Piero

Yes the hair dryer would temporarily break the boundary layer of warmer air above the primary mirror. It should be used in "cold" mode, so it would act like a fan, which moves air but does not warm it. What Magnus described: "nip back to the eyepiece to watch the image get utterly destroyed. Then keep looking, as it gradually settles and clears to reveal startling clarity." is correct. The image is looks very bad first, due to the created tube current, but few minutes later it sharpens considerably. I don't know how long this would last though. Maybe it lasts for 1/2h or 1h. Mine works because the mirror box is low enough that the air coming into the telescope from the gap mentioned previously does not allow this boundary layer to (re-)form. Note that this does not work in the same way with ultra-light dobsonians like Sumerians because they don't have a mirror box. Therefore, pulling up the light shroud, will increase stray light and potentially dew formation on the mirror surface as this would be completely exposed. Another approach is the installation of lateral fans. See Teeters for truss dobsonians. For solid tube, the principle shown with this solution works too: https://skyandtelescope.org/astronomy-equipment/beating-the-seeing/ One thing I learnt from my 12" Lukehurst dobson is that this airflow for tackling the boundary layer must be directed to the mirror top surface, not the mirror edge. An unbalanced airflow directed to one side of the mirror edge causes astigmatism and spherical aberration due to temperature differentials. Here is an example. The opening (oblique cut) at the bottom/front of the mirror box in truss dobsonian was added by David Kriege (Obsession telescopes) to allow one to decrease the overall height of a dobsonian telescope for a few inches. This cut goes from the bottom edge of the mirror box to almost the back of the mirror. Lukehurst clearly interpreted this in a very different way (which is odd as Kriege is very clear about this feature in his book). In particular, he uses that gap as a way to slide off the mirror cell (with the mirror on top), discarding the principle of reducing the height of the telescope, completely. Whilst this seems attractive to him, and I believe some people, it has an important disadvantage. To allow this, the cut is made much deeper and it goes a bit beyond the top of the mirror, exposing the whole bottom edge of the mirror. Doing so, unless the telescope is pointed at the zenith, the outside cold air reaches that edge of the mirror and impairs the temperature of it. It does not have any effect on the boundary layer because the airflow does not reach the mirror surface effectively. Thankfully, this problem is fixable. One needs to close that opening (e.g. with a light plywood panel attached with some heavy-duty velcro). Doing so, the mirror box becomes similar to the design of John Dobson, except for the fan installed at the back of the MB. My point is that experimenting is a great thing because there are many things which can be optimised and some of these can give surprising results. Said this, one should at least try to understand existing solutions, rather than just applying them blindly. Anyway, in line with John Dobson's thought, in telescope making errors can be fixed (with some extra time and dedication). Compare MB opening between these two:

Nice report, Magnus @RobertI I just searched for `Colchester` on Google. It seems a really beautiful city with a lot of historical architecture. I will plan a visit, maybe next year. Anyway, coming back to your question, an increase in aperture allows you to: collect more light and use higher magnifications keeping the same exit pupil. The latter benefit is critical. Small apertures a great for observing large targets, large apertures are great for observing small targets. .. and there are a lot of these small targets in the sky. In general, any object benefits from additional aperture. Objects are not all the same though. For instance, small galaxies and planetaries can simply be stellar (or invisible) at low/medium magnification (even under dark skies), whereas they pop up at medium/high magnification. The "companions" of NGC 7331 are an example. They are not really companions, but about ~300 mly away from us. I spotted 3 of them using at least 320x. Below that, I was unable to determine whether they were actual galaxies or not. To use high magnifications, seeing becomes more and more critical, but I find that this threshold is higher than the 200-250x that most people repeatedly state. A larger aperture is more susceptible to a variety of things though. For instance: a large mirror is much heavier than the lenses installed on a refractor. Therefore, adequate bottom and edge support become more critical a large mirror is generally thicker. Therefore, cooling time is longer. This is not just the cooling time from indoors to outdoors. It is also the cooling time occurring during the night. The boundary layer of warm air 1 inch above the mirror surface (often looked at as atmospheric seeing) is related to this too. There is no free cake here.. a thin mirror improves cooling time, but is picky on mirror support and viceversa. larger and therefore heavier mirrors require more solid supports and mechanics in general otherwise collimation will be affected (a large bucket for many issues here) faster optics require a coma corrector, otherwise the comatic blur can make the already faint object, invisible. The lack of adequate mechanics / cooling can cause astigmatism, but also spherical aberration. Although annoying, astigmatism is easy to analyse and determine the source. Spherical aberration is trickier because it can be caused by a variety of things and some spherical aberration, although minimal, is also present in the mirror optical figure. If you want to use high power with a large dobson, you need to take care of the points above, otherwise you limit yourself to use your dobsons within the range of magnifications typically used with refractors. By taking care of the issues above, I noticed that I can use my 16" with magnifications up to 400-450x quite frequently, and a few times even more. Dealing with the boundary layer alone allowed me to increase the mags for about 150x from the previous "seeing limit". I have tested this many many times now and the result is consistent. When the light shroud is fully down, the telescope performs like a solid tube and the boundary layer of air lays above the mirror. When I pull the light shroud 3" up from the mirror box, the incoming air flow is sufficient for dissipating the boundary layer of warm air above the mirror, without hitting the side of the mirror as this is still protected by the mirror box. The effect on stars and planets at 270x-320x is just striking. Bright stars shrink in size becoming dots. Planets like Jupiter and Saturn becomes covered with features. Their moons become clean disks and 1-2 Saturnian faint moons pop up. It's a bit like with cars. You can drive a Ferrari in a little town and it does work, but it does not really shine. A utilitarian car does the job perfectly. Then of course, you might want the Ferrari for the pleasure of it, but that's another story.

