Piero

I have a rigel finder and an Antares VS-60 RACI finderscope on my 16" f4 Newtonian. Whether or not using a magnifying finder depends on the darkness of your sky and well as your observing style. In my case, I generally keep my (12.5mm) medium power eyepiece in the PC2 when star hopping. So the latter is done at the eyepiece of the finderscope first, but then at the eyepiece directly. After observing at medium power, I go for medium high. My low power eyepiece is used almost only for large DSOs, like extended nebulae. The large exit pupil plays a role in those targets too. If you get a finderscope, I would suggest one which allows you to change eyepieces. Personally I find that the default eyepiece delivers a too large exit pupil. They are also not particularly well corrected. My long term plan is to use something like a 16mm 70-80 deg eyepiece with my VS-60. That would give almost 15x, a darker sky due to the lower exit pupil and still sufficient FOV.

As long as you can move it around easily, the larger aperture will show you the same objects brighter at the same magnification. If built decently, it will also offer more resolution. I would also go for it. 🙂

I have noticed that as well. Also the classified area seems overcrowded by adverts regarding imaging equipment, far more than what I remember. Personally, although I worked as a scientist for most of the last decade, regarding astronomy I look for the observing experience and the surprise of seeing what something much much bigger than us looks like. I largely reduced my posts due to lack of time.. very very busy at work and lots of telescope making in my free time. The latter is complete now, so more time for galaxies and planetary nebulae. 🙂

I recently received a 9mm and it performs very well in my TV-60 (which is an F6). It does show some marginal chromatic aberration with the brightest stars when viewed close to the edge, but this doesn't bother me much. On axis and most of the field, the eyepiece is quite sharp, certainly on par with the naglers and possibly a tad more. It performs well in my F4 dobson + PC2 on axis. It just delivers a slightly but noticeable less vibrant image than the docter (which is a legendary eyepiece though). According to the data provided by Ernest on CN, a few Morpheus are in line or better than Pentax XW, but slightly behind than Delos. I don't own either of these two eyepiece types, but would be very interested in knowing how they compare in a fast telescope.. Last night I used the Morpheus on a few targets, including Jupiter and Saturn, with my dob. I honestly think that the vast majority of users would be very pleased with the views it delivers.

I don't know what kind of eyepieces you will use with this telescope, but as far as I can see I think you are constraining yourself even with a 75mm. If I remember correctly, the baader diamond focuser is quite tall (63mm?) And you will need some back focus (1/2" at least) anyway. In my opinion the main issue with a small-ish secondary is not really the fully illuminated field (FIF), but the fact that one adds a lot of constraints to the UTA design and collimation. And if all does not work very precisely, there is a chance that the secondary mirror does not reflect the whole primary mirror, which is like having a smaller primary. All these constraints can be relaxed with a slightly larger secondary. In your case, a 75mm secondary gives an obstruction of 18%. With a 82mm secondary, you get 20% obstruction. I really doubt you will notice any difference, but it will make a substantial difference in the design. It will also give you a bit more than ~7mm FIF. Generally, one should aim for 1/2" ~ 12.7mm. Also, in one of the images you posted, it says that you plan to have tube thickness of 3mm. That adds a lot of weight but not much stiffness. What really increases stiffness is the tube diameter, not much the tube thickness. Obsession telescopes use 1.2mm thickness (6061 alu). In the UK, I haven't found aluminium tubes 6082T of 1.2mm thickness, but you can certainly find tubes of 1.6mm thickness. The poles of my 16" f4 are 1.125" diam and 1.6mm wall thick. The struts in the UTA are 1" diameter and 1.6mm wall thickness. Both 6082T aluminium (structural aluminium). Looking at the design of your telescope, it seems to me that you will have longer poles. I would use 8 poles of 1.25" diameter and 1.6mm wall thickness. A "+" orientation of the UTA spider / struts is stiffer than a "x". This because the poles of the telescope directly supports the UTA 4 struts, which directly support the secondary mirror. Here the rings only keep the UTA together, but don't really support extra weight apart from the focuser and finder boards. In the "x" design, which derives from the Obsession telescope design, the secondary mirror and spider are supported by the rings, and that can affect collimation at different altitudes (to minimise this, Obsession telescopes uses obsessively undersized secondary mirrors and UTA ID). The "+" orientation also allows you to place the focuser at a certain angle (e.g. 15-20 deg), rather than 0 deg. The pole location slot in your design frees the pole when this is moved towards the edge of the mirror box. However a couple of poles form a triangle, so the weight of the UTA exerts a force in that direction. Having a pole location slot freeing the pole when this is moved inward would be safer, just in case that the hand nut loosens. Said this, for the lower truss attachments of my dob I used a positive lock.

