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Jason D

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About Jason D

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    California, USA
  1. Jason D

    Collimating the telescope

    Can you share the "after" photo for comparison?
  2. Collimation looks good. To further perfect positioning the secondary mirror, I would give it a slight (and I mean a slight) counter-clock twist looking down the tube then recollimate only with the laser collimator. If this step will cause some headache then leave collimation as is and enjoy the views after star test confirmation. Again, the step I mentioned in this paragraph is a nice-to-have step -- not a required-to-have step. Jason
  3. Hello Alan, Refer to the following old post. It includes suggested steps to help you with your alignment: Once you complete the proper laser collimator steps (laser hits primary center then retraces its path back to the source), you are guaranteed to have the primary mirror reflection centered with respect to the focuser edge -- no guarantees with respect to the secondary mirror edge . Think of the secondary mirror as a window to the primary mirror reflection (or the frame of the wall-mirror I mentioned in my last example). Your job is to move the secondary mirror (the wall-mirror frame) in such a way until it is centered with the primary mirror reflections. The proper steps are in the post I referenced. Bear in mind that once proper collimation is met including a well-centered/rounded secondary mirror under the focuer, the secondary mirror silhouette (aka secondary mirror shadow) will appear shifted towards the primary mirror. In addition, ignore the reflections of your spider vanes. The vertical ones will appear slightly shifted towards the primary mirror. Here is a photo of what you should see (I took this one for my well-collimated scope) Jason
  4. Hello Alan, Imagine yourself standing in front of a wall-mirror. Imagine you are wearing a pair of special glasses that emit a laser beam. Imagine you position your head to directly look at your reflection with the laser beam emitted from your glasses reflecting back to its source. But something does not look rightt!!!! Your reflection does not seem to be centered with respect to the mirror frame. Why!!!! Because the laser beam does not interact with the mirror frame to tell you whether the mirror frame is centered with respect to the laser or not. Your friend comes along and slides the wall-mirror until your reflection is centered. While your friend is moving the mirror, the laser beam continues to be reflected back to its source. The moral of the above example is that "conventional" laser collimators can't be used to center the secondary mirror under the focuser because they do not interact with the secondary mirror edge. There are endless positions of the secondary mirror that will allow the laser beam hit the primary mirror center and retraces its path to the source but only in one position the secondary mirror will appear centered. A sight-tube is the best tool to use to center the secondary mirror. Do not get me wrong. I am not implying that the laser collimators are useless. On the contrary, they are good tools to use for collimation. Let e clarify, Collimation consists of three independent alignments: 1- Positioning the secondary mirror for optimal field illumination -- achieved when the secondary mirror appear centered under the focuser 2- Fine adjusting the secondary to eliminate focal planar tilt -- achieved when the laser beam hits the primary mirror center 3- Adjusting the primary mirror to eliminate coma -- achieved then the laser beam retraces its path back to the source "Conventional" laser collimators do a great job with #2 and #3 alignments but a poor job with #1 alignment. Interestingly, #1 alignment is the least important. Looking at your photo, it is clear that the primary mirror reflection is not aligned with the secondary mirror edge. If the photo was taken after completing alignment with the laser collimator then it is clear that the secondary mirror is not well-positioned under the focuser -- it is not centered/rounded under the focuser. Jason
  5. Jason D

    Collimation

    With regards to the crescent in the 2nd photo, the following diagram illustrates what I meant. Once the secondary edge appears concentric with the primary mirror reflection when looking down the focuser axis, the secondary silhouette is expected to appear shifted towards the primary mirror. If you make the secondary silhouette concentric with the primary mirror reflection then the secondary mirror edge will appear shifted towards the OTA opening. You can't have both the secondary silhouette and the secondary edge both concentric with the primary mirror reflection simultaneously. With respect to making fine adjustments to the secondary mirror, consider the following DIY hack that I came up with many years ago only if you are little adventurous: Insert or slide two washers that are cut from a soft plastic container -- preferably with a bumpy surface. Here is the USA, milk jugs make a great material for this hack. It will make secondary mirror fine adjustments much easier since plastic will give you little elasticity to make these fine adjustments. In general, the secondary mirror placement under the focuser is the least critical collimation alignment. Just do your best and accept the results. Honestly, your secondary mirror placements as shown in the two photos are not too bad. You can leave them as such and enjoy your scope. A slightly misaligned secondary mirror as shown in both of your photos will NOT impact the sharpness of your views. The only impact, which I doubt even the most experienced observers can notice notice, is a minute imbalance of brightness at opposite edges of the FOV when using low power EP. No need to take measurements as you stated. Relax, collimate, and enjoy your views Jason
  6. Jason D

    Collimation

    Photo in the first post has the signature of what is called rotate/tilt error. The secondary mirror silhouette is oval in shape with the major axis pointing in the 5:00 direction. Fixing it requires a slight clock-wise rotation of the secondary mirror looking down the tube. Photo in the sixth post has the signature of having the secondary mirror slightly high in the OTA. The mirror silhouette is concentric and you can see a small dark crescent on the secondary mirror in the direction of the OTA opening. Interestingly, the secondary mirror placement in the first photo is better than the second though both placements are not too bad. With respect to the last post, star test is mainly used to evaluate the axial alignment of the primary mirror. It is not ideal for evaluating the secondary mirror placement. Jason
  7. Thank you for your collimation help!

