yuklop

I really enjoyed this challenge and approached the challenge as setting myself the lofty ambition to mount a 50mm lens to my Atik Horizon Camera. I spent a few hours carefully crafting a mounting method out of the various adapters, found it wouldn't focus, so crafted another that seemed to focus far away in the day time. At night though, it turns out it wouldn't quite manage focus at infinity. So I had to very quickly hack it apart and use a glue gun to glue the lens to a M42 spacer ring. Then with some fiddling it worked. Oh, and it was held to a dovetail with electrical tape. After the initial construction there wasn't a clear night in about 8 weeks in Southampton, but Thursday night was clear so I went and took a couple of pics. All I could manage was about an hour on M45, then an hour on both an Ha and OIII filter on NGC2244. There was a load of tilt, and other artifacts, but I had much fun with my fantastic contraption, and the results are below. Maybe we'll have another clear night in Feb, and if that is so I might make a better mounting solution that perhaps allows me to mount any old lens with any mounting as I have a few random old manual lenses. That'd be nice, and SGL, fantastic challenge, I'll look out for future ones - I don't usually image much, so it was quite inspirational! Thank you. M45, 200 x 10sec subs. Full image had more artefacts than the British Museum, so a hefty crop happened. 40 x 1 min Ha subs + 60 x 1min OIII subs. Med Gain.

Oh, it has been a poor show. I saw the nifty fifty challenge and thought I'm up for that. Spent a happy hour working out how to tape a lens onto the front of my horizon mono, and it's been clouds ever since. Competition is open to the end of Jan I think. Be lucky to get a single sub by then at this rate!! It'll need extending.

I finally got the hump with my old Vixen / Orion dovetail not fitting on my larger mount with a 4 inch dual saddle. I was going to mount a homemade Losmandy bar under it, then realised if I put that on, it would no longer fit on the Sky-Tee. If I was loaded, I'd buy more Losmandy saddles or more vixen dovetail bars but what I lack in cash I make up for with an angle grinder. I guess this is cast iron, as it was a right resistant swine. Whatever it is they should make bike locks out of the same material. Looks a bit of a mess, but it works just fine. A bit of Brasso and it'll be good as new.

A FC-100 Classic. Its a superb luxury item. The standard by which I judge all other telescopes that come and go. I have the matching original focal reducer to go with it if I ever want to take photos.

The C6R is pretty good for the planets too!

So its seems this isn't a question without the answer anywhere online, so I'll post it here in case anyone else ever runs into the same issue. I asked Takahashi and they said 83.5mm. Helpful folks they are. They also said that their new fc-35 reducer would work better with modern cmos cameras. I am sure it would, and I'm sure its superb, but its quite expensive, so that's one for another day. In the meantime some analysis on the old original fc-100 reducer. It is a fantastic chunk of glass, with a 60mm clear diameter and weighing 366g. I ran my own measurements and got the result below by using the relationship that focal length in mm = (206*pixel size in micron) divided by pixel scale in arcsec per pixel. Plate solving gives the pixel scale. The graph below shows the results and inserting 590mm as the desired focal length (for f5.9 reducer on 100mm scope), the calculated backspacing is 83.572mm. A pleasing if probably unnecessary confirmation. The question that remains is whether or not it is any good vs a modern system. To my eyes a full frame looks great at all the tested spacings. Siril's aberration inspector takes a close look at the corners. They aren't bad. Best at a range around the optimum length. I compared it to a modern imaging rig using a ES80APO and a 0.8x reducer and it compared favourably under pixel peeping conditions with both setups showing similar amounts of distortion (the distortion with slight comet shapes pointing inwards to the centre). The tests were done with an Atik Horizon 4/3 sensor in both cases. I tested the FC-100 scope with no reducer and that has a slight distortion with the starts smeared perpendicular to an imaginary line pointing to the centre of the image (apols I forget the names of these distortions, perhaps someone can chip in below...). The final observation is that the 83.5mm backspacing is good compared to many refractor reducers at 55mm. The FC-100 back is M72, so a M72 to whatever you want (e.g. M54 or M42) is ideal for making most of that back spacing. With a 2'' nosepiece that I use into the Tak visual back that 83.5mm disappears quick, and doesn't leave space for my SX OAG and wheel. Best go buy yet another adapter I suppose. Hope this is useful to someone one day!

