Piero

Mine is just a thought based on what you wrote. I see a substantial overlap between C8 and 180 Mak in terms of capability. Both require a decent mount and have a rather small FOV due to the long focal length. I would not see these two as "good all around", but actually quite specific-purpose instruments. What is your main interest: visual or imaging? Visual and imaging have very different requirements. I also see a substantial overlap between your 120 ST and a TS 152 F5.9. They are both wide field refractors, the latter a bit more powerful and bulkier than the other. The wide field refractors have very different eyepiece requirements than the C8 / 180 Mak. The SW ED 120 is in between. Therefore, it seems to me that your plan will be quite expensive in terms of telescopes and eyepieces, but it will also have redundancy, meaning that you will end up not using some instruments, I feel. Assuming that you are interested in visual astronomy, my recommendation would be an 8" or 10" dobson with about 1200mm focal length (e.g. skywatcher or bresser (the latter is slightly more expensive, but much better mechanically)), possibly coupled with a 100 ED refractor. Then, some money should be invested in a decent adjustable chair (e.g. Berlebach), Bob's collimation knobs if you get the Skywatcher dobson, and a decent collimation tool. Mastering collimation with a dobsonian telescope is feasible. It's just a matter of practice. The dobson will cover dso, planets and moon. The 100ED refractor will enable you to observe for short sessions, larger targets, planets, solar / lunar, etc. Following that you take care of the eyepieces. You might just need one set of eyepieces for both telescopes as the focal ratio is not too dissimilar, particularly if you decide to opt for an 8" dobson (generally f6) and you choose a 100ED ~F7.5.

The 30mm APM UFF is a great eyepiece optically and a joy to use in terms of ergonomics. Having said that, eyepieces work as a set not alone. You have a 20mm 100 deg eyepiece which shows slightly less FOV than the 30mm APM UFF, meaning that the major difference between the two, apart from the experience, is the exit pupil. Exploring eyepieces when you don't know what your telescopes are going to be can be an expensive exercise.. My suggestion is to spend more time figuring out what telescope you really want based on your lifestyle, targets of interest, finances etc. Then it's the time to build a set around it.

Yeah sure, an eyepiece set is something which is built up over time. A 24 Pan would also work, but 1) it is more expensive, 2) it is quite close to your 17mm Morpheus, and 3) it is a small eyepiece whereas the others are medium size, so balancing can be affected. 30-17-10-7-5 is a very functional set in many telescopes. If one day you decide to get a 10 or 12" f5 dobson, that set of eyepieces is excellent. You've got a keeper there..

Nice sets! If your focuser accepts 2" eps, I would recommend the 30mm APM UFF as a low power / wide field ep. It is as comfortable as the other eyepieces in your second photo and the views are really good. With that in set, you are pretty much done.

A very good 30mm eyepiece which is affordable and weighs like the morpheus / delos / XWs ranges is the 30mm APM UFF. It gives me clean views even at F4 with PC2. It has a very comfortable eye relief too. Of course, the FOV is smaller than your 82 deg Axiom, but it is about half the weight and so it will balance better with your refractor. Congrats on the VIP barlow. It's an excellent piece of equipment, both optically and in terms of configuration. If you have focus issues with it, here is how I modified mine: https://photos.app.goo.gl/CNhQQwLormMRftYi6

This is the current set I use with my 16" F4:

My home made 16" f4 dobson (built the chair too): Substantial redesign of my Lukehurst 12" f6 dobson:

Thanks Stu The 12" is not generally left assembled. I need to install the remaining P-clips to the poles so that they work as pairs, but I stupidly ordered 4 clips instead of 8! In the new house the 16" will live in the conservatory and the 12" in the garage, just because it will be easier to take them out from there. The refractors will have the honour of living in the living room instead. There is no plan for a larger telescope anyway (for now at least!) These refractors... always in the way!

Mm.. I believe my girlfriend DID notice my toys in the living room a while ago, but no protests thankfully! 😁 (You can see the ramps and wheelbarrow handles near the corner too). Having said that, I currently keep the refractors in their cases (see TV60 in the bookcase and tak in its red case 😇).

Generally by central obstruction, it is the linear central obstruction that people refer to. That is secondary minor axis divided by primary mirror diameter. For a choice of secondary mirror sizes, you might want to check Mel Bartel's calculator: https://www.bbastrodesigns.com/NewtDesigner.html#diagonal

For the time being I hang them like this: I never observe at the zenith so, the weights never touch the ground and always stay behind the MC (so less weight to use). I might replace the weights with a bungee cord at some point in the future. For the time being this solution works fine. 🙂

Thanks @Stu 🙂 In my opinion, the best way to sort out balancing issues is to use larger trunnions. However, depending on the telescope, one might have to compromise. Therefore there isn't a single solution here. I did consider redesigning the trunnions as the current ones are a bit small (15" diameter, whereas 18"-20" would be more appropriate even for a classic design). Having said this, larger trunnions also increase the weight.. my current ones are 440g each! How much weight do you need to add to balance the telescope?

