coenie777

If you own the above telescope and are comfortable taking it apart to clean etc, would you mind checking if your telescope has the following on the optics: - Primary mirror: serial number plus two vertical lines drawn on the side of it. - Corrector: on the inside of the center hole, a vertical line. This can be seen without having to remove the fitting by looking at an angle at the fitting. On my telescope it is very light but definitely applied. If you have a factory original setup between the primary mirror and corrector, is there any relation between the two vertical lines on the primary mirror and the one vertical mark on the corrector? Is the corrector's vertical line perhaps in line with the two vertical lines on the primary? Lastly, on the corrector; is your vertical line closest or furthest from the focuser, or maybe at a random position? I am suspecting that this line will either be on the side of the hole closest to the focuser or the opposite side. I would really appreciate it if anyone with a factory original setup could have a look and let me know. Best, Coenie

No, it is the 64mm secondary version. The DS version seems to be a cosmetic upgrade from mine. I think the 52mm secondary version had the single speed focuser as well. The dual speed focuser was used for all 64mm models. That storage multiple should come in very handy for your future MN190 images. I will keep an eye open on Astrobin.

You are right, one would expect them to have corrected any mistakes in the later DS models at least From your signature it would appear that you own one of these. Mine is an earlier version. One interesting thing I have noticed is if you look at advertising images of the MN190 on various sites, there is definitely not a set and fixed position for the focuser bolts on the slide. This tells me that there is possibly some unique setup for each scope that is derived from maybe setting the specific secondary at a specified point. As the focuser is then moved to complete the setup, it ends up in different positions. There seems to be either a screw in the center of the slide gap, or close to the meniscus side of the slide. On my current setup I ended up very close to the upper end of the slide. I agree that SW would make the world much easier for a lot of us if they released their procedure. I once made contact with SW US and requested assistance. Their answer was that the secondary position is very critical and should best be done on a bench. Good luck with getting your ssd to work on the Pi.

My money is on neither. In my case it was not on the geometric center and at 2.5mm odd from geometric center, it was also not the expected offset. That said, my secondary was also definitely not offset by 3.8mm on the stalk originally. Maybe it was offset by the 2.5mm distance. I unfortunately did not take decent measures before I removed it. Since this dot placement can be found on a number of posts around the web, there had to be some reason why Skywatcher applied it as such. If you have the factory offset on the secondary, that dot should corresponds with it. I can recall when I still had it factory installed, when that dot was under the cross-hair it always resulted in a high secondary as the original poster here shown. If you rounded the secondary under the sigh tube, you would be around 1.5mm away from the factory dot. Your dot may still be where it should be relative to the amount of offset the secondary was given. At 2.5mm you will still show the slightly off-center illumination that you said you show. It would be highly unlikely in my view that you will find your secondary being offset by 3.7 or 3.8mm and the dot being where it is. For some reason SW only applied 2mm odd offset and marked it accordingly. It does make me wonder if someone could have gotten it wrong in the factory and used the raw offset number and not factoring in that you need to multiply it by 1.4142 to get the actual physical distance you need to move on the stalk. Just wondering... The flip side is also possible in that we are missing something about this optical design and how to offset its secondary.

Thanks for sharing Wim. That 2.7mm is not the distance your dot needs to be from the center of the mirror but the perceived distance when viewed at 45 degrees, The actual distance on the mirror that it has to shift is roughly 3.8mm. It was my experience that my factory fitted secondary was not offset on the stalk.

My 3.7mm is the actual distance on the face of the secondary. I mainly use this formula: A * (D-A )/ 4 * (f-H) A: Secondary minor axis D Primary aperture f: Focal length H: Intercept distance secondary to focal plane This gives the offset as 2.7mm which is the distance it would be when viewed from the eyepiece, along the diagonal face. I think the hypotenuse would then be 2.7mm with the adjacent side being 3.7mm (I see it is actually 3.8mm rounded up). One thing about this that I could not yet get clarity on is whether to use the 190mm aperture of the meniscus or the 200mm of the primary mirror in the calculation. The difference in this is 2mm.