I would also make sure that those two telescopes can pass through the doors in your house. The base of both the Meade and SkyWatcher dobsons is quite large (more than necessary really..).

Increasing the size of the pads will give you more friction, not less. Do you have a fourth pad where the pivot bolt is? A quick and cheap solution is to cut an area from a milk plastic bottle and add it to the fourth pad located at the centre. If one is insufficient, you could add another one. Milk plastic bottles can have many applications in ATM, as the plastic is soft, but strong enough. Another application is to improve the secondary mirror collimation by adding one layer between the plate of the secondary mirror holder and the collimation knobs. These two mods are by JasonD on cloudynights. Another application is the structure connecting the triangles in a mirror cell.

I would: - get a decent collimation tool (Cheshire/sight tube is fine). - an atlas to get acquainted with the constellations and location of the bight DSOs (e.g. Messier objects). - consider 3 eyepieces with focal lengths of 24/25mm, 10mm, and 5/6mm assuming the 10" you are considering is something like F4.7 . The 25mm which comes with the telescope works quite well. - skip filters for now.

And yes, if you want aperture, to me, a dobson is the way to go.

My advice is that an adjustable chair improves comfort, and therefore the observing experience, in any small to medium size dobson. That's for dobsons with focal length between about 1 and 2m. After that, a ladder is generally needed. The uncomfortable alternative is that you lie on your knees and / or bend your back and neck to reach the right altitude.. With my 12" F6, I use a three steps ladder: 1st step is when standing when the telescope points at the zenith, whereas the other two steps are for sitting when the telescope points at low altitudes. In between, I stand. With my 16" F4, I stand when pointing at high altitudes and sit on the three steps ladder otherwise. All of this is going to change after completing my adjustable chair which is based on a Catsperch chair style. With this I will observe sitting all the time. If you have some basic skills, you can build your own. Look up: "Denver observing chair". Berlebach also makes some adjustable chairs, but they seem quite flexible to me... There are also some metal chairs like the Starbound, and its US and EU clones. Some observers use chairs for ironing clothes.. There is a quite large range of options.

Aha, no problem! 🙂 After reading on CN that kydex can deform if exposed to modest heat, I decided to go for what people (including some professional telescope makers) use nowadays over there: ABS. Mine was bought here: https://www.plasticstockist.com/ "2500x1250x1mm Black Smooth ABS in Matt finish" As you can imagine, I still have a large part of this sheet, enough for a few other telescopes or a giant dobson... feel free to PM me if you want some. The matt finish is on one side. I decided to use that side for the outside of the UTA and the reflective side for the inside wall. This because my plan was to use black flocking paper on the whole internal wall. This added some weight, but it wasn't a concern for me, due to the large bearing diameter (28") which allowed me to raise the horizontal COG, while still keeping the mirror box reasonably shallow. One of these days I will attach the astrosystems dew heater to the secondary mirror, completing the UTA. 🙂

Kaldex? Not sure what you mean. Kydex maybe?

Really beautiful project!

I just realised that your media is actually a video and not a photo. It seems to me that your telescope has insufficient azimuth friction. When assembled, a modest breeze could be sufficient to rotate it horizontally. Of course, it is down to one's tastes, but I'm not sure that is practical..

