Piero

I am not sure you can rely on the mirror on the floor. The returning beam will depend on the angle of that mirror. If the beam goes through the hole of your template, which is supposed to be at the centre, that's a good indication that the focuser is collimated. In any case the tolerance for the focuser axial alignment is 0.03xD, where D is the aperture and here no coma corrector is used. That is 3mm for a 100mm refractor. For checking collimation of the lens, see Suiters' book.

Whilst it is true that a miscollimated telescope can show astigmatism, in this specific case astigmatism dominates over coma when the strehl is below the diffraction limited threshold (about 0.80). At that point the images are quite bad. If your focuser is miscollimated, you should detect coma on axis. That is the predominant aberration. If you are certain you see astigmatism, it could be in the eyepiece or the optics are retained too tightly. Also, make sure the telescope has cooled down properly. For test collimation I use the Glatter square attachment with my Glatter's laser. As others said, the diagonal must be removed.

Mm.. some interesting feedback from this conversation. Thank you all. Food for thought!

In an ideal world, I would love a Vixen HR 5mm, but it seems this cannot be made due to the inherent design of the HR line. So, options are: - 11mm +/- 2.25x barlow - 5mm and use the 24 Pan +/- 2.25x Barlow Yeah.. a Pentax XO 5mm would be great! ... So would be the XO plus the 10mm ZAOII... Tough to find them and on the wallet though..

I currently have a Nikon MC1 zoom 21-9mm which works well although I had to re-do its barrel as it is a spotting scope eyepiece. In terms of optical quality it matched my Vixen SLVs.

Thank you all. From your comments, it reminds me of the Orion super Plossl Sirius 10mm I bought back in the nineties. Good eyepiece, but tight on eye relief.

I am considering this eyepiece for my refractors. It would be used with and without Baader barlow 2.25x. Has anybody tried this combo? In the past I tried the BCO 10mm, but I let it go.

Under my skies, I enjoy 5mm exit pupil. I've tried more, but I found that 5-5.5mm is about the largest I would use.

@BGazing Thank you for the info. I was not aware of the ADC, and it seems interesting. Will certainly study it a bit more. For the time being, I decide to postpone the purchase of a coma corrector. After removing the sling due to its incorrect installation which caused a massive amount of astigmatism (it squeezed the mirror because it passed through the mirror edge supports and also the wrong installation did not allow it to remain parallel to the mirror plane), I noticed that there was still some astigmatism, this time due to the mirror edge supports which tightened the mirror too much. To fix this other issue, I had to cut off 1mm from the padding of each of the 3 edge supports. Now the mirror can move and rotate more freely, without being trapped by its supports. The primary axial alignment is still retained after using the telescope, so no collimation issues thankfully. Hopefully the glue used to attach the secondary mirror will not cause astigmatism this winter, otherwise I will have to fix that too. As astigmatism is more visible than coma (square vs linear), its solution had a priority. Said this, I haven't abandoned the idea of a coma corrector, but I'm not that keen towards the one sold by Televue. In addition, I have postponed the decision about selling my 20mm Lunt as I don't get along with 100 deg eyepieces very much (I find the huge field quite distracting). Generally, I move from the 30mm UFF to the 12.5mm docter and coma is not too much annoying. It might be that I get the Baader MPCC and use it with the 30mm UFF only. 🙄

The Vixen 42 LVW I had was probably good in a F9 or F10 telescope, but in my Tak f7.4 showed quite a lot of astigmatism in the last 20% of the fov. The Meade maxvision 40mm is another good eyepiece after removing its armour. In any case an eyepiece giving 7mm exit pupil is very specialised and requires deep skies. In my 'long focal length' journey the TV panoptic 35mm was much better than the above two eyepieces. It was replaced by the APM UFF 30mm which gave me a more useful exit pupil and its weight is closer to my other eyepieces.

I have never used that collimator so, cannot comment. It's a 1.25” model, so it's fine if you use 1.25" eyepieces. If you use 2" eyepieces, you should make sure that your 2-to-1.25 reducer does not affect the focuser axial alignment. Examples of reducers that do not affect the alignment are the Baader clicklock and the Glatter's reducer.

Regarding your question about collimators, I use the 2" Glatter laser with 1mm aperture stop and 2" Glatter tuBlug for adjusting the focuser and primary axial alignments. I prefer the Catseye Telecat XL for centering the secondary mirror under the focuser, a once in a while adjustment.