Hi All, Hoping an expert on here has some idea of the backspacing of the tak f5.9 reducer for the original FC100 (image attached). I had a go at trial an error last night and realised that was a fools errand. I'd guess the spacing is too large as I get comet shaped stars in the corners. Anyway, help greatly appreciated. Is there way to determine correct back spacing in the daytime? It was cold last night! Dan

You are spot on. The details of the mount are in this thread:

Hi All, I have a fantastic old telescope shown in the picture below. Its does a great job, but it is 31kg, and as such one can't just whip it off the mount and use shorter focal length OTA's on a whim. I used to really enjoy experimenting with different OTA's. So I really want to mount a second OTA, e.g. a 4'' refractor for wide(r) field views and the odd photo. I could try and piggyback it on the Cass, but that adds even more weight, and even more counterweight. What's your thoughts about mounting a second scope on the counterweight side of the mount, so it effectively acts as a counterweight itself. Or does that result in balance hell? Anyone done something like this before? Thank you. Dan

I have signed. Security lights are daft. They use energy, and help crooks see what they are doing. I am lucky enough that my fence blocks the light show from the flats at the end of the Garden, and I spoke nicely with my neighbours and managed to get theirs turned off. Our neighbours on once side had lights that were not well shielded and shone about 1,000,000 candela straight into our bedroom window! They were pretty good about it when that was pointed out. If you see the effects of the lights in your bedroom, you have a stronger case I think. Otherwise, you'll have to make your own light shield if possible.

No problems there. Its an ONSTEP system so I can change the steps per degree to account for any changes to the worm drive.

I have a very nice old mount. It does struggle a bit with guiding even small scopes. I also want to use it with a really heavy telescope, est 35kg worth of OTA. I have tried that telescope on the mount before and it works, but it didn't last too long before the cork gaskets were split. I changed those to PTFE gaskets, and these work better. Trouble is, I can't find suitably sized needle thrust washer bearings. The mount shafts are 1.5inch on Dec and 2 inch on RA. One of the RA bearings is 2inch shaft and a 7 inch surface. I have found these: Search Results: needlethrustset - Simply Bearings Ltd These are not very wide though, so although they would fit around the 2 inch shaft, they'd only use 3/4 inch of the surfaces. So do you think changing the PTFE gaskets on this mount would benefit me. There are 3 gaskets on each axis, one each sise of the work wheel, and one at the base. Should I change all of them, or would changing just one be enough to benefit from reduced sticktion, even if the weight was still tricky. I'd also like to change the worm wheels. The ones on there are 4.5'' and 7 inch for RA and Dec, but they use a very small worm. A larger one would likely be more precise. But I think that is an expensive job and likely one for later. Anyways, any thoughts or inputs are welcomed.# This is the mount: Dan

Love it. Its so very British! I'm thinking of all that American Space X, Blue origins, Kennedy space centre stuff, and then see this image, and think... how wonderful it is to be a plucky Brit, achieving useful stuff on a sub-1% budget. (At least next time, I'm sure they will do something useful). Go Skyrora.

I recently replaced my monitors. Got a 34'' ultrawide at 1440p. Its very good indeed, especially for astro editing. Can always make use of the extra screen real estate. 4k at that size would require a better graphics card, and that gets expensive. Specifically, this one: https://www.cclonline.com/90lm06f0-b01170-asus-tuf-gaming-vg34vql1b-curved-gaming-monitor-34-inch-3440-x-1440-uwqhd-va-panel-amd-freesync-black/?cq_src=google_ads&cq_cmp=13175109251&cq_con=125042006760&cq_term=&cq_med=pla&cq_plac=&cq_net=g&cq_pos=&cq_plt=gp&gclid=CjwKCAjw3qGYBhBSEiwAcnTRLnM3dXBpo2ihMqOcscI8VMeGvo3ibcADK1Ujlqj4T_x6KLkUN6K-ehoCOkMQAvD_BwE Although of course, I spend too much spare change on Astro stuff... so got mine second hand for a lot less than that. Defo consider an ultrawide if you have space though.... highly recommended.

Agree with the above. I had a LX200 12'' for quite a while, and had a lot of fun with it. Cooldown took ages though, even when stored in an unheated garage. At least 90 mins. Which meant most of the time I was using the thing unstabilised. I also felt I spent half the time collimating, and the other half wondering if the collimation was out. In truth, I was probably trying to collimate before the scope was properly cooled. But kids, work, and weather conspire to keep my observing between about 20:00 and 22:30. Barely enough time to cool it down. Sold it a while back. Do I miss it? Not really. I think the scope was a lot better than my skills, and at a different stage of my observing life, I'm sure we'd have gotten on a lot better. Would like to try a C9.25 one day. Would also like a decent refractor one day. But that's for the future. I'm presently messing around with a classical cass and a DK that I somehow ended up acquiring with more-or-less wise auction site bidding. First impressions are that they are sharper than the SCT, and easier to collimate, but they are also both open tube, and so cooldown is quick.

Yes, indeed it is a Rob Miller mount. It doesn't have clutches as such, rather a gasket between the metal surfaces that allows the turning. When I got it had milk bottle plastic as the gasket, which probably works quite well, but I replaced that with cork. The cork has been fine until now, but doesn't seem to have enjoyed the pressures of the added weight of this scope, and has squeezed itself out one side of the gap! I think I'll swap the cork out for PTFE. Only question is... will I do a single 1mm thick PTFE sheet between the surfaces, or two 0.5mm thick sheets. Not really sure which option would work best. Probably both would work just fine. The focus position changed a lot! I think about 20 inches. Physics has been my friend this time. For the few minutes I had at the eyepiece last night all looked well with a star. I probably won't go to the lengths of Ronchi testing... unless something looks wrong. First tests I do will probably be looking at Saturn and seeing how happy I am with it! Then I might take a photo of a starfield and see how flat (or not) the frame is. I also have some hope that a Meade 0.63x focal reducer flattener would work OK at this FL, so I'll test that too. By then I'll have a decent idea of how much the spacing change has messed with the correction.