My dobson has 4 square teflon pads sliding under a sheet of FRP. The 4th teflon really helps (see pivot bolt in the photo). Each teflon was sanded at the edge. The three teflon pads above the feet have a vertex pointed towards the central pivot. This also reduces friction. Friction can also be reduced by reducing the size of the pads as I'm sure you already know. It might also be worth trying to slide a solid soap bar on the laminate where the pads slide. This works well for reducing friction on the altitude bearings.

That's linear obstruction, which does affect contrast, but not via light scattering caused by dirt on the optics. The reduction in light grasp reduction is minimal, as this depends on the quadratic (non linear) obstruction. Said this, linear obstruction is something to consider, but, to me, not something to get obsessed about. Reducing linear obstruction from 20-21% to 16-17% has a very limited gain on the views - assuming it is implemented and maintained correctly -, whereas it adds a lot of technical constraints. These constraints not only can make your life more difficult, but can also damage the views. With this, I don't mean we should favour large central obstructions. Not at all. What I mean is that choosing the smallest possible or obsessing with this parameter, has more disadvantages than advantages. Examples of constraints due to (obsessed) minimisation of the secondary mirror (NOTE: these constraints are not just at design time.. they live during the whole lifetime of the telescope..): - focuser must be perfectly squared. - focuser axial alignment must be perfect, otherwise not all the primary mirror area is reflected, effectively acting like an aperture stop (=> smaller primary mirror..!). This means that there is no shift in the focuser axial alignment over any altitude or in the drawtube (hard to believe this is the case... e.g. Moonlight focusers do have an inevitable shift in the drawtube as the stainless steel bearings dig into the aluminium drawtube...). - the whole OTA must be top notch and retain collimation perfectly at different altitudes, otherwise, a minor shift in orientation in the primary mirror will cause this not to be in the exact centre of the secondary mirror, which again can cause a loss in aperture. - the edge of the secondary mirror is also used for reflecting the light from the primary. The edge is generally the worst part of the secondary mirror. It is generally "trimmed" in interferometer tests for this reason. It is also the part of the mirror touching the holder if this is a "container" holder like astrosystems.. This means that it can also be more sensitive to thermal differentials. - the focal length can be described as the sum of (a) Primary-to-Secondary and (b) Secondary-to-FocalPlane. An undersized secondary constraints the measures for (a) and (b), significantly - an undersized secondary constrains errors in UTA diameter, the length the focuser drawtube can be ranked out / in (which affects eyepieces/PC2..) - an undersized secondary reduces the FIF (fully illuminated field) diameter for large / medium field stop eyepieces (basically those used in low / medium power deep sky observing..). This is another parameter people often obsess about, but it is actually far less critical than all the constraints above. The UTA is all about compromises. Because of this, having some degree of freedom is much better than constraining yourself and being forced to accept side effects. Not all optimisations are golden. To me, undersizing the secondary mirror is an example of these. Just my two cents.

Exactly. More light scattering means faint stars washed out in the back background sky, which becomes brighter, and a general reduction in contrast. Here's a direct example: take your sight (or sun) glasses off and intentionally add dust on the lens surfaces. Then put them on and look around. Then blow the dust off with your breath and put your glasses on again. I experienced that so often, when I was making Nunki, my 16" dob, and was covered with wood dust... 🙄

I think one should clean it when it is clear that the mirror needs a wash. This depends on how much time the telescope has been used. Hard to say how often. The problem is that there seems to be a phobia associated to scratching it. It's just about learning a good way and be careful during the process. Said this, a large amount of dust / dirt is certainly worse than a tiny scratch. Both cause light scattering = noise, not visual aberrations or distortions.

