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Jason D

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About Jason D

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  1. 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
  2. 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
  3. 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
  4. Yesterday I found out that Catseye is now officially offering new versions of the XLKP autocollimator with longer barrels for astro-imagers. I could not help but to recall this interesting thread which is quite relevant to the new offering. All the details are on Catseye website https://www.catseyecollimation.com/ Jason
  5. Can you clarify what looked a bit off? Was it the collimation cap pupil reflection with respect to the primary mirror center spot? Was it the secondary mirror placement under the focuser? You might the following post (and the one below it) helpful: Jason
  6. To OP, Laser collimators can't be used to center/round the secondary mirror under the focuser. You need to use your sight-tube to center/round the secondary mirror under the focuser first then use the laser collimators as you did. You might need to re-iterate. Check out the following old post for your reference: Jason
  7. collimation fast scope

    To the OP, In the photo of your first post in this thread, you were trying to align the cheshire/sight-tube tool cross hairs with the pupil reflection of the tool. This is incorrect. You are supposed to align the cross hairs with the reflection of the primary mirror center spot. With respect to the secondary mirror silhouette for a well-collimated scope, it is not an elliptical silhouette but rather a shifted circular silhouette which gives the illusion is it elliptical. See attachments below. The secondary mirror silhouette is actually circular in the photos as well as the illustration. The shift is always towards the primary mirror. The reason the silhouette appears shifted is because it shows the secondary mirror from a virtual point behind the primary mirror. It is a misconception that only fast scope will show a shifted secondary mirror silhouettes. All Newtonians will show the same; however, it is just more apparent for fast scopes. The reason the secondary mirror appears as a silhouette is because it reflects the dark area around the focuser opening from inside the OTA. If you were to attach a template around the focuser opening from inside the OTA as shown below, the secondary mirror will no longer appear as a silhouette: Jason
  8. Hello, I am not sure if you heard back from Jim yet -- it was a national Holiday here in the USA yesterday (Labor Day). A while back Jim stated that he is planning to offer an autocollimator with an extended barrel specifically for imagers . Check out this page from Catseye official website: http://www.catseyecollimation.com/autocol.html#xlkfeatures Refer to the info under the section titled "ATTENTION IMAGERS WITH ASTROGRAPHS" Jason
  9. Collimation Help.

    To emphasize, when the secondary mirror is mounted without an offset then the spider veins reflection will be centered with respect to the "shifted" secondary shadow as shown in figure 2. When the secondary mirror is mounted with an offset then the spider veins reflection will be centered with respect to the collimation cap reflection and the center spot reflection as shown in figure 1.
  10. Collimation Help.

    You are fine. The cross-hairs do not need to be centered with your "circle 2". It depends on how the secondary mirror was mounted. If it was mounted with an offset then the cross-hairs will not be centered -- just like your illustration. If the secondary mirror was mounted without an offset then the cross hairs will be centered. The general rule is to ignore the cross hairs reflections when collimating. As far as "circle 2" being shifted, it is normal. I am including few illustrations to explain the reason for the shift: Edit: "Cross hairs reflection" term used in this post should have been replaced by "spider veins reflection". It should not be confused with "cross hairs" of the cheshire/sight-tube collimation tool.
  11. That makes sense. XLKP is easier to deal with for telescope with longer focal lengths. I am glad to hear that the results are looking better -- though not perfectly -- for your other scope with shorter focal length. Report back once you get a chance to use the scope for astrophotography Jason
  12. Can you re-iterate between P&2 via offset (secondary) and Hotspot&ring via Telecat (primary)? when done, check the central pupil of the XLKP It is late here in California -- almost 1:00AM... I will continue this exchange in the morning. Jason
  13. First attachment explains the concept of axial alignment Second attachment explains how the P&2 stack via the offset pupil of the XLKP is only sensitive to the tilt between the AC mirror plane and focal plane Third attachment explains how the HotSpot alignment with the Telecat or Blackcat is only sensitive of the displacement between the Telecat/Blackcat center and the focal point Both alignments are "independent" and should be able to achieve