis in the centre of the platform I think that the scope will need a very low COG to be safe at +/- 15 degrees of tilt ( 2 hours) unless it is firmly attached to the platform. Nigel

You can also try SRB Photographic: https://www.srb-photographic.co.uk They do specials, you just have to find the link on their web site. Nigel

For me the best description of the "Star of Bethlehem" is given by Michael R Molnar in his book: https://www.amazon.co.uk/Star-Bethlehem-Legacy-Magi/dp/0813527015. It turns out to be Jupiter. The circumstances fit pretty much all the details from the Biblical stories provided we view these from the society of 2000 years ago and not from our current times. It doesn't need any unusual/unexplained events in the heavens. None of the other "obvious events" to our eyes (Novae, comets, conjunctions etc) can be regarded as fitting anywhere nearly as well - hence the continuous debates. Nigel

Every angle of lines on the mask will produce two spikes at 180 degrees to each other. Therefore if there are three different line angles you will get 6 spikes and with 4 line angles there will be 8 spikes. Nigel

I think you will have to have a longer "back working distance" than that in your Cass calculation as it has to accommodate the radius of the tube plus the working distance outside the tube for focuser and camera. Nigel

I think that one of the telling comments in that announcement is that we only know, on Earth, of anaerobic micro-organisms that produce Phosphine and HUMANS. I am sure that it was not suggested that we produce the gas via our bodies but in chemical reactions in labs. This process requires that we bring together the right chemicals at the right conditions and we get as much Phosphine as we want. Chemistry does the rest. I, personally, believe that chemical reactions will be found on Venus that explains this result despite the offered analysis that says it can't. Nigel

If I was designing something like this I would make sure that it was easy to maintain/repair using the simplest, readily available, components possible as a first requirement. Any friction implies that there will be wear that will eventually cause the component to malfunction so the easier it is to replace the better. Nigel

While it is fun/instructive to design the telescope structure and analyse it's performance on a computer, I would have thought that the fundamental question in this project is "can you make a mirror to the specifications desired?" Personally that is the first thing that I would do.....make the mirror. If it works out fine then it will wait for the rest of the structure to be made. If you make the structure first, or alongside the mirror making, and the mirror is not up to your specs then it could be a big disappointment. Nigel