Hi, There are many factors determining the balancing of a dobson, not just short f-ratio or heavy mirror. The size of the altitude bearings as well as the number and how teflon squares are installed, are also very important. Being a truss design, the telescope is very portable at both F4 and F5. Between the two I would opt for an F5 (you did not mention, but you will also need an adjustable chair). 12" F4 is as tall as a 8" F6 or 10" F5. For that size, a solid tube offers more advantages in my opinion. F4 dobsons are nice but many factors become critical aside from eyepiece quality and coma. Mirror cell, misalignments, mechanics, etc are all more demanding. A pyrex mirror takes more time to cool down, but if the outside temperature continues decreasing, it can easily be that the mirror will never cool down enough. A decrease of 1C per 45min is more than sufficient to cause this. When the mirror is cooling, the optics look overcorrected and can show some astigmatism (which dominates coma). Sure, you can use the telescope at low power during these situations, but at some point you will also want to use it at high power. A cooling fan will help decrease the mirror temperature faster and also minimise temperature differentials. A Quartz mirror expands less, which can be beneficial particularly if the temperature in your location changes a lot during the night. Regarding a coma corrector, the general advice is that a coma corrector is needed at F4, whereas one should wait and see before buying one at F5. The region where the optics are meant to have a Strehl value >= 0.80, namely the coma "free" region, is very limited. An overestimation of the angular coma free region is about 16 deg AFOV for an F4 and 25 deg AFOV for a F5. Therefore, although it is possible to live with coma, the previous sentence means that the views through an Ethos eyepiece and F5 mirror with 0.99 Strehl (excellent), will be <0.80 Strehl (average and less than average) from 25 deg AFOV to the edge. In my opinion, if one decide to go for fast optics and UWA eyepieces, a coma corrector should be included. Why would you get a Ferrari, and decide to get the tyres from a Fiat Punto for saving money? I would also check that the secondary mirror is well supported. This is often the main cause of misalignment when the telescope is moved in altitude as the weight of the secondary moves from 4 supports to just 2. A good focuser is also important in order to maintain this axial alignment. If you don't have one, I would suggest the purchase of a good collimator - a one-off purchase -. The latter can help you identify many potential weaknesses of your telescope and therefore how your can make improvements. Piero

Here are some shots taken during a nice session observing the Moon. And here a video: VID_20190909_222618~2.mp4

This thread is updated here:

It depends. The angular coma free region is constant across magnification, although coma is often perceived at low power rather than high power. I believe the reason is due to how coma is revealed, as the star is a point source - brightness increases with mag -, whereas the comatic blur is more like an extended object - brightness decreases after a certain mag -. For bright stars and planets, coma is well noticeable at high power, unfortunately.

I agree with Paz. LP is a major filter unfortunately. My TV60 on the dolomites (21.2) showed much more detail on anything but planetary targets than my Skywatcher 8" F6 dobsonian used in a location affected by severe light pollution (18.4).

If it's not offset, the downside could be an unbalanced field illumination. This is generally more critical in imaging than visual. There's nothing perfect in this job (collimation), just a requirement to remain within tolerances.

I can move the sec mirror closer to the primary mirror along the axis, but cannot slide the secondary mirror along the 45 degrees axis. I would not do the latter anyway.

This is what I've done. It helps with balance too

Congratulations! Need to try to spot Triton.

I found out the "correct" size of my secondary mirror! Minor axis: 57.15mm Major axis: 85.725mm It was supposed to be 62mm minor axis. Anyway, with the new correct measure, the central obstruction is 18.73% . On a positive note, contrast should be better. Using the laser I could also notice that the secondary is currently not offset along the mirror axis and that it's about 2mm titled. Not a big deal as my larger ep field stop is 38mm, but I will adjust it when back home.

Thank you gentlemen! I will take advantage of this clear evening as rain is expected tomorrow! Wish you clear skies in the UK!

I'm currently on my annual leave in Italy and the sky has been clear all day with no air turbulence. The telescope is out - of course - and I managed a session before dinner. I thought about sharing some photos taken with my phone and a couple of sketches.

The square attachment is for the secondary mirror alignment as shown in Jason D's posts on CN.