Piero

By 'tipped' do you mean off-axis towards the primary mirror? 1. A primary mirror axial misalignment will cause coma on axis (coma due to misalignment). 2. A focuser axial misalignment will cause the stars to focus at different points across the focal plane. 3. A secondary mirror (severe) misalignment will cause unequal field illumination. It's rather obvious that only the first one degrades on-axis, therefore 2 and 3 reveal nothing on a star testing as this is conducted on-axis. Of the 3, the last one is the less critical for visual astronomy. Without coma corrector, the first one is the most critical. With coma corrector, also the second becomes rather critical. All of them become more critical in faster newtonians. There is nothing to fear in this process. If wrong, it can be fixed and rather easily.

If collimation of a, let's say, f5 Newtonian telescope without coma corrector is checked with a star test, the star must perfectly be on axis, otherwise the coma-dependent misalignment is also visible. This cannot be really distinguished by optical misalignment. So, again, to me at least star testing is not the right way to check collimation as it is too sensitive, unless the seeing is very good, the high power eyepiece is good, coma corrector is used, and the mount tracks automatically. Even so, this test doesn't tell you anything about secondary and focuser alignments. Coming back to the topic, my HG laser and Catseye telecat give the same reading consistently.

Not sure what some members above meant by star testing in this thread. Do they mean "checking" or "collimating" with a star test? In my opinion, I wouldn't suggest the latter, particularly with manual driven mounts. Regarding the former, the procedure can be rather complex as it is easy to get errors due to other factors which don't have anything to do with misalignment. Of course, if one knows how to star test, the feedback given by this tool is incredibly useful in order to understand what does not work properly in a telescope. Regarding the importance of collimation, well Suiter's book on star testing offers some quantitative data on this subject. Of course, a slightly miscollimated telescope still works. Said this, if one pays for premium optics and is happy to have an "okay-ish" collimation, the same person should reflect that some okay-ish optics with excellent collimation could probably give the same results in terms of view quality.

I tend to stand while observing as that's my preferred way. This with both my dobson and refractors. The refractor mounts have an extensible column which I love as it avoids contortionism or "yoga postures".

Yup! Congrats!

Last Thursday night the seeing was great with very good transparency. Observing Aristarchus crater and plateau from 160x to 630x was amazing. In particular, I was impressed by the kind of "river" approaching the crater to the left and how the light played fantastic shades inside. From Wikipedia: Aristarchus plateau (NASA).

Found these books in the second hand market.

I observed Copernicus past night with my dobson up to 533x without any sign of image degradation. It was wonderful!

Last night I had a chance to try the telescope following the last work on the mirror cell. The sky conditions were clear, windless, and rather stable temperature between 8.20pm and 9.20pm. After that the temperature started dropping. The telescope was left outside with the fan on since 6.30pm. The light shroud was also fixed in order to reduce internal and nearby turbulence. Both telescope focuser and primary mirror axial alignments were adjusted with my 2" Glatter laser 650nm. Several stars were used for star testing: Betelgeuse, Procion, Rigel, Pollux, and Aldebaran. This was done in order to have a range of colours and different altitudes. I could not spot any trace of astigmatism. Spherical aberration was highly corrected too. This was at about 300x. The rest of the time was spent observing the moon which was outstanding. Copernicus at 533x (zoom plus VIP barlow) was really impressive, showing fine and minute features (e.g. tiny craters inside and ground detail) that I didn't even imagine were possible to see. Very happy about this result. From sky and telescope lunar map (at the eyepiece I could see far more detail that this):

I thought you already used a RACI finder with your dobson. Nice finder! At the moment I have an Antares VS60 with no illuminated field.

I worked another bit last night and this morning: replaced temporary nylon pins + 2mm pad on top with a stack of 2 x 5mm thick pads (20mm diam). This raised the mirror 1mm. the milk cartoon structure to maintain the triangles in position without affecting their movement was moved underneath marked the position of the sling (COG) on four points. These are about equidistant. cut velcro strips in two segments and placed them under and above the COG where the sling will pass. The external border of the velcro strip (which is slightly flat) was placed towards the sling cable. Advantages: the velcro strips do not attach to the sling. Therefore they cannot stretch it. they remain attached to the mirror. Therefore there is no need to remeasure the correct position of the sling removing the sling is much easier as the velcro strip do not attach to the sling, it is possible to rotate the mirror when placed on its mirror cell. one velcro strip working as a bridge to make sure that the sling remains in position Here are the photos Mirror cell: COG marking (see previous posts for weblink on how to calculate this). The number is the sling position from the bottom of the mirror Velcro strips and placement: Make the sling tight, but blocking the mirror position using two felt pads placed opposite to the sling: Add velcro strip to keep the sling always in position. Note, these attach to the velcro strips on the mirror. The sling can slide through these strips. Therefore the glue on the velcro strips does not cause any issues (e.g. stretches). Mirror on top of the triangle pads: Mirror cell put back into the mirror box. Gap between sling and lateral support pad. View from the bottom Video showing how the mirror goes back to the mirror cell increasing the mirror altitude. There is still a minor gap, but I will leave it as it is. This is probably 0.5mm or so. VID_20200201_123921.mp4

I ordered a set of felt pads of different sizes and 5mm thick. After taking all the measurements, I need to stack two of these in order to replace the pins. Will do the work tonight or tomorrow morning. What's the diameter of your pads installed by Randy? 1inch, 20mm, or more?

