How to Collimate your Newtonian
Posted 30 November 2020 - 03:15 PM
Posted 20 December 2020 - 01:15 PM
My last collimation. I didn't mess up with the secondary this time, as Vic said it's probably ok for visual.
The primary isn't 100% since I took this photo at almost horizontal. It's actually centered at around 60-70 degrees.
Posted 20 December 2020 - 04:44 PM
Doesn't look bad horizontal (all annotated circles are concentric)...
Edited by Vic Menard, 20 December 2020 - 04:50 PM.
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Posted 22 December 2020 - 07:08 PM
Is that good? My 12 inch secondary's spider is a bit of centered in mine right now. I have the circle right on the spot but I think I forgot this step. Sorry, when it comes to this, I'm a newbie, since my 8XTi never really needed collimating after the vendor at the Observatory ( 'Scope & Astronomy store I now think sadly is defunct..) It always held, for many years. But not the XX12i, it can go out fairly easy too. I need to learn to do this both quickly & then the more refined ways, like with Lasers. Also will try a Star test. The Manual supplied with my scope is not entirely great here.. Help would be appreciated!
Sorry, I know its the 900 millionth request!
Posted 23 December 2020 - 07:04 PM
Thank you , Don. I don't have any of the tools yet, except the included cap, so I'm just trying to do a rough ( field?) collimation. I'll have to order the Laser sight, etc. after the New Year. I do want to do the Manual instruction & also the star collimation. My only problem with that is I cannot really get a good view of Polaris, which every one says to use for this. I assume because it doesn't move much, but here its lost in the DFW light dome.
Posted 24 December 2020 - 01:24 PM
Maybe apply a small amount ofgrease to the ends of my bob's knobs where they contact the secondary's mounting plate to keep them from spinning the secondary?
Edited by SeaLint, 24 December 2020 - 01:27 PM.
Posted 24 December 2020 - 02:00 PM
You have a couple of choices, but first, check the tips of your Bob's Knobs screws and make sure they're slightly beveled/rounded so they don't gouge the contact side of the secondary mirror stalk.
Now, to fix the gouges you will likely find on the contact side of the stalk you have a couple of choices. The quick fix is to sand down any hard edges and then place a large stainless steel fender washer that covers the entire contact area so your Bob's Knobs have a brand new push plate. Or, you can use the milk jug washer fix Jason described some years ago https://www.cloudyni...asher-question/
(OR, you can use both, a stainless steel washer and milk jug washer. Just don't use grease! )
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Posted 24 December 2020 - 06:47 PM
I'm going to have to keep fiddling with it until I get it right. The Secondary is the one that makes me somewhat nervous frankly. I could screw this up. I can get the Primary's dot centered any time. It got badly off after I moved the tube around, so I had to move it back. But I believe its my Secondary that is not quite right at the moment.
Posted 26 December 2020 - 02:21 PM
Posted 26 December 2020 - 02:39 PM
You should see an image like this through the focuser. Your secondary outer diameter will probably appear larger and closer in size to the inside of the sight tube.
You needn't worry about offset, since the process of collimation will automatically build it in, whether the secondary is centered or not.
Just note you cannot and will not get the outline of the secondary silhouette concentric to the other images:
Turquoise: inside front edge of sight tube
Green: outside of secondary mirror
Red: outside edge of reflection of the primary mirror
Purple: outside edge of reflection of secondary silhouette (shadow)
Yellow: reflection of tool in the focuser
All are concentric except the purple.
Edited by Starman1, 26 December 2020 - 07:11 PM.
Posted 26 December 2020 - 05:04 PM
I'm unable to achieve a concentric view through the focuser draw tube.Im beginning to wonder if i need to offset. I have the Orion 10" f/3.9 astrograph but Orion is no help. If it has offset,is it axial,radial or both,and how much. Please help.
Offset will cause the reflection of the secondary mirror to be off center. Three edges should be concentric. The edge of the focuser draw tube or combo tool, the actual edge of the secondary mirror, and the reflection of the primary mirror.
The edge of the primary mirror is concetric with the focuser draw tube or combo tool when the focuser axis is aligned to the primary center. When the secondary mirror is centered under the focuser, your field illumination will be centered in the field of view.