It was great to meet you too, Iain. Hope you will have a chance to take your telescope to dark skies soon, maybe this autumn. Clear skies

Fair enough. Good to read that for the vast majority of members, the lack of skills is not an actual blocker. Those kind of skills are always handy to have anyway, particularly after retirement with 50-60 extra hours per week. 🙂 Not sure, I follow the cost argument, considering the equipment of some members posting in here, but if others see some reasonable maths in there, that's excellent!

What is blocking you from making your own dream dobsonian?

In my opinion, we, amateur astronomers, should start considering making our own dobsonians, in the same way as American amateur astronomers have been making heir own for a number of years. I don't want to hijack this thread, raising different questions. So I created a new one:

This is how I take my 16" to my garden. It's about 20 meters each way and the telescope remains assembled. Assembling the telescope involves attaching the trusses, the UTA on the trusses, and put on the light shroud. That's about 5 minutes. Collimation with HG laser and 1/4” white Catseye triangle (which helps indicate which bolt to turn) takes less than 5 minutes, whereas day or night time. Assembling those ramps takes another 5 minutes (the actual difference from solid tubes). Lifting the telescope via wheelbarrow handles is like lifting about 6-7kg.

Will you install the spider directly to the thin vertical plywood panel or will you install 4 struts for holding it? If the latter, it was probably easier to recess the strut base on the rings to lock their alignment and stiffen the structure: two star nuts per strut and bolts passing through the rings. Then, the thin plywood sheets could be glued or nailed to the internal wall of the two rings. From the photo, the wood of the top ring seems different from the one in the bottom ring.

I don't see where the problem with checking collimation before observing is. It takes a fraction of time and it guarantees that the telescope optics are aligned. For me that's standard procedure. Said this, if the trusses are numbered, the telescope assembling will only require a small collimation touch at most. If one is concerned with preserving collimation while using the telescope, well then that person could opt for a truss dobsonian that is sturdy. Ultraportable truss dobsonians, like nearly all the European ones, are optimised for reduced weight and compactness, not for sturdiness. One can optimise a truss dobsonian for sturdiness. This will be less compact, but the weight of each component will still make it portable with minimal or no flexure. Also, collimation can be affected by other aspects which go beyond trusses. Primary mirror cell, secondary support, focuser are also very important. For these, a solid tube doesn't offer any advantage. Actually, the very small clearance between mirror and tube wall is more a disadvantage, particularly in larger apertures.

To me a solid tube makes sense up to 8" - 10" with f.l. of 1200mm. These sizes fit the back seat of a car. With a focal length longer than 1200mm, I don't see any advantage in using solid tubes, whereas I do see many advantages in a truss design. I am aware that this comment will be confronted, given the fact that here in the UK people seem to prefer solid tubes no matter what. I wonder whether this preference is primarily due to the design of the UK Orion optics VX solid tube dobson, and people willing to stick to UK brand / choice.

