Jump to content




Jason D

  • Posts

  • Joined

  • Last visited

Everything posted by Jason D

  1. "single diagram is going to take away the simplicity I was trying to achieve" My suggestion is to show a diagram without mentioning whether the secondary was mounted centrally or with an offset. Just don't mention how the secondary mirror was mounted since such an info will have no significance on collimation. That will keep it simple. "But not if the secondary reflection (x2) were elliptical indicating a secondary error?" Well, the "error" has to be taken within context. Achieving axial alignment is the most important goal of collimation. Axial alignment is achieved when the eyepiece axis points directly at the primary mirror center and the optical axis of the primary mirror points back at the eyepiece center. The secondary shadow elliptical "error" you have referenced has nothing to do whatsoever with axial alignment. In that sense, the "error" impact could range from nothing to something. If the elliptical shape is minor and the scope is used for visual observation then the "error" can be safely ignored. If it is major and/or the scope is used for astrophotography then it needs to be corrected. The elliptical "error" only impacts the illumination of the FOV which impacts astrophotography more than visual observation. But sometimes the elliptical "error" is not an indication of a secondary mirror rotation/tilt error but rather an indication of a slightly off-centered secondary mirror in the OTA and/or a non-squared focuser. In this case, achieving axial alignment will demand rotating/tilting the secondary mirror to achieve axial alignment which will give its shadow an elliptical shape. Check the attached animation. In each frame, my scope has achieved axial alignment yet only one frame includes the optimal placement of the secondary mirror. My scope happened to be mechanically aligned so the optimal placement will not show an elliptical shadowy shape of the secondary mirror. If I intentionally adjust the vanes of my scope to shift the whole secondary mirror assembly to the left or right perpendicular to the focuser axis then the optimal placement of the secondary mirror will show a slight elliptical shadowy shape. Jason
  2. You stated: a=b - That is my goal but IF a user had chosen a non-offset arrangement then it is right to say that IF i=j THEN a≠b SHOULD be true? It is true if i=j then a≠b. But this statement is true regardless whether the secondary mirror was mounted centrally or with an offset. You stated: e=f - Incorrect labelling on my diagram. See below We are in agreement. You stated: g=h - Added to represent a non-centred secondary and therefore highlight an error. Above statement is true regardless whether the secondary mirror was mounted centrally or with an offset. For the most part, there is no difference in the collimation view between a scope with its secondary mirror mounted centrally or with an offset. Interestingly, the only difference is the apparent relative placement of one set of the vanes. For a scope with a secondary mirror mounted centrally, the vanes will appear centered with respect to the secondary shadow (figure 2 below). For a scope with a secondary mirror mounted with an offset, the vanes will appear centered with respect to the secondary mirror (figure 1 below). For example, I can tell the secondary mirror was mounted with an offset in your illustration. Below is the view through my scope with the secondary mirror mounted centrally: Whether the secondary mirror mounted centrally or with an offset, the optical axis of the primary mirror will intersect the secondary mirror at the same spot, hence, the view (with the exception of the vanes) will look the same. "New Model" figure corresponds to a scope with a secondary mirror mounted centrally "Classical Offset" figure corresponds to a scope with a secondary mirror mounted with an offset.
  3. a=b (or b>a for a centred alignment with no offset)* [[ a=b should be the goal regardless whether the secondary mirror was mounted centrally or with an offset ]] c=d e=f [[ This will always be true no matter what you do. ]] g=h [[ This should be ignored. If collimation is done correctly for a mechanically aligned scope then g=h will be true. That is, there is no collimation step specifically for g=h. It is an automatic result of a good collimation ]] i < j IF a=b [[ a=b should always be the target. i<j will always be true for a good collimated scope regardless whether the secondary was mounted with an offset or centrally.]] or i=j IF *b>a by the correct offset amount [[ This statement should be dropped. It is an incorrect target ]] Jason
  4. I believe the issue reported by the OP could be (and I am speculating) due to the holographic optical piece mounted on its metallic casing at an angle. Referring to the OP's original photo, note how the pattern looks OK along the 2:00<->8:00 line but looks at its worse along the 11:00<->5:00 line. Jason
  5. That does not look right. I would ask for a replacement. Here is mine. Don't mind what I was trying to do in these photos -- just check the uniformity of the rings.
  6. Your collimation as shown on the above photo looks good Jason
  7. Secondary mirror silhouette will always appear skewed towards the primary mirror. It is just more paramount for fast scopes. The cross hairs in the photo are the spider vanes. These should be ignored when collimating. Refer to my photo in the pre reply. My scope's spider vanes intersect slightly to the right of the center spot.