  8. No equations are required. You need to figure out the approximate distance between the camera's sensor and the OTA. You will need a 2" extension tube that will position the AC mirror at approximately same distance. You will need to improvise to make these measurements. Placing the AC mirror exactly at the focal plane (where the camera sensor is positioned to obtain focus) is not essential. Being very close to the focal plane is good enough. Jason
  9. Are you asking about how to determine the height of the 2" extension tube that can be used with your autocollimator? Jason
  10. The visual autocollimator is adequate for imaging reflectors. I have already explained in earlier posts that the error is small. I also provided a technique in this thread to further reduce the small errors. Catseye instructions do call for the autocollimator mirror to be positioned at or near the focal plane. I am not into astro-imaging and I do not know how common for imagers to also own a 2" extension tube to be able to use their imaging reflectors for visual observation. If owning a 2" extension tube is common with imagers then the extension tube can be used to eliminate all errors -- assuming the extension tube is of good quality. The new product ensures the overall mechanical quality of the autocollimator is perfect. Jason
  11. Hello Rich, When you mentioned the free evaluation stock for those involved in this thread, I assumed you got a free one for evaluation. I misunderstood. It has always been recommended to position the autocollimator mirror at or very close to the primary mirror focal plane for optimal precision -- it is written in Catseye instructions. Meeting this requirement is highly doable for visual reflectors but can't be done for imaging reflectors without using a 2" extension tube. Stacking a visual XLKP autocollimator on the top of a 2" extension will not have the same mechanical precision as using a single calibrated imaging XLKP autocollimator that comes with a elongated barrel. Jason
  12. Just saw your response -- two weeks late Hmmm, I am involved in this thread and I did not get any free "evaluation" stock Actually, since I do not own an imaging reflector, the new product is not for me. Rich, did you get one of the new XLKP products? If yes, which one did you get (barrel size)? How well did it work for you? I am very much interested in your feedback -- assuming you got one. Jason
  13. There are several misconceptions about collimation. One of these misconceptions is that secondary mirror adjustments via the set screws are completely independent and orthogonal to secondary mirror rotation. That is, starting off with a perfectly collimated scope, once the secondary mirror is physically rotated by a small amount there is nothing that can be done to undo the impact of the rotation -- incoming star light will never be perfectly aligned with the EP. Mathematically speaking, adjusting the secondary mirror via the set screws does have a "rotation" component and therefore it is possible to undo the impact of the secondary rotation and restore perfect axial alignment -- without physically rotating the secondary mirror back to its original position. I know this does not make sense. Here is a hypothetical example: Assume the secondary mirror stalk has a hinge. Start off with a perfectly collimated scope. Now flip the secondary mirror to face the opposite side of the focuser via the hinge. The mirror is still at 45 degrees but facing the opposite direction. This setup is equivalent to physically rotating the secondary mirror 180 degrees. Similarly, by adjusting the secondary mirror using the set screws we cab achieve a an equivalent effect to small amount of rotation. The rotation component is small; therefore, it can only undo small amount of physical rotation to the secondary mirror. If the secondary mirror is physical rotated by a large amount then we will need a huge amount of tilt adjustment. You might wonder if I start off with a perfectly collimated scope, rotate the secondary mirror by a small amount, recollimate then the final result can't be exactly the same!!!! You are correct. Even though axial alignment is exactly the same (incoming star light is perfectly aligned with the EP), the secondary mirror will not appear perfectly circular under the focuser. It will be slightly oval but that has no impact on the central area of the FOV and a minute impact only on the illumination of the stars around the FOV edge -- which might be noticeable via astrophotography but not visually. Jason
  14. Suppose I start off with a perfectly collimated reflector then I follow these steps: 1- I loosen the secondary mirror set screws by a tiny bit 2- I rotate the secondary mirror by a small amount 3- I insert a qualify laser collimator then adjust the secondary mirror to redirect the laser beam back to the primary mirror center but by only using the set screws -- I do not hold the secondary mirror stalk and rotate it whatsoever. Adjustments of this step are only carried out by touching the secondary mirror set screws. 4- I finally readjust the primary mirror to redirect the returning laser beam to its source. In your opinion, can you describe the negative impact of the above secondary rotational error on the view at the EP? For example, will coma get worse? Jason
  15. Just curious. Can you clarify what you meant by secondary mirror rotational errors? And how do you use the collimation cap to minimize this error? Jason
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