Piero

Congratulations! I bought my docter from a member here on SGL. That was the only time I saw that eyepiece in the second hand market! It is not easy to find a new one either. As far as I know the only European sellers are okularum.eu in Denmark and apm-telescopes in Germany. If you mostly use 2" eyepieces, I suggest to get the Baader pushfix adapter and semi permanently attach it to the docter. You won't have any focus issues with this compared to the native 2" mode of the Docter which will require outward travel. In addition you can use the docter in T2 mode, which can be handy if you mod the VIP so that this connects via T2. https://www.baader-planetarium.com/en/accessories/telescope-accessories/focusers/baader-pushfix-reducer-2"-to--1¼"-(t-2-part-15a).html

I compared the Docter 12.5mm Vs Delos 12mm and in my opinion the docter was a bit better: https://www.cloudynights.com/topic/539425-docter-and-delos/ My copy of 10mm Baader BCO was not at the level of my ZZ or Docter, suggesting that there is some variability unfortunately.

I never looked through a Zeiss zao or tak abbe. Those two eyepieces are among the very best I tried. The only one which might have a minor edge is the Vixen HR. Unfortunately the focal lengths for this line are very short. It would be great if they made a 5mm, but not sure whether scaling the design is possible. The Docter (now called Nobles) works very well with the Baader VIP barlow. I modified mine to attach via T2. I didn't know that our Zeiss zoom is now discontinued. Hopefully, its replacement will be another good eyepiece. The eyepieces I use the most (almost always) are: APM UFF 30mm, Docter, Zeiss zoom, modded Baader VIP barlow. Time to time I also use the Lunt 20mm 100deg, great eyepiece, but I am not a "ultra wide" person, and tent to use only its 70-80 deg AFOV.

This is the right one: https://www.google.com/amp/s/www.bhphotovideo.com/c/product/678909-USA/Zeiss_52_80_68_DiaScope_Vario_15_56x_20_75x_Eyepiece.html/amp This is a photo of mine with the Baader adapter and my modded Baader VIP: It gave me some cracking views with my Tak and recently, after telescope mods, in my dob 12". I still remember how it opened up M42 and its trapezium in my dob 8"... Here the VIP with my docter plus Baader pushfix adapter: Time ago, I asked Gerry which of the two he preferred and he said "both!" I cannot agree more. Both are keepers in my eyepiece case.

No it isn't. The one Gerry meant is the 20-75x one, which goes from 25.1mm to 6.7mm focal lengths. Super eyepiece! You will need a Baader adapter and some minor DY work to use it with your telescope without having focus issues.

My favourite one is the docter 12.5mm with Baader push fix adapter to make it 2", nearly parfocal to my other eyepieces, and attachable to the Baader VIP via T2 connection. Generally I prefer 70-80 Deg AFOV EPs, with 15-20mm eye relief, large top lens, and weighing around 500-600g. In my dob I use only 2" eyepiece (very similar weight). In the Tak, both 2" and 1.25", depending on what I feel. Oh I also love zoom eyepieces. I have two: one in 2" mode for the dobson and one in 1.25" mode for the refractors.

I just checked and slightly titled my secondary mirror holder and focuser axial alignment. Here's an updated photo.

This is not perfect (e.g. there are asymmetries between the sight tube circle edge and secondary mirror edge as well as between the secondary mirror edge and primary mirror edge), but it works just fine, particularly because my secondary mirror is oversized. Thank you for asking! You reminded me that I need to redo this, now that I improved my primary mirror cell!

As far as I can see, your secondary mirror is titled and too low with respect to the focuser axis. As not all the primary mirror is reflected on the secondary mirror, there is also some aperture loss. Coming back to your questions, yes, it is absolutely possible to see the three clips of the primary mirror (and you should see them!). Your collimation issue can be solved. Could you take a photo of your secondary mirror holder from the front of the tube, please? Secondly, which tools do you use to collimate your telescope?

Thanks guys for your comments. Yes, I spent a lot of time trying to figure out the source of the problems and this effort has been quite frustrating at some point. Whilst I think that every telescope is not perfect and can require some adjustment, it should still be functional. Unfortunately, this was not the case for this telescope. There are other things that could have been implemented in a different way, more in line with most of the other designs. For instance, the truss pole attachments, the plate holding both the focuser and the finder, the wood plane making the primary mirror cell, etc. At the time I asked for these changes, but these requests were simply refused.. Anyway, the issues here that compromised the function of the telescope could have been easily sorted out if the telescope had been tested before consignment. It is a shame because I offered to pay more and wait more time to make sure that the telescope was working fine. Again, this was refused.. Thankfully, these issues have been solved. I'm glad that Sidgwick wrote that page in his Amateur Astronomer's Handbook (a bit old, but still a great reading. Highly recommended like Suiter's star testing). As the whole book can be read online, I think it is okay if I report that page here. If this is not possible, please, let me know and I will remove it.