It would be a very expensive one!

Thanks Mark, weather permitting, I will do a star test during the platinum jubilee.

Thank you! The cell was not welded by me, but by a local welder here in Cambridge. It's not a super elegant job, but it works. Good point regarding the balance. I built a spreadsheet for the COG equation and other things. The telescope was built with the idea of adding some counterweight at the base. The reason is that I prefer to have some movable weights but a lighter / more portable telescope than the other way around. As you can see, I am not going to use wheelbarrow handles here. At the horizon, the telescope needs 3kg counterweights, but I don't plan to use this telescope below 45-30 deg. My plan is to take it around for DSOs, high in the sky. For altitudes from 90 to 30 degs, only 0.5-2kg are required. I installed a kind of hook at the bottom back, which can be used to hang ankle weights or a bungee cord. For the time being, I will use detachable ankle weights. I also ordered pole clips (like in Highe's book) to pair the poles so that they align for assembling the UTA. Regarding the paint, I used Rustins BLAB1000 1L Quick Dry Blackboard Paint - Black: https://www.amazon.co.uk/dp/B001OX9YAE/ref=pe_27063361_487055811_TE_3p_dp_1 (many coats).

And finally, here is a summary of the main changes (side by side). I am pleased with these changes now. I wanted to spend as little as possible as this is not my main telescope, but the new design is considerably easier to handle. Oh, forgot to say... I could not find a more appropriate name to this re-born telescope than PHOENIX!

Here is the final telescope next to my 16". They look like too buddies who wish to go out! The red bag between the two contains my 4" Tak refractor.

TRUSSES Despite what physics laws say, there is a belief in the EU/UK that one can use small diameter long poles whilst retaining adequate stiffness. The original poles of this dobson were 7/8" SWG 16. Personally, I never felt they were adequately stiff, despite the fact that they were rather short given the depth of the original mirror box (22"). In addition to choosing slim poles, the original builder also built the tube connectors too. Guess what.. the pole inserts were made out of plastic and the pole ends connected using a non-stainless steel eyebolt which were subjected to bending due to the lack of stiffness. The pole inserts were held in place by a mini bolt. It was so mini that was not even capable of holding 3 of the 8 inserts in place. I had to replace them. Anyway, here I replaced those poles with longer 1 1/8" SWG 16 poles, like those in my 16" f4. I used star nuts and 1/4" aluminium angle brackets as pole connectors. These brackets were made by me. They are the same as those used in my 16". The lower truss attachments of the original telescope were retained instead. For the top ends, the poles were squashed. The design for these was copied from Highe's Portable Newtonian Telescope (amazing book BTW). Like in Highe's book, I cut a channel connecting the holes in the squashed plate, so that the UTA can be slotted in. That's much more comfortable than the original design. The new structure is very stiff.

Installed fan grills to the MB side holes.

VARNISH The original telescope was pseudo- lacquered. I write pseudo because certain parts were not lacquered at all. As this made me incredibly nervous, I did lacquer it (see page 10, I think). Nevertheless, I never liked lacquer. It gives a pale / dead tone. After trying CPES + clear matt polyurethane on my astro chair (see my profile pic), I decided to apply this combination to the telescope. Now the black paint.

UTA I decided to get rid of that weird focuser / finder panel and the magnifying focuser itself. Not shown here, but I will only use a Quick Rigel finder. The Moonlite CR2 will be sold at some point this summer. This was replaced with an Antares helical FOCH focuser. With this telescope I decided to use only 24 Pan and Nagler T6 eps which are very light, powerful and parfocal with each other. The original focuser panel is 18mm thick (yeah, no kidding ! the one on my 16" is 12mm thick which is the norm..). The photos below do not show this, but I actually removed a lot of wood from the back of the focuser panel to make it lighter. I also removed a couple of plies from the front. Oh, another curiosity... when playing with the UTA, I found out that the focuser holder is made of plastic (either nylon or ABS). A really unique design.......! For the time being I will keep this UTA. In the future, I might ditch it.

MIRROR AND ROCKER BOXES AGAIN One of the issues of the original structure is that it is so heavily built with wood panels that the primary mirror not only struggles to cool down properly (the fan at the back is nearly useless as too far away from the back of the mirror), but also the air boundary layer above the mirror surface can be a serious. The new cell and the shallower MB allow the mirror to cool down and remain at ambient temperature more efficiently, but they do not tackle the air boundary layer issue. Here are the changes to improve this. On the other hand I also wanted to reduce the weight of the rocker box.

Feet and ground board The original feet always reminded me of wood clogs heels.. These were shortened as all that length was not needed at all. I also added a fourth pad. I am not a great fan of this ground board to be honest. This structure is in direct contact with the ground and humidity... it's basically the place in which one should limit the use of screws.. None of those screws are stainless steel either! Anyway, for the time being I will keep it as it is. In the future, I might ditch it.

Here is the shorter MB on the shorter RB.

ROCKER BOX Now the time for the rocker box!