Piero

I took the mirror cell out. The mirror clips seem to be glued.. why this?! How does one take the mirror out if the clips are glued? Detaching the edge supports is even more difficult as there is no way to pass a key through that.. 

A nice post by Nils Olof Carlin: "[..] To design a support for the largest mirrors, the sling may be worth the effort - for the next-to-largest, a whiffletree arrangement is fine, but for more moderate sizes, a 2-point support is fine by far, and easily made well. This includes a "normal" 18" mirror and a bit up. If you want an upgrade, practical and economical considerations come into play - Howie G-s sling might be the easiest to add to an existing mirror support." (https://www.cloudynights.com/topic/570249-whats-so-special-about-glatter-slings/#entry7768997)

Nice mirror cell! How much gap is there between the sling cable and wood edge limiters (clamps?) of your mirror cells?

The pads can be widen a bit to allow the cable sling to move with the mirror. They are attached to the clips, not part of them. I'm just not sure I want to go for that route or take the Glatter's cable sling out completely. Velcro would be needed of course. This is an easy mod. Said this, velcro does not replace the padding. The bearings do and make sure that the mirror does not move to the left or right of the cell. This is actually quite important if the ground is not exactly flat.

For now my cable sling is loosened in such a way that it does not touch the mirror. In the coming months, I will think about whether taking it off completely. For a mirror of this size and weight, the current mirror edge supports do work well and do not cause any astigmatism nor pinched optics. This was proved in my last observing session. I like the Whiffletree with rolling bearings proposed in Lockwood website. To me it seems a much nearer approach than having a sling. Being this a public forum, I think it is also fair to state that the whole telescope works really well, and I am more than happy with it. We all can be picky at the details, and being critical is a good thing in my opinion as it allows us to improve things and make them work better. I did have a problem with the sling but this can be solved easily and in any case a sling is not required for the correct functioning of the telescope. When observing planets at low altitude I always collimate the telescope at the altitude of the planets before observing anyway. In my opinion, this is the most accurate way. In my last session (with loosened cable sling), this telescope gave me the best view of Jupiter I have ever had. Considering the intricate and delicate details on that planet, I think this is a sign of very good optics and telescope structure.

The main cause of astigmatism is due to the incorrect installation of the Glatter's cable sling. There are no bearings allowing the sling to follow the mirror axis. Instead, the cable passes through the pads of the lower mirror edge supports. Therefore, when the telescope is pointed at low altitude, the sling is not always parallel to the mirror plane axis. Instead, being trapped by the existing lower edge supports, the sling squeezes the mirror at the bottom, effectively disfiguring it, causing astigmatism. This also explains why there was no astigmatism when the Glatter's sling was loosened. The two low mirror edge supports do work, so does the Glatter's cable sling. They don't work together, though. In all this learning process, I also read an article written by Lockwood, an American experienced mirror maker. Interestingly, he considers Glatter's cable sling a rather outdated approach to mirror edge support. References describing this issue: https://www.cloudynights.com/topic/570249-whats-so-special-about-glatter-slings/#entry7767543 http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html http://www.jpastrocraft.com/cells.htm

I think I found the main source of the problem..

Thanks Niall, great post.

The three dab of glue I mentioned were referred to the secondary mirror. I don't think these are the source of astigmatism, because if they were, astigmatism would be present all the time. I think the same for the primary mirror. To me the fact that astigmatism is transient and last time there wasn't any (and the view was very good), suggests that a long cooling time can be the cause of this issue. This particularly considering the double layer of wood (mirror box + rocker box) and the current long daylight in the UK which causes the drop in temperature starting quite late. In addition, the seeing was not good either in the first two sessions. As I said previously, in the third session the airy disc was irregular only for 1/3, and the stars showed a kind of "half astigmatism". Considering the mirror box opening showed in the second photo of my first post, this suggests uneven cooling for the primary mirror to me. Anyway I need more time for observing and testing.