It's kind of shocking how much that school of thought is widespread.. I won't mention the location, but some years ago I had a chance to observe through a - what I believe was - 20" F4 Newtonian on an equatorial mount in an observatory. On the primary mirror the was a large (easily 4" diameter) bird squirt - possibly dropped by a seagull. Aside from that, the whole mirror was covered by plenty of dust and dirt. By the look of it, who knows when was the last time it was washed, if ever. A few guests pointed that out, and guess what, the person responsible for the observatory simply replied that, that dirt did not affect the views. We observed Saturn that time and the view was just a bit better than my prehistoric Celestron 114mm F8 Newtonian telescope (the one mounted on that kind of hyper shaky German "EQ 0" mount).

Agreed. And light scattering is pretty much noise in the optical train. Effect? Star light spreads on a larger area, and faint stars can even become invisible as squashed with the background. In addition to that "structural" noise, there is the actual noise due to dust, each particle acting like a small scratch.....

I also used a large magnet at the bottom of the tube, opposite to the focuser. No balance issues even when I the ES 30mm 82 deg was in the focuser. 10" is a nice aperture. Televue and Pentax XW?

Your comparison between a 10" and 10 m is unrealistic.. Cite one person in the UK having a 10m telescope.. It is hard to find one owning a 1m! I think both quality and aperture are important. However, if the two instruments are on par in terms of quality (as I said before, not just optics, but the whole thing), the larger is the better. This is said in a realistic context though. I don't know what the upper bound is, but I certainly disagree that 10" is the limit and beyond that, targets are just brighter. Seeing, transparency, and darkness are other critical factors to be taken into account. Some locations are better than others, but there are a few decent places in the UK where one can take serious advantage of aperture. As far as I can see amateur astronomers owning medium to large dobsons (15-30" aperture) are people who are not interested in more resolution.. They don't get those telescopes for observing Jupiter, the Moon, or double stars... They get them for catching SMALL FAINT planetaries and galaxies. These targets require DARK SKIES and EXPERIENCE. An 8" refractor, even with the most exotic and best polished optics, is still an 8" aperture. Nice all-around, despite the cost reaching the sky, but a few classes of targets are out of its range due to its limited aperture. My bottom line.. telescopes with different apertures have different use cases. Understanding the different use cases, your own interests, and your life context is a good starting point when choosing a telescope.

Yeah, those are not for wide-field.. those are for catching small faint galaxies and planetaries with your feet on the ground or at most on a step. Storage is another benefit. Something like that could be stored in a garage. The same aperture but f4-5 requires a dedicated house! An F4 mirror is easier to make, but not necessarily better. Achieving an excellent smoothness is determined by the process the mirror maker has in place. You can have a terrific F3 and a so and so F5 for the same aperture. Of course, the former is going to cost more. Both f-ratio benefit from a coma corrector. I would not do that. Attaching the fans to metal brackets will cause vibrations. This is certainly not wanted while observing. In addition, fans for defeating the boundary layer should be off while observing. If on, the chance is that they will cause a lot of air turbulence. I would find a way to temporarily (=e.g. consider velcro strips) mount 2 fans to the front panel of your mirror box (the one which is shorter). I would place them so that their point at the mirror surface, but not the mirror edge. Warm air moves up, not down, so those locations are sufficient. Then you turn them on for a few minutes, then off and observe. It should last for probably 45m / 1h . Then you give another blow for a few minutes while you have a break from the telescope. In addition to that, I would raise the light shroud up for about 10-12 cm from the bottom. Not sure how much distance your telescope has between the top edge of the mirror box and the top mirror surface. It might be that raising the light shroud up is sufficient for blowing the air boundary layer away. That is not really a laminar flow, it is the fact that the air boundary layer cannot form (which to me seems an even better solution).

Have a look at the section "Tools for observation" on this website: http://takitoshimi.starfree.jp/index.htm Toshimi Taki's free star atlases are really good in my opinion.

Absolutely, health is first. I too have sore back time to time, that's way the whole use case was designed so that there was no need to dismantle Nunki (the telescope). Of course, if needed, it can be dismantled anyway.