Piero

Interesting. That is the kind of feeling I have when looking through my Docter 12.5mm. I did not feel similarly for the Morpheus 9mm, even though I consider it a very good eyepiece. Regardless of brands, observing whilst forgetting about the optical path, is a truly beautiful experience, a connection with the universe.

When I started the construction of my dobson, I largely used the book The dobsonian telescope by Kriege (= American, imperial units for everything). I thought that finding components in imperial units in the UK was easier than finding the same in metric units. The truth: it was a serious headache. Whilst one can find some bolts, nuts, drill bits, etc in imperial units, the available choice is considerably smaller than the equivalent components in metric units. A few components are just available in metric units. During the development of my reflector I switched over to metric. For instance, my mirror cell uses 3/8-24 UNF collimation bolts, but M8 bolts for mounting the cell to the MB. The stainless steel collimation hand knob were only available with metric threads. So I had to buy them as threaded for M8, and then I had to break the thread and tap it for 3/8-24 UNF as the holes in the rungs were already tapped for this kind of bolt. Useless work. I do prefer the inch to the centimetre because the former is larger and I find it easier to see the ticks. Said this, when / if I'm going to refactor my 12" f6 Lukehurst dobson, I will use metric units from the very beginning.

I believe you are already aware that the TV PC2 also has a minor barlow effect (1.15x), so your f/5 telescope will become f/5.75 when used with the PC2. This barlow effect was added in order to minimise the additional inward travel usually required by coma correctors. Personally, I am actually happy about that minor barlow effect, but those observers interested in observing at very low power (e.g. IFN, etc) do not like it much. Anyway, just a comment.

The TeleVue paracorr2 is the best one out there. I wouldn't use my dob without it! As you already use TV eyepieces, choosing a TV coma corrector would make sense. Here are the specs with eyepiece settings: https://www.televue.com/pdf/literature/Pacacorr%20Type-2%20Instructions.pdf The ES version might also work okay, but it requires much more inward travel (if I remember correctly, this is 35mm). Given the design of the Baader, I would consider this for imaging applications, but not for visual.

Yes, those work fine. I used them too. There might be better options and I would be interested in knowing more. 🙂

Just checked my data. The fan I mounted is a Noctua NF-R8 redux-1800, High Performance Cooling Fan, 3-Pin, 1800 RPM (80mm, Grey) and was bought on Amazon. The specifications regarding the 3-pin is available online. I cut the small cable which is redundant for us and connected the two small cables to a plug. I use a Tracer 12V 7Ah LiFePO4 battery. This connects to a Lynx Astro 4 Port Dew Controller. The channels power: 1) the fan, 2) secondary mirror dew heater, 3) Rigel quick finder dew heater, and 4) a 2" dew heater which I use with the finder, eyepiece, etc whenever needed. While in use, I reduce the speed of the fan to half using the dew controller.

Well.. there are two distinct problems: cooling down the primary mirror removing the boundary layer above the mirror surface To tackle the former issue, a fan installed behind the mirror and pushing air to the back of the mirror is the most effective way. Lateral fans pushing air on the mirror surface (not the mirror edge!) can be used for tackling the latter issue. cooling down Leaving the telescope outside to cool down for 1 or 2h before observing matters but it is not sufficient. The temperature continues to drop over night. The issue is that a mirror takes more time to cool than the surrounding environment. Astigmatism and spherical overcorrection will appear as soon as this temperature discrepancy is 1-2C. A fan will help the mirror cool down faster, approaching the ambient temperature effectively. boundary layer It does not matter whether the dobson is a truss or solid tube. The boundary layer can be present in both. I believe the S&T articles mentioned by Merlin are these two: https://skyandtelescope.org/astronomy-equipment/thermal-boundary-layers-in-newtonian-reflectors/ https://skyandtelescope.org/wp-content/uploads/Adler-NewtThermal.pdf As you can see in the second one (by Adler), a solid tube is used. The mirror box of my dobson is quite shallow (11" deep). If the light shroud is lifted up 4-5 inches from the bottom, the flow of incoming air is sufficient for breaking the boundary layer and preventing its reformation. For deeper mirror boxes, the installation of fans would do the same. In my opinion, the boundary layer 1 inch above the primary mirror surface is one of the unconscious reason why people state over and over that refractors are better than reflectors and that larger apertures suffer from seeing conditions more than smaller apertures. It's also the same reason why people say that they cannot push the magnification much with their reflector (despite the fact that they say they can with their refractors). BTW, my dob is only few months old.