As Don said, don't expect the dark reflection of the secondary mirror to be centered on the focuser axis. It's likely offset away from the focuser, already. So, it sits lower in the tube and it looks offset away from the focuser as seen reflected in the primary mirror.
Edited by Asbytec, 26 December 2020 - 05:07 PM.
Posted 05 January 2021 - 06:42 PM
I had a few questions about collimation that I'm hoping to get some help on.
1. As far I can understand, FAE and PAE are independent of each other, and can be exclusively made to be 0.
-- after aligning the focuser axis to the optical center of the primary, FAE = 0, PAE ≠ 0.
-- after aligning the primary axis to that of the focuser, FAE = 0, PAE = 0.
-- tip the eyepiece slightly (which would happen when eyepieces used after collimation is thumb screwed hard), so the its axis and that of the focuser aren't parallel, FAE ≠ 0, PAE = 0. But, in this case, is PAE still strictly 0? PAE is defined as the distance between the primary mirror's axis and that of the eyepiece at the focal plane. With the eyepiece tipped, I don't think the primary mirror's axis is centered on the focal plane anymore.
2. Vic Menard has listed collimation tolerances down to fractions of a millimeter. Are such tolerances achievable merely by eyeballing using Cheshire+sight tubes? Are star tests then needed to check this?
3. In reflectors, and probably in refractors too, the primary isn't held down by their housing (that is, there is a few mm of play between the primary mirror clips and the upper surface of the mirror.
-- Won't this cause the mirror to tip slightly when the OTA is close to horizontal, while either viewing or collimating?
-- Can the tolerances mentioned in the above question be achieved with such play in primary housing?
-- My OTA always remains horizontal when stored. Is this a bad idea, especially when its on the back seat of the car since it might keeping bumping along during the ride?
-- Is there a recommended altitude the OTA needs to point to during collimation? What would happen to the collimation tolerances when the OTA is brought to horizontal? Would it be retained when it is brought back up to vertical?
4. Instructions about collimating Newtonians mention that the secondary's geometric axis should be offset from that of the OTA by causing it to appear circular through the focuser. Then, the optical axis of the secondary need to be made to fall on the center of the primary by tilting it away from the focuser. I'm guessing this latter operation should be done only by unscrewing the secondary screw that lies along the spider vane coincident with the focuser axis, and screwing the other secondary screws the opposite way.
This implies that before performing this operation, the secondary mirror should appear round through the focuser while not having any tilt. How can this be ensured without having the OTA vertical while centering the secondary mirror?
5. My Astrosystems Lightpipe has what appears to be silicone blobs on the edge that contacts the focuser. This causes the meeting point of the crosshairs to revolve when the Lightpipe is turned in the focuser. Is there a way I could eliminate this while not compromising on how the Lightpipe registers at the focuser?
6. How can I ensure that the physical center of the primary mirror is also its optical center, apart from star testing it?
Many thanks for your time.
Posted 05 January 2021 - 09:46 PM
Let's see if I can help.
1.) Tipped eyepiece. The lateral movement is small, perhaps a fraction of a hundredth of an inch, so impact on PAE is likely to be insignificant. Potential tilt error (FAE) could be significant (the reason behind Howie's Parallizer). Your question then becomes, are PAE and FAE still within tolerance. And that depends on the scope, and the application.
2.) Let's say the allowable PAE is 0.02-inch. This means the allowable Cheshire read is 0.04-inch. If you have a donut center marker that's 0.6-inch diameter, and a Cheshire perforation that's 0.7-inch diameter, a corrected PAE annulus would be 0.05-inch wide, and a 0.04-inch error would make one side of the annulus nearly 0.1-inch and the other side 0.01-inch, a ridiculously easy read. And I always verify the read on s star, at least once--then I trust my tools. (I still assess every image in the eyepiece, center of field and edge of field, it's just common practice when you're looking for the best possible image performance.)