A while ago, I decided to spend some time in re-design the mirror cell supports of this telescope, in order to fix the issues of astigmatism and spherical aberration once for all. I have tested this new design on a few sessions in the field, with very positive results. The telescope delivered very crisp and details views of the moon up to 300x (limit caused by the seeing, really). Stars were puntiform and uniform. A lot of small faint stars were visible. Off-axis views have improved massively too. In addition, the telescope behaviour has become consistent between sessions and within the same session. The main issue was due to the design of the mirror edge supports: too tall, thick, and wide. In particular, they: tightened the Glatter sling cable which squeezed the mirror, causing severe astigmatism. See Figure 1, but please, keep in mind that that installation of the Glatter sling is incorrect, as the sling should not pass through the edge supports; tightened the primary mirror too much, squeezing the mirror, causing astigmatism; The second issue was due to the screws at the edge of the triangles. In particular the screw threading could block the triangle vertical movement. Here is a picture of the original installation: My first intervention to solve the severe astigmatism issues was the removal of the Glatter sling. This largely reduced the astigmatism, but did not solve the issue. Therefore, I decided to cut 1mm from each edge support pads. While this reduced astigmatism a bit more, again, the problem was not yet solved. On the other hand, the reduction of astigmatism, highlighted the moderate presence of spherical over-correction. I spotted this spherical over-correction before as the diffraction rings were distinct with a bright external one outside focus, but not visible inside focus. Said this, the impact of this spherical over-correction on the views was overshadowed by the severe astigmatism before the support adjustments were applied. In order to tackle the remaining astigmatism, on Gerry's suggestion ( @jetstream ) I removed the screws at the edge of each triangle. These screws were installed to prevent the triangle from rotating. However, the threading could cause the triangle to get stuck in a certain position causing mirror support impairment. Therefore, I design a simple structure from a milk carton and applied to the triangles. Milk carton is great because is rather strong, but still flexible. The triangles can move more freely now. The central nut was slightly loosened to allow even more vertical movement (which is now almost to the underneath wood plane). This work left some marginal astigmatism, but did not affect the presence of spherical over-correction. For a few months, no other modification was applied, as it was not clear to me what could have caused that visible spherical over-correction. The views were not satisfactory unfortunately. On-axis, bright stars appeared rather bloated, and faint stars were essentially not visible. Off-axis the views were much worse. Not being a telescope maker or an expert in optics, the only thing I could do was studying. One morning on the bus heading to work, J. Sidgwick gave me the information I needed: poor mirror central support can cause spherical aberration, due to the mirror sagging on itself. Therefore, I took the mirror cell out again, and after a few tests, it was noticeable that despite being reduced in thickness, the edge supports could still prevent the mirror from returning to its original position after the telescope altitude was changed. In addition, if the mirror sagged towards the two edge supports at the bottom, more than half of the top mirror and most of the centre was not supported at all, despite of the triangle pins being very close to the glass. Therefore, I decided to get some packs of felt pads generally used to reduce gaps underneath chairs/tables legs, and replace the current edge supports completely with these. The less padding surface was applied the more the mirror was equally supported by the underlying triangles. This raised other two problems: how much padding was necessary and at what height of the mirror the padding should being applied. One of the features I wanted in this telescope was that the primary mirror was supported by a Glatter sling cable. On Google, there are many photos of the original Glatter sling design. The one originally installed on my mirror cell is different, in particular, the sling attachment do not move vertically upon mirror collimation. In most mirror cells, this could cause severe astigmatism as the sling would squeeze the mirror edge if too much collimation travel is applied. Thankfully, in my mirror cell, the triangles are fixed on a plane. Upon collimation, this plane moves all together. As the sling attachment are attached to this plane, the sling cable always horizontal to this plane which is pretty much horizontal to the mirror plane. Therefore, I decided to re-install the Glatter's sling cable, making sure that: the sling was installed slightly under half mirror thickness and attached to the mirror edge with some velcro strips; the sling does not hold more than half mirror circumference; the distance between the mirror and each edge support was equal (mirror completely centred); 2mm were left between the edge support pad and the mirror or sling able (mirror was able to freely move on the cell and that the edge supports as well as the top clips were only used for precaution, rather than actual support); Here is a photo after the clips have been put back on the mirror edge supports, and the mirror cell back into the mirror box. After a few tests, the mirror is adequately and proportionally supported by the underneath triangle pins. Primary mirror axial alignment is well retained during a session. I never felt the need to recollimate after changing azimuth and altitude several time. Most importantly, stars have shrunk in size massively and as a consequence have become much brighter. The secondary mirror spider spikes are now prominently visible on bright stars. Detail has significantly improved. In particular lots of very minor craters popped up easily on the lunar surface, as well as the contrast between shadows and bright surfaces was well defined. Light scatter control seemed also improved. In particular, the contrast between the Moon and the background sky was striking with the Docter at 150x. I am very glad that the views through this telescope have largely improved. In the last few sessions, I have got some of the very best observations with plenty of details to keep me busy on few targets. On a side note, I measured the laminated diameter of my mirror and this is ~300-301mm instead of the 305mm on the specs. Not a big deal really, but this makes my telescope a 300mm F6.04 instead of 305mm F5.94. Signature changed accordingly. Hope you enjoyed this update and found it useful. Piero

You don't see the 3 clips because your focuser axis is not aligned properly. You collimate this adjusting the 3 screws behind the secondary mirror holder. If you don't have done it already, I would suggest to replace those screws with Bob's knobs (FLO sells them), to make the focuser axial collimation much easier. Once the 3 clips are visible, you can proceed with the primary mirror axial collimation.