Thanks Niall, I haven't taken the mirror cell out of the mirror box, but I think yours is a good point. As far as I know, if the mirror gets somehow stuck in its mirror cell (e.g. due to excessive heat, frost, or same position for a long time) this can cause astigmatism as the mirror is essentially deformed. The primary mirror should be able to rotate easily without friction in its mirror cell. If the problem won't be solved with the fan, I will take the mirror cell out and see. Regarding the collimation shift, I don't see how this would cause astigmatism though. Apart from the autocollimator technique which is for very fine adjustments, the three things to collimate are: - secondary mirror position and tilt so that it faces the focuser - focuser axial alignment to the center of the primary mirror - primary axial alignment to the secondary. In case of miscollimation, the first one causes an uneven illuminated field, the second one causes differential focus positions across the field, and the third one shows on-axis coma. In star test collimation, one aims to remove that residue of visible on-axial coma using a high power eyepiece. This because the airy disc is larger and so the comatic tail is more apparent. Astigmatism is due to optical deformations or issues in the optics themselves.

Thank you for your reply, Paz. I'm not an expert, so my answer should be taken with a grain of salt. Yes the figure of the secondary mirror can change with temperature, but the mirror is considerably smaller and thinner than the primary mirror. I doubt this requires a long time to stabilise. As far as I know secondary mirror can cause astigmatism in the optical system when they are glued to the holder by a large amount of glue or silicon rather than three small points. The issue arises because in a large amount the outer part of the glue gets thicker much earlier than the inner part. In the process the glue can slightly deform the mirror, causing astigmatism. This astigmatism is visible on axis and in the whole field. If this happens, the mirror must be taken apart from its holder, removing the glue and repeat the attachment.

Hi Gerry, I will rotate the mirror by 90 degrees and see if this has any effect. The thickness is 1.45", not very thick but not thin either. I haven't yet tried a very high power observation due to seeing conditions and limited time due to work commitments. Last time the airy disc and diffraction rings looked very good as described previously, but this was only at 150x.

Hi John, I would say that there are a few additional variables compared to a solid tube dobson, plus my level of inexperience. What I am trying out is to isolate each potential issue and maintain an unbiased attitude (as much as possible) to the causes of the problem. The potential issues causing astigmatism (described here: https://stargazerslounge.com/topic/337091-new-dobson-arrived/?do=findComment&comment=3682534) I can think of are: poor seeing thermal issues due to primary mirror cooling zone of warm air trapped around the primary mirror, instead of dissipating (including light-shroud) secondary mirror attachment (glue) poor collimation Glatter's sling primary mirror support (mirror cell) three clamps preventing the primary mirror from falling mirror figuring I can exclude (4) because last time the telescope worked really well as testified by a great view of Jupiter and star testing on Vega (5) because I checked the position of the donut on the primary mirror (it's precise) and I paid considerable attention on this. Result: see (4) (7) for the same reason stated in (4) (8) because I checked that there is a small gap between the clamps and the primary mirror. (9) for the same reason stated in (4) My current thought, but I try to be open minded as possible, is that the real causes are mirror cooling (2) and trapped warm air (3). I am not convinced that the Glatter's sling is one issue. After checking this more carefully, It seems to me that it was tight in terms of supporting rather than squeezing the primary mirror. As far as I can see (2) and (3) work differently between wood truss and solid tubes. In a solid tube, the whole tube cools down, and the mirror is separated from the back environment by its cell only. In my telescope, the whole wood structure "wraps" the primary mirror considerably and wood is a bad conductor. In the 2 photo posted in my first thread, you can see that there is an open area at the bottom of the mirror box. My thought is that area helps the mirror cooling down, but the part of the mirror which is opposite to it will cool down more slowly. Hence, thermal differentials around the mirror surface. I read a fair amount of comments on CN by people having classic dobsonians and the problem seems very similar to mine. They solved thermal issues using fans, also left on through the whole session. It would make sense as fans would not only cool but flow away warm air trapped around the primary mirror.

The socket arrived today and I've already installed it at the end of the cable. The fan can be turned on, very silent and vibration free.