3.) Will the primary mirror tip? Only if you observe below the horizon. Properly configured, there is virtually zero play in the primary/cell registration. I wouldn't "store" a large Dobsonian OTA horizontally unless you remove the primary mirror first. A smaller (10 inches or less?) OTA with a reasonably thick primary and a decent cell would probably be fine. But an easily removable mirror/cell is preferable. Typically, routine collimation should happen during cool down with the scope pointed upward around 45- to 60-degrees. The normal observing range is usually about 15-degrees to 90-degrees (unless you're looking at Eta Carinae from the Florida Keys). It's unlikely that the collimation will hold to better than +/-0.02- to 0.03-inch the closer you get to the horizon, but magnification usually drops with the move, relaxing the tolerances (a bit). For high magnification planetary work, I can quickly assess (and correct, if needed) PAE (and FAE) with the scope pointing at the planet (usually it's just PAE).
4.) Don't worry about offset precision or any other geometry precision (spider/focuser/UTA/etc.). Just make the three circles (bottom edge of the focuser, actual edge of the secondary mirror, reflected edge of the primary mirror) concentric. This will center and balance the field illumination. The axial alignments deliver image performance.
5.) I'm not sure. If you have a 2-inch focuser and a 1.25-inch LightPipe, you might try a Glatter Parallizer.
6.) While a star test is the final arbiter, the general consensus is that a good optician will deliver a figure of revolution with its center within a hundredth or two of the measured center of the mirror blank. My 22-inch f/4 primary has it's center spot located within about 1/2mm (0.02-inch) of the measured center. My high magnification PAE tolerance is about 0.35mm (0.014-inch), but I rarely use more than about 500X (moderate magnification, so relax the tolerance a bit), and a calibrated Cheshire seems to consistently get the job done.
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Posted 06 January 2021 - 02:50 AM
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Posted 12 January 2021 - 11:25 AM
I posted this quite a bit earlier (7/23/04 "Secondary Mirror Alignment", now in "The Best of Reflectors" thread.) There are, of course, other ways to align the mechanical focuser axis. And even though the focuser axis is the reference axis for optical collimation, this "mechanical" adjustment is really just a good starting point...
This is more or less the procedure I use.
There will always be a question of how important it is and I guess that depends a great deal on the application..
For imaging we can see from the OP that there was a problem, and the focuser being tilted could indeed give the symptoms mentioned.
Another application where the focuser tilt is critical is the use of binoviewers. Because the binoviewer light path is very long, if the light cone enters at a slight angle, it can cause internal reflections off of the prism walls and the collimation would appear to change when going from the most inward focuser position to the most outward position.
I struggled with off axis glare when using binoviewers when I first got my dob and in the end, I had to go back to the focuser and use a proceedure very similar to the one above, and this fixed the issue.
The problem is simple: If anything is off, it is off, and the only way to compensate for it is to make something else off so that in the end, you get a center dot in the laser collimator, but to do so, you usually have to tilt the secondary from side to side or to the point where the primary does not sit exactly centered in the Cheshire view of the secondary when the laser shows collimation, or the laser dot does not hit the exact center of the primary when the Cheshire shows a perfectly centered primary.
For visual use, as long as the in focus star at the center of the field shows no coma, then this is all that is needed. It is generally not at all necessary for everything to be exact as long as this condition is met. For imaging or applications like binoviewers, focuser tilt can have very serious effects and should be eliminated.
Posted 12 January 2021 - 01:25 PM
...For visual use, as long as the in focus star at the center of the field shows no coma, then this is all that is needed. It is generally not at all necessary for everything to be exact as long as this condition is met. For imaging or applications like binoviewers, focuser tilt can have very serious effects and should be eliminated.
I was with you until I read this paragraph. For visual use, a tilted focal plane can be overwhelmed by off axis coma, so the high magnification tolerance is about 3- to 4-percent of the primary mirror diameter (linear distance of the laser dot from the center of the primary mirror). But with a coma corrector, the off axis coma can be eliminated (down to about f/4) and the high magnification tolerance is significantly reduced (0.5-percent of the primary mirror diameter for a Paracorr 1, about 0.2-percent for a Paracorr 2).
Note that this doesn't require a "squared" focuser--just an accurate focuser axial alignment (relative to the primary mirror center marker).