A bit late to this long thread. For what is worth, I use two sets with my FC-100DF: (1) combo: 30 APM UFF, Zeiss zoom +/- modded VIP Barlow (just 2" nosepiece), OR (2) 24 Pan, 9 nagler, 5 ortho, 3.4-2.0 vixen hr. I tend to use: (1) for DSO (2) for double stars For planets, solar, lunar observing, I use the Zeiss zoom + VIP (2" nosepiece and push fix adapter). The zoom + VIP is tall, but once the telescope is balanced I don't have issues with it. I use a wrist weight on the dovetail to counteract the torque when pointing at high altitudes. I've used the 24 Pan + N9 on DSO successfully too. Other times, I just leave the ZZ without VIP for the whole session.

There are different ways to make dobsonian telescopes, but in general, one does not need a lot of skills to make a good one. My 16" f4 was my first wood / metal project. I literally never drilled before, and my only tools were / are: drill (+gator drill guides), router (both bases), jigsaw, and sander. The whole project can be successful with: patience, careful research / study, attention to details.

The triangle to the left is misaligned. Not sure why, but it looks like those triangles can rotate freely. Whilst they should float freely, they should not rotate, otherwise this will unbalance the mirror back support causing astigmatism and spherical aberration due to changes in the mirror figure. I don't really know why manufacturers / telescope makers still opt for a 9-point over a 6-point mirror cell. The additional RMS error of the 6-point mirror cell is completely negligible vs the 9-point and the 6-point mirror cell is considerably easier to make as it does not involve any triangle (easier => less chance to get it wrong => more consistent support...). Anyway, the proper fix to the problem above would be to create a flexible ring structure connecting the triangles so that these can still move but not rotate. Whatever solution you go, PLEASE don't screw the triangle to any support in order to prevent their rotation. That non-solution - unfortunately used by some professional telescope makers.. - introduces a hell of issues as triangles won't be able to float freely causing the mirror figure to deform due to the mirror own weight. Rather than doing that, pay a quick look before using the telescope and align them manually if needed. Piero

I don't have any "handle" attached to my dob, as it is moved with wheelbarrow handles 🙂

The adjustment I described previously will move the focus plane slightly inward, meaning that the focuser drawtube shifts towards the tube, not the other way around. I don't think the drawtube will protrude into the tube though. This adjustment is minor - I doubt you will be able to move it more than 6mm or 1/4” inward - but the gained shift could be sufficient to see the whole primary. I would just try it 🙂

The bottom bolt is for locking the mirror in position. The top bolt is for collimating the primary mirror. If the spring is decent, the lock bolt is not required. Anyway, if you loosen the lock bolt, you should be able to screw the collimation bolt clockwise. This will move the primary mirror towards the back of the tube and compress the spring. As a consequence the distance P between the two mirrors will widen, making the reflection of the primary in the secondary smaller - or the secondary mirror to be larger. In addition this will slightly move the focal plane inward towards the secondary mirror (L distance). Decreasing the L distance will make the secondary mirror to appear larger. By decreasing the L distance you also increase the FIF (fully illuminated field) a bit. This adjustment can be applied until the focuser drawtube doesn't enter in the telescope tube using your eyepiece with the most inward focus travel. That's why low profile focusers exist. I would not worry about this with your Skywatcher telescope. I'd suspect that if you move the primary mirror cell few millimeters towards the end of the tube, you should be able to see the clips and the edge of your primary from the secondary.

I would run a star test on Polaris using 150-200x, first. F11 is quite forgiving.

Is your primary mirror pushed all the way in by the collimation screws / knobs or is it at about half way of the travel? If it is all the way in, there are two potential downsides: 1) the primary mirror touches the bottom of the 3 clips - which can cause astigmatism, 2) not all the primary mirror is reflected by the secondary mirror - which is what you described. Also, have you offset your secondary, in particular moved it forward to the primary? Regarding your questions, the WHOLE primary mirror surface is supposed to be reflected by the secondary. Ideally, there should also be an additional margin. If not, the telescope is operating with an aperture stop. For instance, if the clip is 1/4” inside, and you just cover them, the effective aperture is not more than 7.5". This does not affect contrast, but is a minimal reduction of light grasp and therefore resolution. The other issue in this context is the secondary mirror. The edge of the secondary mirror is the weakest part - this is well known in high end secondary mirrors, so much that the provided interferometer analysis of these mirrors usually discards the edge automatically. If the secondary mirror does not reflect the whole primary mirror, it means that the secondary edge is used. This can affect the views at low power. Before replacing the secondary mirror, I would check how the primary is reflected by the secondary mirror when the focuser is ranked out with your eyepieces, rather than fully in / fully out. Then, I would try to pull back the primary a little if possible. I would avoid offsetting the secondary toward the primary too. These changes are all minor. For a commercial telescope, I wouldn't upgrade the secondary, unless damaged - it is not flat or the coatings are ruined. The former shows as tube-aligned astigmatism, the latter as light loss and scattering. The former is a quite common problem. It can be due to the mirror itself or by its support (including incorrect gluing).