Great information Wim, thanks so much for taking the time. Nothing wrong with your "tech" here. You mention: If you have a large enough secondary mirror, you can slide it along the plane of its reflecting surface without upsetting collimation or the Maksutov criterion. This point is what I tried to convey as the possible reason for Shibby's high position of the secondary. I am suspecting that there is a margin of movement available in the 64mm secondary option for the MN190, to move the secondary past the normal Newtonian collimation's optical center position. In so doing, because of the extra space available, you can collimate effectively and possibly only sacrifice the position of the fully illuminated field. I had a factory set MN190 that displayed exactly what Shibby is seeing. I also had what I assume would be the same factory applied secondary dot which is not the offset distance for these scopes but a number close to 2mm. I suspect that Skywatcher apply these dots to the secondaries after they have set the scope up on a bench. They get the primary and meniscus optical axis aligned by some method and then adjust the secondary and focuser position to intercept this. When it is done, where the secondary finds itself is marked with that dot. It does not follow a normal collimation procedure but it delivers the correct position for the rest of the scope's setup. From what I could see with my factory secondary, it was not offset on the stalk. The offset was worked in by shifting the illuminated field across the face of the secondary as it had sufficient room left to do this. That is why the dot is where it is. It is not the traditional offset dot you find on a Newtonian. Lastly, there is a reason why the focuser can slide. There is a sweet spot (I still need to figure it out), where the secondary position is optimal but not necessarily under the focuser. You then should not tilt the secondary to make up for this but rather slide the focuser over the right intersection. I need to read through your explanation again in the light of day and will add some additional comments tomorrow. This is a really great discussion in solving the enigma of the MN190. I hope Shibby does not feel like we completely hijacked his post.

Hi Wim, I follow your work on Astrobin as well. Nice work with the MN190! With the MN190's focuser being able to shift up and down the tube it is clearly there to ensure focuser placement in the correct place without moving the secondary. I was told by many other users before that the distance the secondary is dropped from the meniscus is very critical. I could not find anyone yet that could explain to me how you determine this distance though. The telescope tube's construction is such that you will always have the primary mirror and meniscus at the same distance from each other, unless you raise the primary mirror with springs or larger rubber o-rings. So getting them at the right distance to each other should almost always be as set by the factory. The problem is the secondary. But maybe it is not that difficult. The secondary needs to bring the light cone out from the tube to the eyepiece/camera. To do this in the most efficient way, it needs to find itself at 45 degrees inside the tube while capturing the whole light cone. If the whole light cone is caught with a center 5mm away from the secondary mirror's center, and if there is sufficient secondary mirror area available, and the secondary mirror remains at 45 degrees, you will meet the required condition. If alternatively you capture that light cone right in the optical center of the secondary, your collimation or details delivered from the primary/meniscus light cone will be identical to that from the 5mm mark. I am really struggling to understand how the secondary mirror's distance from the meniscus can influence the image if you meet the condition that you reflect the full light cone at exactly 45 degrees to the eyepiece. By rounding a fully offset secondary mirror under the focuser you would take care of the offset towards the primary mirror. Where the secondary sits on the center screw away from the meniscus should not influence the image as you have the ability to shift the focuser to compensate for the center shaft movement. As stated before, the only critical component in terms of the meniscus is that it should be on the same optical axis as the primary. Once that is met, you need to just introduce the secondary as described. Or am I completely wrong in my simplified view of it? I have been struggling to answer that the past four years with this scope, without much luck. Your views?

Your secondary position looks similar to what mine did before I removed the secondary and offset it by 3.5mm on the stalk. Yes I moved the secondary position... I still regret it although I now start to think that it is not such a bad thing. My factory fitted secondary had its dot applied roughly at 2mm away from center. Since the offset for this scope calls for around 3.7mm, centering the secondary dot under the chesire/sight tube, will result in the secondary sitting slightly higher up in the tube towards the meniscus (as viewed from the eyepiece. You should also find that a flat frame will show the image spot off to one side away from the center of the camera chip (unless you use a very small chip). If you round the secondary under the focuser (the secondary's outline), you are effectively aiming at a spot on the mirror roughly 3.7mm away from center. That happens automatically as you rounding the secondary mirror under the focuser is offsetting it by the desired number towards the primary. So doing that would not get you to point at the factory applied secondary mirror dot. And as noted, if you aim at the factory dot you end up with the secondary not rounded under the focuser. I personally think that this is not a problem since you have an oversized secondary mirror to work with. As you can see from your images, your primary reflection do end up concentric to the sight tube barrel/eyepiece barrel. Having the secondary sitting high up in the view only affects your image spot position. You can get perfect collimation with an oversized secondary without the secondary outline also concentric in the eyepiece in my view. Having moved my secondary further on the stalk now results in me getting a rounded secondary when I am aiming at my new secondary mark applied at 3.5mm. I also get perfectly centered illumination. I am not sure what camera you image with but using a DSLR with APS-C sized chip should not be able to get pin point stars in the corners. You should get round stars up to about a diameter of 80% of the field of view. Then the stars become elongated. This is especially noticeable in the corners. If you use a chip with a size below or up to 20mm, you should have round stars all the way to the corners. But I digress; your images taken with your scope clearly show that you have mastered the scope itself. Despite my views above, I would not recommend anyone move their secondary mirror if it has not been done before. I still need some answers from optical savvy people before I will start suggesting that.