Just to close this thread out.... over the last weekend, I modified the secondary holder to allow primary - secondary spacing adjustment, and be collimatable. I had to drill two extra small holes to mount a bracket on the back of the holder, but these would be completely hidden if the scope was returned to original condition. I also had to tap an M5 thread into 3 existing holes to allow the collimation screws to fit. The collimation screws have pointed tips and engage with existing holes in the secondary holder. The centre screw is held by a top nut and has a spring to tension. The image attached makes it look a bit scruffier than it really is! As I completed this work, the clouds turned up, and I couldn't test it. Last night looked possible at about 19:30, so I got it all set up and the clouds came again. By 21:00 I was all but ready to strike the evening off as a failure. I went inside, came out again 15 mins later and a large hole in the cloud revealed Arcturus, a really good bright test star. And it all worked beautifully. After many hours faffing around... I had it centred and focussed inside a few minutes. I had to lower the secondary just slightly to allow focus. The gap to the primary has been shortened by 27mm, and this allows focus using a 2'' diagonal in the big focuser I installed. Very happy indeed with the results, and now I know it works, I might refine and neaten it all a bit. Next job, is to replace the cork gaskets in the mount that haven't liked the weight of the thing... over to PTFE I think. The job list doesn't end! Thanks very much for all the support and ideas. Dan

Ooops. Farted.

I seem to remember getting ready to go to work at WH Smiths in St Austell (Cornwall). I remember the hype around it all and the dodgy little cardboard glasses with black film. I also remember standing in the garden, in my uniform and it being cloudy. Saw the secondary effects, shrugged the shoulders and went to work.

I agree with that. Actually, they are slightly smaller than M4, not M3 as I said earlier. So I think they are probably 4BA screws. There are some on ebay at 13/4 in length that should do the job.

The secondary is 90mm diameter and measures 17.5mm at the edge and 19.7mm in the middle. The secondary holder is comprised of two parts screwed together, one bracket part that fixes the assembly to the main tube, and a second part that holds the mirror. It will be really pretty easy to put a spacer in between these. With the holes that are already present I might even be able to fit three screws with springs over, so it is collimatable. The screws are some imperial thread that is very close but a bit smaller than M3. I may just retap these to M3 so I can use screws I have. Happy that it looks fairly achievable.

You know, I think the massive hole is a weight saving exercise. Since the secondary is 90mm, and with the metal around, it would shadow that area of the primary, so why not cut it out. The weight saving is important for space flight! I'll measure the secondary when I get home this evening.

This thread suggests that the spacing causes an effect of the, secondary magnification squared, on the focal plane. https://www.cloudynights.com/topic/73676-classical-cassegrain-spacing-question/ As I think my secondary has a magnification of 4.6 based on the design docs here: https://articles.adsabs.harvard.edu//full/1971IAUS...41..304B/0000305.000.html I would only need to move it an inch, to get 20 inches of back focus shift. On the design the focal plane is just at the very back of the primary. I do wonder if a half inch secondary spacer went missing somewhere in its long life. The design also shows the huge hole in the primary. I will see if I can add a simple spacer and let you know how I get on. Thanks for all the support.

I get the idea I think. Would it be two equal lenses placed back to back with the extension amount between them? One lens converts the converging rays to parallel, and the second reverts them to converging... if that makes sense. Can you buy such things... I imagine they'd need to be pretty customised to the cone angle of the light from the secondary? I think, from the post above with the link to the Cassegrain notes, and similar pages, only a relatively small shift in the secondary will have a fairly substantial effect in the position of the focal plane. Gregorian focus is an interesting idea. I have always thought I'd never grind a mirror (I make myself enough work with odd telescopes and mounts to engineer). Perhaps I can find one already ground and silvered to buy and test. Newt configuration would also work fairly well although I'd have to deface that nice titanium tube to make that happen, and I usually try and keep my shonky amateur engineering to a minimum. I can move the secondary forward easily enough with a spacer though. Ideally I'd make it adjustable if possible to find the best spot. Might be able to manage that with a single threaded hole.

Thanks for the response. More details on the telescope here: But I haven't posted all the dimensions. The hole in the primary is huge. I'd estimate about 6 inches, which is a slightly odd feature. The secondary seems oversized too, at about 90mm. I think it'll all be about experimentation and trying to move the secondary forward. Thanks for the mount comment. It is an amazing mount, and another slightly risky auction site gamble. Turns out it is a late Rob Miller prototype from end of the 80's. More details on that one here if you're interested: