Astrobits

Well, this is a very small scope with built-in erecting prisms. In the flat configuration it gives telescope type images ( upside down etc): while folded it gives correct images: It is an 8 x 21 monocular. Nigel

Go to 'home' on that page and you can then click on 'lenses' to see the options for ( mostly) positive lenses. At the bottom he mentions that he has more ( there might be some negative lenses amongst his stock) , just contact him with your requirements. Nigel

You could try: irpoyser.co.uk and see if he has one to suit. As he doesn't list everything he has got on his website you will need to email him. Nigel

Way back in the 90's I decided to try importing telescopes from GSO and ordered a small shipment as a trial, both for the product quality and company performance. The two 90mm achro's both had their objectives in the wrong way round and there were problems with the order which they seemed unable to grasp. I didn't order any more from them. I think they must be doing better these days. Nigel

I had a 4" spherical f/9 mirror telescope and it showed me detail on Jupiter and Saturn, fine views of the craters on the moon and dso's like the Crab nebula. The comment that a 4" spherical mirror needs to be f/10+ is for the writers own satisfaction. A 4" f/10 mirror is only 1/5th wave from parabolic and f/9 is only a bit worse. At f/12 that mirror is only marginally better than 1/8th wave. (Just for a horror moment, when the Chinese started producing telescopes in earnest some 25+ years ago they supplied 5" f/5 spherical mirrors. That is close to 2.0 waves off parabolic. Not only that, many had severe zones and some still had grinding marks.😱 These were sold around the world. I had a batch of 25 of them, 23 needed corrective polishing/grinding, one I passed as acceptable and one was binned with a large crack. I think that their manufacturing is better now!) I would suggest that you try to contact a local astronomy group so you can meet up and view through their scopes to decide what you want. If you really want to make your own mirror then go for a 4" spherical f/9 dobsonian. If you do a good job, you will be able to sell it ( if necessary ) to fund your next project and with the experience gained you'll do an even better job of that. In the meantime use binoculars for your astronomy fix. Nigel

There are many 4" f/9 telescopes working well with spherical mirrors. They might not be "perfect" in some eyes but they will give acceptable views. A pair of mirrors for a binoscope do not need to be EXACTLY the same focal length, within 1% is acceptable. Use mikeDnight's method ( above) to get the mirrors close to the same fl. In my experience, quite a high level of skill is needed to build the mechanical structure to collimate everything ( both optical tubes separately and together ) and keep it collimated as you move it around the sky. Once done the views will be amazing. Nigel

Putting a 0.5mm thick wire into a 25mm eyepiece it will appear to be 5mm thick visually as that focal length eyepiece is a 10x magnifier. I have found that such a thick crosshair will occult a focussed star making it difficult to get it onto the centre. The only way round this is to defocus the star sufficiently so that the crosshairs show up against the bright disc. It does make seeing the star patterns a bit more difficult but it means that you can have any reasonable thickness wire/thread that you like. Nigel

If you are looking at objects far away, end of garden, stars etc then the arrangement is as I have crudely drawn here: A relay lens does just what it says, it relays an image from one place to another. The top arrangement is having the focal point of the second ( relay ) lens at the focus of the first lens and it is producing an image at infinity. In the lower arrangement the second lens has been moved back so that the final image is now a positive image that can be accessed with eyepiece or camera. Vlaiv's arrangement would be found in a spectroscopy set-up where the leftmost focal point is at an image produced by another lens ( telescope ) and a diffraction grating will be placed between the two lenses. It is also the basis for the new, high-end light microscopes which have infinity optical arrangements that allow more interesting optical effects to be used. In microscopy the object at the leftmost focus would be a small specimen, flea, fly's wing, plant cell etc. Nigel

I have used silk threads for this application.Easily obtained from any bit of silk cloth, thread, haberdashers or silk moth cocoon ( where I got mine from). The best spider threads are those from the Orb Web spider as they are remarkably strong. I have yet to find any wire thin enough and readily available in small quantities for my liking. Nigel

You can't change the focal length without re-grinding the glass components but you can use one lens as a transfer lens to place the final image at a different location and size. Take one lens ( I am using the term 'lens' to mean the doublet ) and with an object at infinity the image is placed at the focal length the other side of the lens. This image can then be used as an object for a second lens to produce a secondary image. If the second lens is placed so that it's object ( the original image) is at it's focal length, then the second image will be at infinity and infinitely large. If the distance between the second lens and it's object is twice the focal length then it's image will also be at twice the distance from the lens and will be the same size as the image produced by the first lens. Varying the position of the second lens will produce an image of varying distances and sizes. Use the formula : 1/object distance + 1/image distance = 1/focal length ( rearrange as required) to get some spacing data. You will, of course, need to measure the focal lengths of the lenses, if only roughly, to get meaningful data. Image size is a direct ratio of the two distances. For an example using focal lengths of 300mm ( yours will almost certainly be longer): 1st lens to image =300mm image to 2nd lens =500mm second lens to 2nd image =750mm ( from the formula) Total length 1st lens to final image = 1550mm and the image will be 50% ( 500:750 =1.5x) larger than the original. Nigel