Hey Gerry, Discard the z function (green) as not necessary here. What we care here is the part of the functions where x is from 0 to pi/2 rad (90 deg). This is the movement of the telescope regarding altitude. Basically, depending at 0 deg telescope altitude the weight force is all on the sling (see cos function) and this decreases as the altitude grows to 90 deg (pi/2). Follow the sin function for studying how the weight force works on the triangles. Note that the intersection is not at 0.5 y. Source: Wikipedia.

Here we go You don't need the actual W vector for this, so it can be assured to be the unit vector. What really matters are the sine and cosine functions at different angles alpha. The alpha angle is 90 - telescope_altitude. Sine and cosine receive angles in radians, so you need to covert: 1deg = pi/180 = 0.01745 rad.

Thank you, Gerry. Very helpful. Placing a pad on top of the pin is not a final solution, but only a quick way to test the idea. I certainly need to consider either taller pins or taller pads, once I get the the measure of the exact height. If this is exact there is no friction caused by pins or pads.

Gentlemen, you are too kind! I haven't come up with new products or design, but just put some information together and been trying to analyse / address some issues which are often ignored. Hopefully, this thread will become a useful resource for those of us having to deal with these issues or want to improve their telescope performance. That would be great! I feel the thread title should change. Currently, it is not really informative and this might affect searches. Probably something like "Lukehurst-Nichol classic dobsonian mods" would be more appropriate? If happy with this, could @John or @Stu change this, please?

It's late here Gerry! What's your humblest suggestion? I'm curious!

I don't think it is linear. I suspect it is sin/cos, but haven't thought about this properly.

Yes, please

Sling and triangle supports play orthogonal (=perpendicular) forces AND they should do so, otherwise the result is astigmatism (at the least). Triangle support is 0 at 0 deg altitude and 1 at 90 deg altitude. Sling support is 1 at 0 deg altitude and 0 at 90 deg altitude.

Quick solution,for the time being they are on top of the nylon pins. See video.

Absolutely! The problem is not really the lack of contact between 0 and 5-ish Deg of altitude. The problem is the difference in support that the mirror receive from the bottom. The point is that even if the mirror is supported by all pins of the triangles (all pins touch the mirror), this does not guarantee that the mirror is well supported. If the pins touch the mirror at the same time at the same altitude (0deg ideally), then the support is expected to be identical or close enough.

Very low altitude, probably 5 degrees. You need to raise the telescope very gently. I laid down with the mirror box above me, and then pulled it towards me very gently, raising the telescope. The video shows this. The mirror touched the pins of the 2 lower triangles, but the telescope needed to be raised another bit before the mirror was supported by the pins of the top triangle. Adding 1 pad reduced this gap, although there is still a little gap left.

John, here is a photo of an original Glatter sling attachment. As you can see, it is attached to a roller which can slide up and down according to the mirror plane. Together with the velcro strips, the sling remains in place and in the correct position. It's a great design really. The sling does the job, not the person who needs to assess the correct mirror height. Also, that design prevents the sling to squeeze the mirror (and therefore astigmatism). My telescope also works, but this just because of the wood platform holding all the components (mirror, sling attachments, etc). Still in this case it is necessary to find the exact height of the sling attachments and mirror so that these are in agreement. Once this is done, it is done though.

Thanks Gerry. That's an interesting idea as it minimise the amount of strips, and therefore potential side effects from these. I should do the same. At what angle are these two cruces attached to the mirror? Thanks John. Yes, in my telescope the collimation bolts adjust the alignment of a wood base on which lay the triangles, mirror, sling attachment, sling and edge supports (for safety). This is not generally the case though. In almost all mirror cell designs I've seen, the collimation bolts move the triangles up or down. I think the range can be up to 1 inch. The sling attachments are fixed (they don't move with the mirror), hence the rollers invented by Glatter so that the sling always remains parallel to the mirror plane independently of collimation adjustments. The clips on the lateral edge supports are for safety. The edge supports are for safety too. They don't touch the sling - there is actually a gap of 1-2mm between them.i copied this last setting in my mirror cell. In the original Glatter sling, there isn't any sweet spot because the sling height follows the mirror height (and most importantly the mirror plane). If the sling is attached with eyebolts like in your mirror box, yes, there is a sweet spot for sure. To check this you need to make sure that the cable is parallel to the mirror and perpendicular to its attachments when the mirror is collimated. If you keep the mirror height like that, it should work well. If you raise the mirror too hight or low, the mirror will slide a bit on the triangle supports and could not be equally supported by these. There is also a chance that astigmatism is introduced due to friction of the pins of the triangles on the mirror. Spherical aberration is also likely. If you raise the mirror too high, the sling position will go slightly down on the mirror side. Therefore it will squeeze the lower part of the edge, making the mirror figure hyperbolic (overcorrection). If the mirror height is too low, the sling cable can slight a bit too high, changing the paraboloidal mirror figure to ellipsoidal (undercorrection). As I said above, I don't think this is a problem at all. Just make sure not to ease the mirror so that this touches the clips. How are the sling supports attached to the structure of your telescope? Can the sling move up and down following the mirror plain upon collimation or are they fixed? Also is there a bit of gap between the mirror edge supports and the sling?