Thanks for your reply, Gerry. I haven't rotated the mirror nor take it out from the cell. In the first two sessions, the astigmatism was the classical tangential on one side, sagittal on the other side. In third session astigmatism was skewed towards one side, a kind of half astigmatism. Of course in these three sessions, the degree of astigmatism increased off axis towards the edge. Yesterday, there was no astigmatism in the star pattern. Just a minor variable flickering on the external circle. The thickness of this flickering was about like one ring. After the view I had yesterday, I am not worried at all, to be honest. Actually, I'm quite excited to see how the views will be when the mirror is uniformly cooled with the fan. I only need more sessions to get acquainted with this telescope and learn how it works under different conditions. Do you still use your Catseye tools or just the Glatter's? I'm working out my way on the many excellent posts by Jason D. A new book is also coming..

Oh man, so hot today! So.... The sling seems installed correctly to me and it is placed right in the middle of the mirror edge. In the first two sessions, I could see astigmatism on off-axis and on-axis stars also at low power (60x, 90x). In the second session, the star pattern was a bit better, but astigmatism was still visible in the lower part of the diffraction rings (about 1/3 of the circle). That lower part coincides with the sling position and the main entrance at the bottom of the mirror box where air can flow into it. In both these two sessions, the light shroud was used and closed completely. Seeing was average. Collimation was done without the 1mm aperture stop attachment. In the third session, I did not use the light shroud and the astigmatism at the bottom of the star pattern was noticeably reduced. Seeing was still average and collimation was performed like before. Last session (yesterday), the light shroud was not used, seeing was very good, collimation was done using the 1mm aperture stop attachment, and the Glatter's sling was slightly loosened in order to exclude sling-dependent astigmatism. The astigmatism at the bottom of the star pattern was completely gone. The views were very good - I could see the GRS hollow with the Docter 12.5mm + VIP barlow ~2x (~300x). Only a minor residue was detectable (an inconstant flickering) on the outer border of the star pattern. To me this also excludes secondary mirror-dependent astigmatism due to strong glue otherwise I would have seen this on axis. The visible residual astigmatism (flickering) seems due to mirror cooling in my opinion. The fan will test this. Air turbulence can also be excluded due to the lack of light shroud. This Friday I will receive the socket for the fan, so that I can test its effect. My prediction is that the fan will reduce that flickering as well as improve air circulation. This is somehow expected considering that warm air is almost trapped by the wood structure (very different from a steel / aluminium tube!). With the fan on, I want to try to tight the Glatter's sling as it was before, and see whether this causes astigmatism or not. If it does not, it means that that astigmatism was mostly due to mirror cooling and air turbulence. Regarding the light shroud, there are many advantages in using one - in particular it prevents something from falling onto the primary mirror. Thankfully, I can still use it and just leave it partially open on the bottom of the dobson (my shroud has some kind of "buttons" which can be left open). Anyway, this will be tested. Thoughts?

I'm cycling back home. Will reply asap.

I believe the sling was just too tight, but have to check this better. Possibly it was just air turbulence. In any case, being rather loosened last night it does not seem very critical for this 12" scope. Still happy to have it though.

No, it was not touching the clips and no, I do not have velcro on the mirror edge. The sling stays in position pretty well. The laser dot remains in the correct position when racking the focuser in and out. My concerns are more due to astigmatism which I saw in the previous sessions. I am tackling the potential causes, e.g. imperfect collimation, too tight sling, air fluxes, light shroud, mirror cooling. In the last two sessions I made very good progress. Yesterday there was no astigmatism at all, and Jupiter was really gorgeous to observe. When the fan will be used, I will have another bit of improvement. This due to the current star pattern showing minor cooling issues on the outer circle.

I am still experimenting with this Dobson. Last night I slightly loosened the glatter sling as my suspect was that it was too tight. Still early to confirm this, but the result was noticeable. On Friday, I will receive the socket to be installed at the end of the new cable. With this, the fan can be turned on and help reduce temperature differentials on the primary mirror. Last night, I collimated the focuser and primary mirror axes using the 1mm attachment. This made collimation much more precise in my opinion. This attachment is meant to be removed when collimating the primary mirror axis. Dunno.. I think the device is more precise with it attached.... Result: a superb view of Jupiter!

I had one of the very best views of Jupiter last night, particularly just after sunset. GRS, hollow, moon shadow, and so many other details between the main two belts and on the belts themselves.

Really nice, indeed!

The new battery sits in the rocker box nicely. The socket is coming this week too, so the fan will be operative soon.

For my dobson:

Thanks! In daytime the colour is more plain wood actually.