Newtonian telescope collimation
Posted 23 September 2020 - 10:36 AM
Posted 23 September 2020 - 10:46 AM
I don't see the secondary mirror reflection as a perfect circle.
Posted 23 September 2020 - 12:29 PM
That appears to be "not bad" and usable. You'll want a slightly better picture that shows the end of the focus tube better to get better advice.
Assuming you are a newbie and this your first telescope, I would suggest using the telescope as it currently is for a while. Just enjoy using the telescope for a while and get familiar with it and the skies. Right now is great time to have and use a good telescope to observe the summer primary planetary parade! Don't miss it.
Unless you have very clear dark skies any additional collimation improvements would likely not be be noticeable to most newbies - AND the risk of misaligning the optics is quite high if you are new to collimation. If you muck it up right now, you'll miss a lot of the primetime planetary viewing opportunities that are currently available.
Best way to learn how to collimate properly is to join a local astronomy club and find a experienced telescope owner to be your mentor and adviser. If they can come and show you how to collimate your scope directly, it might take an hour of your time and that's all you will need. That could be iffy during the pandemic, but they can still act as a direct adviser via email or video chat, which might take a couple of sessions.
If you want help via our forum, that can take weeks and you will have a variety of different suggestions from multiple people (with varying levels of knowledge and skills). The ultimate guide on CN for precise Newtonian collimation is posted here. This specific approach requires a Cheshire and a laser tool. It is well worth reading and trying to understand what they are doing - unfortunately words don't work nearly as well as being shown how to do it.
Everyone considers Vic Menard as the ultimate CN guru on collimation. He will very likely see and comment on your post. Always listen to what Vic; you cannot get better advice.
For newbies, I like to keep it simple and have them learn to first use just a Cheshire to help them understand what each of the rather complicated and precise steps actually are doing and how they affect the optical path.
My one big No-No advice for newbies is to avoid trying to adjust the secondary mirror until you have someone right there with you that really knows how to collimate a scope. Adjusting the secondary mirror is where collimation nightmares begin! Secondary adjustments require a very specific sequence of steps that have to be done properly and in order. Fortunately, once you learn how to do it properly, it becomes quite easy - a lot like learning to ride a bicycle.
Edited by JohnBear, 23 September 2020 - 12:32 PM.
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Posted 23 September 2020 - 01:49 PM
OK--here's what I see.
The focuser axial alignment looks pretty good (your sight tube's cross hairs are aligned to the primary mirror center marker) and the primary mirror axial alignment also looks good (the primary mirror center marker is centered in the bright Cheshire ring). These two alignments deliver good star images. What I can't see is the actual edge of the secondary mirror.
I checked your previous posts and see that you have an ES 8-inch Newtonian--I'm guessing the f/3.9 astrograph. If that's the case, it looks like your secondary mirror isn't fully offset. If your secondary mirror offset is incorrect, it will have a measurable (CCD Inspector, etc.) impact on the illumination profile, and a (probably) small impact on your final image.
If, on the other hand, you have the ES 8-inch f/6, you should be good to go.
Posted 23 September 2020 - 01:52 PM
But to answer the original question: the silhouette of the secondary in the primary's image is not meant to be centred with respect to the rest, due to perspective effects (the close end of the secondary appears larger than the far end, which ultimately has an effect on where you place it and indirectly on where the optical axis hits the secondary).
Edited by sixela, 23 September 2020 - 01:53 PM.
Posted 23 September 2020 - 02:21 PM
But to answer the original question: the silhouette of the secondary in the primary's image is not meant to be centred with respect to the rest...
Good catch! I read right over that question in the second post. It would be helpful if we knew the scope's primary mirror diameter, the focal ratio (maybe even the secondary mirror minor axis), and any modifications. It would also help if we could clearly see these three circles: the bottom edge of the sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror*. If that's impossible with the Cheshire/sight tube combination tool being used, then a view through a collimation cap would also work.
* A white piece of paper placed against the inside wall of the tube assembly opposite the focuser (behind the secondary mirror) will make both the secondary mirror and the bottom edge of the sight tube much easier to see.
Posted 23 September 2020 - 02:39 PM
So I decided to replace the whole secondary mirror holder with this unit.. It is solid and the secondary mirror Is dead center now...
It is f/4 800mm ES telescope.
I use hoytech laser collimation but the star test shows very bad shape stars, diffraction is very bad and not even. So it decided to use the Cheshire collimation eyepiece and this is the first time I use it...
Tonight I will take photos of the Pleiades and see how it goes..
Posted 23 September 2020 - 02:42 PM
Posted 23 September 2020 - 03:41 PM
Posted 23 September 2020 - 05:27 PM
Posted 23 September 2020 - 05:55 PM
Quite - but that could be pinched optics. You said you removed the primary clips. What did you replace it with?
But it looks more like simply clipping of the image (in this case the top) by the secondary.
Your second image is from a star very far from the centre of the field, and that's normal, although it means illumination isn't very high at the edge of that field. If a secondary is well sized, you have an area where light bundles for stars are not clipped, and then the bundles are progressively clipped more and more as you stray to the edge of the field, in a radially symmetric way.
In that last photo through the focuser tube you seem to be too far from the secondary, so much that we can't see the whole primary. Which also means we can't see if the primary's image is centred. We can guess that the image of the centre spot is decentred in the secondary, though (it's overexposed but we can still guess it's not in the middle), which would indicate that your fully illuminated field is not centred, i.e. that the secondary is not placed well.
But since the relative placement of the primary's image with respect to the apparent centre of the secondary (which is not its geometrical centre!) changes if you rack the focuser in or out, to know whether the secondary is placed well you'd have to take a picture from where there is a just a visible black edge around the primary (i.e. where you are seeing the entire edge of the primary and a narrow ring of black around it -- as narrow as possible although it should be there over the entire perimeter).
Posted 24 September 2020 - 08:05 AM
Hello everyone this is my collimation eyepiece view... Is this correct? What should I do else? Thanks
Looks to me like you have some parallax from the camera as evidenced from the foreshortened bottom half of the secondary and the fact we can see the root of one spider vane at the upper left (white circle) and not of any other vanes. The camera is probably a bit low and looking upward toward the focuser.
You can deal with parallax error, but the reason this is important is because your cross hair does appear to cross somewhere in the central perforation (best guess blue lines). In other words, the cross hair looks aligned on the central donut from that angle. Almost like you aligned the cross hair with the center marker when framing the picture.
I believe viewing from top dead center of the Cheshire Pupil, indicated by the red cross hair, I think you will find the (blue) cross hair is actually below and outside of the central donut. This means the center mark is also not really centered in the bright Cheshire ring as shown, either.
Try to find the sweet spot looking straight down the Cheshire pupil of the site tube, then tilt the secondary so the central donut moves onto the cross hair as viewed from that centered perspective. Then tilt the primary to bring the bright Cheshire ring directly over and centered on the white donut. Take another picture.
Edited by Asbytec, 24 September 2020 - 08:11 AM.
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Posted 26 September 2020 - 01:46 AM
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