I use a noctua fan and found it really effective. I agree with other members that you should install it with some elastic material, or pads at least, to avoid vibrations. I used an abs plastic panel which is screwed to the back of my mirror cell. Pads were added between the panel and the rungs where the bolts are. My fan is almost always on when I observe and there is no vibration whatever mag is used.

Thank you. Yes, it's a nice experience and is not particularly difficult either if you have some basic tools. I literally had zero experience (=none, not even used a drill..) before making the telescope and chair above. Of course they could have been made better and more efficiently, but my point is that these projects are feasible with adequate patience, study, measurements, and attention to detail. Honestly, I encourage everyone to give it a go. To me these have been very rewarding experiences. 😀

Regarding the cost, my home made astro chair cost me much more than 4-5 quid, and I don't mind admitting it. Although I did have 24mm Baltic birch plywood at home, this was bought at some point by me, so it should be taken into account. Its delivery wasn't cheap either. In my case I also glued some panels with Titebond 3 glue. I had it already, but it should still be taken into account. Regarding the finishing, as it is plywood, I felt more comfortable to make sure that it was sealed properly, particularly on the areas closed to the ground. I used 1 litre of Smith's CPES. On top of that, I applied 3 coats of Ronseal ultra tough polyurethane varnish (matt finish). The bottom edges of the chair (the feet) have 5mm rubber attached, so that the finish on the feet remains in place and the plywood doesn't get water infiltration. Hinges are stainless steel heavy-duty ones. So, I'd say that the total cost of fabricating is roughly around £100-120, excluding my work on it. I'm pleased with the result and don't feel ashamed of admitting the cost figure. Here are some photos:

I use my home made observing chair, but not all the time. Possibly, it's because I work with my computer all day, so standing up in the evening / night is a kind of relief!

Some serious eyepiece collections!

That's a tested mirror, at least we know that it works as it should! Looking forward to reading whether it solves your issues. My advice is to use the provided secondary support if possible, as it would ensure that no aberration comes from the secondary. Therefore, if you see other issues (hopefully not!), you know that these come from the primary mirror cell. All the best! Piero

My aluminium ramps can be used for rolling the disassembled telescope into the boot of a car if needed. I walk on them. They fold in half. https://www.sgs-engineering.com/slr200-folding-aluminium-loading-ramps I paid £82.99 for mine. You might find the same cheaper somewhere else. Alternatively, you can use two wood bars, cut them in half, and connect the two sections with a hinge. Many options.

Among all the available chairs in the market, the ICS Astro Stuhl and the Catsperch PRO were the only ones I considered seriously. I eventually built my own chair as I had some spare Baltic birch plywood from the built of my 16" f4 dob and wanted to give it a go. That's all. So, here is mine just after finishing. It's a mix between a Denver, a Catsperch PRO, and my own tweaks.

Nope

It depends on whether 1) you like 100 deg AFOV eyepieces, 2) your observing eye suffers from astigmatism, 3) your budget, 4) you don't mind swapping the 2-to-1.25" adapter in the case that your other eyepieces have 1.25" barrel. It is also worth saying that with your telescope, the MUCH cheaper 20mm APM HDC would work very well. Without a coma corrector, I am not sure you will be able to distinguish it from an Ethos 21mm, given that any difference between these two eyepieces will be next to the field stop. Personally, I am not a fan of 100 deg AFOV eyepieces, but asked myself the question between the 24 Panoptic and 22mm Nagler T4. Although the 22mm is more immersive, 1) its field stop is only 3mm larger, 2) its weight is 3 times the 24 Pan, 3) I would retain the 24 pan anyway, so this would cost me another £300 assuming that one in the s/h market comes up, and 4) most of my observing is done at medium and high power, not at low power - the 24 Pan is a jolly because it fits in the pocket of my trousers easily - without pulling them down! - and can be inserted into the PC2 without having to change the 2-to-1.25" adapter. I only take the latter off when I go really low power using the 30mm APM UFF, but that is not frequent. Now, people are different of course. If you see yourself buying a set of Ethos eyepieces gradually, then the 21mm can make sense. If I were in the market of 100 deg AFOV eps at that focal ratio, I would add a TV paracorr 2 on top of my list, in order to really take advantage of a clean 100 deg field.

Personally, I would clean it. With patience and gentle method, no harm is done. It can be quite shocking to hear how much dirt would people accept on their mirror before considering cleaning it.. One time I was in a public observatory and the 20 f4 dobson had a large seagull poo on the primary mirror in addition to an awful lot of dust, dirt, fungi, etc. Nearly every guest noticed the status of that mirror and some, including me, asked the guy offering the tour, why that mirror was so dirt. His answer was "it does not matter, it does not affect the views". A classic example of how much people can neglect issues.

It happens to spend some time when things do not work. Yeah, it is frustrating, but it's part of the learning curve. I spent about 6 months to nail down the issues of the mirror cell of my 12" f6 - which was bought new by me 😕 . It gave me a very hard time and frustrations, particularly considering that all those issues were caused by design errors of the telescope maker. Said this, at least they are fixed now and the telescope is functional. Then, when you make your own telescope (for me, it's the one in my profile photo), you design it so that all these issues cannot be present.. 60mm or ~2.4" - I suspected correctly then! Lol! No problem, Mark.

That's very good, Mark. You tested this very well It's kind of shocking that, if you do know what / how to test, a half decent night is all what it takes. What secondary do you need? I do have a 3.1" Antares secondary + Astrosystems holder on sell in the classified section and abs, but that is for a 12" f4 - 15" f5 for visual. I have no idea what it is recommended for imaging with your telescope, but my suspect is that you need something smaller, maybe 2.4" at most.

Thanks for your reply, Mark. I have some other critical questions / checks. These look like small details but they are not at all and can have a lot of impact. Primary mirror Your mirror is supported by 9 points. Could you check that all points are supporting the mirror? I have never played with an Orion Optics mirror cell, but I would believe that the 3 isosceles triangles (1 triangle => 3 points) can float (not rotate!) a bit. Could you confirm this, please? You can run these checks by moving the mirror on its cell and check that no triangle is stuck for some reason. Secondly, from your photos, the nylon pads on the edge supports seem to touch the mirror tightly. They should not press onto the mirror, only prevent a lateral shift. Also, could you make sure that the mirror can somehow "slide" vertically and that those nylon pads do not block this movement? It is important that the mirror (when the cell is inclined) does not sag on its lateral supports and it can still move vertically along the mirror cell axis. If you feel that this movement is hard to achieve, then very slightly loosen those nylon screws mounted on the edge supports, so that the mirror still does not shift laterally, but it can have some movement. Those pads should not cause any friction. Secondary mirror I saw your other post. To be honest, I don't know whether that secondary mirror is sound. It also seems cracked at the back, as if it fell at some point. There is a chance that it is not flat. Does the holder touch the secondary laterally? Usually, when gluing the secondary to its support, three blobs of silicon are applied at about half way from the centre and the edge (in your case you glued it at the very edge because your holder has a hole at the centre. Also the blobs are not really thin. This is important because with the decrease in temperature, the glass contracts and the silicon must not warp the secondary figure. If too thin, there is also a risk that this warp occurs as the silicon dries. >>> 1. "no it’s not classical astigmatism- the stars elongate until they split into a double image on one side of focus and on the other they irregularly bloat and turn into comatic horseshoe shapes." That's not a surprise, you have a combination of aberrations, not just astigmatism. The problem is to break them down. You certainly have some astigmatism in there though, and as it has major effects, it is better to fix this one first. >>> 2: I’ve never looked through this scope 😳 I haven’t got adapters to get an ep in focus. But this “astigmatism” seems to be even across the whole frame >>> 3: I was trying to test without rcc1 last night but the clouds rolled in. I think the same thing happens but i can’t use oag live view to rack focus in and out because of severe coma and using main camera is painfully slow and tedious (30secs per frame!) ^^^ testing on axis is critical. You can discard the rest of the field as this can contain additional aberration (e.g. quality of the CC, coma, eyepiece edge correction, and so on). Testing on axis is where you need to focus. If you have collimated your telescope properly, you should not see coma on axis (although the coma-free region is quite small given the f/ratio of your telescope). Other test to run when you take the telescope out: a) all my posts here assume that the optics have cooled down properly. If not, you are going to have an additional level of astigmatism due to the mirrors cooling down. b) Record the orientation of the on-axis star elongation upon changes in telescope altitude ONLY. In particular, you need to record whether these elongations remain somehow consistent with the OTA axis or they change orientation depending on the altitude. See sketch below. This test can disentangle whether the astigmatism comes from the secondary only or the primary only. Forget the horseshoe, just test with stars defocused on the side where they become elongated. That's sufficient. c) you would really need an eyepiece for these tests.. Is there a chance you can make a temporary extension tube to reach focus? Test checking astigmatism axial orientation:

In any case, it is good to be critical, Mark. Denying a problem certainly does not help and only adds frustrations. Any mounting issue can be fixed with not too much effort / cost.

Hi @markse68 , To me what you described seems like astigmatism mainly (elongated stars), but also some spherical aberration (bloated stars). Astigmatism does not come from miscollimation of the primary mirror unless this is severely miscollimated, which is not the case here. A focuser axial misalignment causes the stars not to be on focus at the same time and / or unbalanced field illumination (for instance if the secondary tilt is very off). The photo of your secondary mirror does not look good to me. It seems like it has been cemented to its support. That alone can cause astigmatism and this astigmatism can become even more severe as the temperature drops. Being the surface not flat, the rays can reach focus at different distances and for a star, at different positions, likely causing some spherical aberration. At this point, I would focus on the astigmatism, discarding the chromatic aberrations that you saw. I have some other questions: Could you confirm that stars elongate when defocused on one side, and broadly elongate at 90 degrees when defocused on the other side? (basically they have a kind of "X" pattern) Does a bright star show astigmatism when placed on-axis, using an eyepiece? Same question, without Baader RCC1 Could you take some photos of the primary mirror cell with the mirror please? In particular, I would like to know 1) if you can say whether the mirror can rotate on it or this rotation is somehow constrained due to the 3 mirror edge supports, 2) if the clips above the 3 mirror edge supports (the ones preventing the mirror from flipping forward) somehow touch the primary mirror do you think the secondary mirror can be detached from the holder? A dental floss can be used for detaching it if it is glued using silicon, but not if it is glued with contact cement (I think).

Yes, I did. See my comment at page 7 in the thread mentioned by Zermelo.

The effect of the boundary layer is very similar to poor seeing. Stars tend to be a bit larger, with variable plumes when defocused slightly. When you try to get a better focus, you get frustrated because the image doesn't get much sharp. The more you magnify, the more you see the effect. Essentially, it is due to the light path being disturbed by a layer of heat released by the mirror as this cools down and adjusts it's temperature. Some astigmatism can be caused by the mirror changing temperature. A drop in 2C can show some astigmatism as well as spherical overcorrection. These tend to decrease as the mirror reaches the environment temperature. I suspect you experienced this. Severe astigmatism is generally caused by optics supports. It's easy to distinguish whether it is due to the secondary or the primary support and which part. All the above assumes that good eyepieces are used (e.g. TeleVue - which don't have astigmatism) and that the optics are sound and of good quality. If you are interested in these things, I would suggest you to get a copy of Suiter's star testing book. It's excellent. Although old, Sidgwick's handbook for the amateur astronomer is a great resource too. In my opinion, Sidgwick is an outstanding writer. It's a real shame that his books are out of print. TBH, I find it awkward that Patrick Moore was made Sir despite of all his rough comments, and a gentleman like Sidgwick seems just ignored. Mad world.