  8. It should look like that (with an offset). Here is a photo of my collimated scope
  9. That does not mean much with respect to collimation. I would ignore that observation if I were in your shoes. Your photo does not provide enough information to evaluate your collimation. The primary center spot is not showing and the focuser edge is also not showing. A good collimation cap will have a reflective (or at leave a white) underneath surface.
  10. As the person who came up with the particular center spot shape (called Hotspot), my main target was to improve stacking the center marker reflections via Catseye XLKP autcollimator collimation tool. The tool will show several reflections of the center marker and some of these reflections will be 180 degree rotated. To stack the center marker reflection against its 180 degree rotated reflection, the radioactive symbol shape (the Hotspot) works best. Another advantage is aligning the Hotspot against the ring of the Catseye cheshire. See attachments. The Hotspot center marker is irrelevant to the collimation cap. Jason
  11. Based on your description, I suggest you leave the secondary mirror alone. Just ensure the primary mirror is adjusted then enjoy the views. Jason
  12. The following statement in the book "For uniform illumination of the focal plane the diagonal must be displaced away from the eyepiece and toward the primary mirror by equal amounts" is wrong!!!! Jason
  13. What you have described only checks for the primary mirror alignment which is the most critical collimation alignment -- the one responsible for coma. However, the method you have described does not evaluate the proper placement of the secondary mirror unless the secondary mirror is grossly (and I mean grossly) misaligned. Jason
  14. I presume the middle figure below matches your concern when mounting a secondary mirror without an away-from-focuser offset , am I correct? That is, as shown in the same middle figure, part of the reflected star light cone will miss the secondary mirror unless it is either properly offsetted away-from-the-focuser or the secondary mirror is large enough to intercept the whole reflected light cone, am I still correct? Well, this is a common misconception!!! If the secondary mirror is mounted with the proper away-from-focuser offset then you will get the left-hand side figure which is the "intuitive" picture we have in mind when we think of the reflected cone. However, when the secondary mirror is mounted without any away-from-focuser offset then we can still intercept the whole light cone as shown in the right-hand side figure. As we go through the collimation steps, we will end up tilting the primary mirror towards the focuser without even knowing it. Now the central star in the FOV is not the one intercepted by the OTA axis and our setup will intercept the whole reflected light cone of the FOV central star. Both methods of secondary mirror mounting will provide perfect collimation if the proper steps are followed as shown below: There is one exceptions when it is desired to mount the secondary mirror with the proper offset which is to improve DSC accuracy but the benefit is small. There are two exceptions when it is a MUST to mount the secondary mirror with the proper offset which are: 1- When tilting the primary mirror causes the incoming parallel light to be clipped by the OTA edge. This only happens with the OTA opening it too tight -- as large as the primary mirror. 2- When the Newtonian has a corrective lens mounted at the OTA opening but most Newtonians do not have it. One more thing, it is typical for off-center stars to have their reflected light cones clipped by the secondary mirror even for a perfectly collimated scope as shown below: Jason EDIT: Added missing last attachment
  15. The slight misalignment you referenced is negligible and has no impact on your views. I would definitely ignore and not be bothered with it. Jason
  16. Hello Tim, With proper knowledge, quality laser collimators are great and accurate collimation tools. The rotational error you have described is hardly noticeable at the eyepiece for visual observation. It does not impact coma or focus but rather it has a "theoretical" impact (that is hardly noticeable visually) on the distribution of field illumination within the FOV. It should be known to any laser collimator owner that laser collimators can only used to achieve axial alignment -- when the eyepiece axis points at the primary mirror center and the primary mirror axis points at the center of the eyepiece center. Unless the laser collimator is equipped with a holographic attachment, it can't be used to optimally center/round the secondary mirror under the focuser simply because a typical single beam laser collimator does not interact with the secondary edge. Jason
  17. To OP, Check this post and the next post https://stargazerslounge.com/topic/186945-collimation-woes-laser-vs-cap/?do=findComment&amp;comment=1946524 Jason
  18. Can you share the "after" photo for comparison?
  19. 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
  20. 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
  21. 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
  22. 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
  23. 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
  24. 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
  25. Are you asking about how to determine the height of the 2" extension tube that can be used with your autocollimator? Jason
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.