I read rumours about this a while ago on CN. Personally, I keep my 30mm APM UFF.

The 32mm TV plossl is a nice and quite comfortable eyepiece. The only reason why I sold mine is because I have the 24mm Pan, which I find more engaging.

Great stuff, Gerry! Hope you find the push fix adapter as useful as I do!

In Italian, we call it sen or sin.

And here giant VIPs for T2 or 1.25" eyepieces if needed:

As I use my VIP only in 2" / T2 mode, I don't need the 2"-to-1.25" reducer. That reducer is a well made one, but it is a reducer in the end. There is a downside if the top T2 is used: the brass ring locking a 1.25" eyepiece is in the light path and it can cause light scatter on bright objects (e.g. the moon). The reducer is also rather heavy. So, here the plan: replace the 2"-to-1.25" Baader reducer with 2" / T2 Baader nosepiece (part no. # 2408150). This weighs slightly less than half the reducer, does not have the issue of the brass ring, and allow me to use the reducer as such (e.g. with the dobson - I don't like the Moonlite reducer..). Another advantage is that the internal has a T2 diameter. As the VIP clear aperture is 28mm, there is some space on the internal wall which could be fitted with some black flocking material reducing light scattering even more. And here with my zoom and Docter eyepieces connected via T2:

Really fascinating thread! Good luck with this project and looking forward to hearing more.

I wanted to buy this a long time ago.. now it's here

If the secondary mirror is slightly oversized, its alignment is far less critical than what one would imagine, for visual work. For imaging, it's more important. Try to get the three rings as concentric as possible: edge of the sight tube, secondary mirror, and primary mirror.

nice one! Great book!

Yes, go for a 8 or 9x50. Much better and it is still light.

Thanks for sharing this, Gerry. I agree with you about the fact that truss dobsonians can be quite demanding. In my case, I saw a noticeable improvement since the telescope lives in the shed. This is a well built one. Temperature inside is about 1 C higher than ambient. In the summer, the telescope dislikes the light shroud, whereas it loves the fan. The effect is rather obvious. As we are approaching the colder seasons, body heat does affect the views, so I started using the light shroud. This works, but does not like the fan on! Therefore nowadays the fan is on when I mount the telescope, but switched off when I mount the light shroud before starting observing. I will try your approach with the light shroud next time I observe. --- Regarding the coma corrector with medium fast newtonians, my position is as follows: 1) it is beneficial in the same way a good collimator can make a difference. 2) if UWA eyepieces are used, coma can damage the views. If a coma corrector is not used, it makes me wonder why one would get premium eyepieces and telescope optics, without wanting to fix an intrinsic issue that certainly (due to physical laws, not personal opinions) transforms that "premium" to "average". Cost is the only factor I can think of. Still, if one spends 3K pounds in eyepieces, 500 pounds in a CC is not a big deal to me, particularly considering that the views through all the eyepieces will improve. 3) the larger the aperture the more coma becomes an issue. Again not my opinion but basic physics, due to the increase in brightness of the airy disk. 4) I think coma is less noticeable under light polluted skies, simply because the comatic smear mixes with the brightness of the sky. Said this, it is still visible on planets and the moon off axis. On planets coma would be less critical if the mount is auto driven. 5) I think that other issues need to be sorted before tackling coma. For example, careful analysis of the mirror cell and secondary mirror attachment to avoid astigmatism, collimation to avoid miscollimation-dependent coma and potential astigmatism when the miscollimation is large, structure robustness (mirror cell, truss poles, secondary mirror holder and spider, focuser), minimisation of temperature difference between environment and optics, tube current minimisation, decent eyepieces that don't have noticeable astigmatism and chromatic aberration off axis. Some of these factors affect the views very negatively (and regarding some I speak by experience..). Finally, once all the above is within acceptable tolerance, meaning that they are not perfect but they don't affect the views significantly, it is the time to assert coma. Compared to the previous points, this is easy to deal with as coma correctors are available in the market. --- Regarding myself, yes, I am not excluding the purchase of a coma corrector, because: 1) I like dobsonians and don't see myself without one in the future 2) I like refractor-like views 3) if I ever had to change dobsonian in the future, the replacement will be faster than f5.94. In fact, below 12", I would only get a 10" Chinese dob which is faster, above 12" it must be faster, and if I move into night vision a faster telescope works better. In all cases a coma corrector will be useful.

In terms of loss in detail off-axis (from about 50 deg AFOV). To my eye it is rather soft when compared to the crisp axial view.