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Everything posted by Piero

  1. Found these books in the second hand market.
  2. I observed Copernicus past night with my dobson up to 533x without any sign of image degradation. It was wonderful!
  3. 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):
  4. 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.
  5. 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
  6. 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?
  7. 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.
  8. 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.
  9. 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.
  10. 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?
  11. It's late here Gerry! What's your humblest suggestion? I'm curious!
  12. I don't think it is linear. I suspect it is sin/cos, but haven't thought about this properly.
  13. 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.
  14. Quick solution,for the time being they are on top of the nylon pins. See video.
  15. 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.
  16. 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.
  17. 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.
  18. 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?
  19. Further tests on the Glatter sling cable. There are several ways to install a Glatter sling. Here I will focus on the 180 deg (or "U-shape") installation. There are a few important aspects to pay attention: the sling must be placed slightly below half of the mirror thickness (see "h" in figure). The exact value can be obtained here http://www.cruxis.com/scope/mirroredgecalculator.htm . For my telescope, this is 17.75mm. the sling must be attached with some vecro stripes, so that it remains in place. These stripes should be minimal in order to avoid mirror distortions. the mirror must exert the same force (see f1, f2, f3 in figure) on each bottom mirror support (e.g. triangles) and that this force is parallel to the mirror axis. the sling must be parallel to the mirror axis (see f4 in figure) or perpendicular to the force exerted on the bottom mirror support. the sling should not wrap the mirror more than half otherwise it exerts extra forces, which will result in astigmatism The original Glatter sling solves (3) automatically, adopting rollers where the sling is attached. This and the velcro stripes guarantee that the sling remains parallel to the mirror plane. In my Lukehurst dobsonian, the sling attachments do not have rollers, but are fixed on the same wood platform where the mirror lays. This platform is regulated by the primary mirror collimation bolts. As a side effect, the sling attachment do not need to have rollers, but it becomes crucial that the sling attaches at the same height "h" (see figure), in order to remain parallel to the mirror plane. In order to check how the mirror lay on its back support after the sling is placed at the correct height "h", I placed the telescope horizontally and I laid down underneath the mirror box. A neckless can be helpful to evaluate that the mirror is exactly vertical. By increasing and decreasing a tiny bit the mirror altitude, the mirror moves towards and backwards its back supports (the pins of the triangles). In my case, it was clear that the pins on the top triangle did not touch the back of the mirror at the same time as the pins of the two bottom triangles. This meant that the mirror was not equally supported by the three triangles, and in particular less supported by the top triangle (about 4-5mm gap). To improve this, 1 adhesive pad (2mm) was added to the pins of each triangle so that the sling became more parallel to the mirror plane. The outcome was a noticeable improvement to the views, with further reduction of spherical aberration and astigmatism. Sirius B was visible at 250x without difficulty. E and F in Orion's Trapezium were also easily visible. Background stellar dust was noticeably more apparent. Even at low power the contrast was improved (e.g. Orion nebula). In addition, a few times while observing stars of medium-low brightness, I had the feeling to observe through a refractor rather than a Newtonian telescope. Some traces of astigmatism and spherical aberration were still present, but I really had to look for it. This was expected though as I estimated that there is still 1-1.5mm gap left between the mirror and the pins of the top triangle when the former started touching the pins of the two lower triangles (see video). In general, I am very happy with this result and the mirror and telescope are working better than my expectations! The next step is to calculate the exact gap and either replace the existing nylon pins with longer ones or add another pad to the pin to raise the primary mirror a little bit more. No doubt, I am going to fix to completely! VID_20200125_160607.mp4
  20. Great idea, John! Here are mine:
  21. Personally, I would upgrade the telescope rather than getting a coma corrector to be used with that telescope. Then, yeah, at F5 I would use a coma corrector, especially if using eyepieces with 70 Deg AFOV or more. Not everyone, and here on SGL very few, would though.
  22. Looks good! Are the plastic covers open at the bottom? P.S. I also keep the small ladder and ramps in the living room, although these are stored in a corner and barely visible (as they are covered by the dob!).
  23. Looks great! .. and I second what Stu said above.
  24. Thank you! i think it is important to be sensible and accept some compromise. If partners have their own space and hobbies, then we can have ours, respectfully, without causing arguments. in my case the living room is rather large so keeping the telescopes there is not an issue. Actually the opposite, as they make it feel less empty. The other side of the living room has almost only things of hers.
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