Simple optics. 1/object distance +1/ image distance = 1/focal length. If you are dealing with a telescope of focal length 2 meters then putting the object ( TV screen ) 2 meters away results in an image at infinity. OOPS! If you can put the TV screen 4 meters away then the focus will be 4 meters from the scope. You will need more than 8 meters of space and how to rigidly connect the sensor to the telescope over 4 meters of length will be a problem. It might work for very short focus telescopes. Nigel

That was my first thought too. Nigel

I prepared my own calculator many years ago. FWIW here's the file: You can change any of the yellow cells. I didn't include the fov but that could be added easily. Nigel flatcalc.xls

In my view the silicone blobs are too large. They are also too near the edge, they should be about halfway from the centre, or, as I mentioned previously, there needs to be only one blob in the centre. As I mentioned previously, my secondary has been held with a single blob for about 20 years. ( Maybe it's time I took it off and had it re-aluminised, then, maybe not). Consider the last image you put up. If the Al sheet warps in a vertical direction then the silicone twists and puts a small "plus and minus" stress onto the back of the mirror over the area of the blob but cannot affect the mirror beyond the blob boundary. Also, the more silicone there is between the Al and the glass the more the stress can be absorbed by the silicone and the less stress is transmitted to the glass. Thus the smaller and thicker the blob the better. However, if you heat the Al sheet then the sheet will expand horizontally and try to move the blobs away from the centre of the mirror putting a stress over the whole of the back of the mirror causing it to bend slightly compared to the front surface. In addition, the radiant heat from the heated Al will warm the back of the mirror more than the front putting further stress onto that back and causing more curvature of the mirror, ( all commonly used glasses expand with temperature, Low expansion Borosilicate glass expands 30-50% of that for plate/float glass). Heating one side of a sheet of glass ( or anything else for that matter) will always produce a temperature differential from one side to the other as the thermal conductivity limits how fast the heat moves from one point to the next and the unheated side will always radiate the heat away keeping it cooler than the heated side. To prevent dewing the mirror(s) should be shielded as in a solid tube (or shroud for open truss designs). Nigel

I suspect that the "hot" backing plate is simply heating the back of the secondary causing it to warp. If there are only three small blobs of silicone, with decent ( 3mm is fine) gaps to the secondary, then any warping of the backing plate should have no effect on the shape of the mirror. Personally, I am against any heating anywhere near a telescope. Thermals of only 1 or 2 degrees will spoil a perfect image. Nigel

It's a very long focus reflector. Schiefspieglers typically have spherical mirrors and are in the area of f/25. They work off axis but with the very long focus they have minimal astigmatism. The primary in the pic would appear to be around 12" and reflects onto another mirror that is not visible behind the panel at the left of the telescope. That mirror reflects onto the small mirror seen just left and above the main mirror which, in turn, reflects into the eyepiece. Nigel

Venus but not comet. Just went out to see if Venus and comet were high enough over my local horizon ( houses ) for a go at catching comet Leonard. As chance would have it there was a short contrail in almost the correct position for the comet. Whipped my phone camera out and this is the shot ( cropped) I got . The plane had moved a bit by the time the shot was taken. Nigel

Also try rotating the binoviewer as there are lots of surfaces within it. Nigel

You could also try RS https://uk.rs-online.com Nigel

I think that rather than rant about it here, a letter to the editor pointing out the negative aspects of the satellites would be better. Nigel

I can assure you that it does work.🙃 I purchased some diffraction film (A4 sized) many years ago ( IRC 500 and 1000 lines per mm ) and put small circles into spare filter holders. The finer the spacing the longer the spectrum. I still have some of that film. Nigel

Looks good. There is a slight curvature to the fringes, particularly noticeable to the right hand side. I suspect this is caused by viewing geometry. With flats you need to be as far away from the surfaces as possible otherwise the gap between the mirror and reference surface becomes greater away from your direct line of sight. Ideally your view should be perpendicular over the whole surface but you would need to be at infinity to do that. Get very close and you will see a bullseye pattern 🙀 and not straight lines even though the flat is perfect. Nigel

Just to add; if they don't meet on the same night go to as many as possible for a few meetings each and then decide which one to join. Nigel

I would suggest that you go along to one for a few meetings and then try others if you think that the first one is not for you. Most societies allow you to attend a few meetings before you need to join. Nigel

It's 80mm MA. 16" f/4.8 primary ( which is held by three blobs in case you were wondering). Done in the early 2000's and still perfectly O.K. No. As long as the backing plate is as large as the blob of silicone there is no point in having anything bigger for the sake of it. Just gone to take the attached photo which shows the flat from the side, so it is about 113mm long in this shot. Nigel