In my opinion, labelling an eyepiece as premium is a bit meaningless. If one evaluates an eyepiece as highly controlled regarding aberrations and distortion across the field of view down to F3, Televue wins hands down. They also offer the widest ranges on several criteria. Televue quality is also consistent over samples. Said this, I would not consider the eyecup of the Delos / delite lines as premium. The optical quality of the Docter/ Noblex is not affected at f4, but shows AMD, which is not wanted by some users. The BCO 10mm I had was fine, but not special on axis at f6 and f7.4. Off-axis was so and so.. Bad sample? Maybe, but if so, then there are inconsistencies between samples, which suggests that the standard could be improved. The Zeiss zoom I have is outstanding on axis, better than all the TV eyepieces I tried. It does show astigmatism off axis, and although this is acceptable to me at f6, I don't tolerate it at all at f4. This Baader Morpheus could be called premium at f4.5-f5, but to my eye, it is clearly behind TV eps at F4. At f10, many eyepieces are much more comparable.. In summary, to me there isn't a premium class. It depends on the telescope focal ratio and personal tastes too (e.g. regarding optical quality only - how much FOV correction is important to you).

Great report Gerry. 🙂 On my Tak, ZZ+VIP is a bit shaper than my Delos EPs and on par with the HR 3.4mm. For planets / solar / lunar observing, I prefer the combo above, as it allows me to fine tune the power for the evening. I seem to prefer the 5mm ortho and the HRs on double stars though.

I'm glad that you bought a new telescope, but I'm a bit confused when reading your first post. - First of all, I don't understand why you thought that the mirror needed to be recoated. Was this suggested by the previous owner? - if the telescope is not collimated, it isn't possible to assess how it performs. - if the coatings are damaged, the effect is more light scattering and a dimmer field of view, due to the reduction in reflectivity. - whether the coatings are damaged (erosion, many many scratches) or the surface is significantly dirty / dusty, the effect on the views is pretty much the same. Therefore, I don't understand your statement when you said that a mirror should be cleaned only when absolutely necessary.. - finally, don't you think that if your mirror hadn't been left covered with dust and particles, fungi etc, maybe, this recoat would not have been necessary? There is nothing wrong with cleaning optics, it's just maintenance really. Recoatings is also maintenance although a bit more invasive. If done properly, there is no harm.

Regarding the central spot, I use the Catseye 1/4” white triangle on my 12" and 16" reflectors. The mirror is rotated as Craig said in a previous post.

I would also flock the internal wall, at least the area opposite to the focuser. I assume the focal ratio is F5. At some point you might want to replace those Philips screws for collimating the secondary with hand knobs, so that the process becomes tool free. Regarding the fan at the back of the primary mirror, well, to me it is required for cooling the mirror faster and keeping it close to the ambient temperature. However, I'm in the group of less than 1% of SGL members thinking so..

The cell holding the lenses is collimated by the manufacturer. You should not try to adjust that as this might void your warranty. It is possible to collimate the focuser. Not all focusers are designed for easy collimation. Focuser collimation involves the adjustment of certain small screws in order to make sure that the drawtube is orthogonal to the light path. Specifically on refractors, the only focuser I collimated was the helical focuser in my TV-60 and the collimation was generally done with a Glatter laser collimator plus square attachment, although it can also be done on a star field. It generally reveals as unbalanced field curvature. When severe, it also shows unbalance field illumination and vignetting. Although focuser miscollimation can be a bit annoying, it does not affect the optical performance of a telescope, it the sense that you can still observe with it. On refractors using standard focusers, I would not generally worry it, unless a star test or general observing shows otherwise.

Quick heads-up! The seeing is incredibly steady right now over here and Jupiter looks superb with a very distinct hollow surrounding the GRS and plenty of details on the belts. The GRS is also showing some rounded features. One moon (not yet checked out - could be Io) has just started its transit. Typing while observing.. 🙂 Telescope: 4" Tak; eyepiece: Zeiss zoom 25.1-6.7mm + VIP barlow operating at about 200-220x