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Collimating a Maksutov-Newtonian with a Howie Glatter Laser Collimator
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Collimating
a Maksutov-Newtonian
with a
Howie Glatter
Laser Collimator
This procedure describes how to collimate a maksutov-newtonian telescope such as those from Skywatcher, Explore-Scientific, APM, Intes or Intes-Micro. The same procedure applies to conventional Newtonians as well, apart from the corrector plate.
6.1 Align laser on the centre of the primary – with a centre spot
6.2 Align laser on the centre of the primary – without a centre spot
6.3 Align primary to secondary mirror – first attempt
6.5 Correct the focuser offset
6.6 Secondary mirror – final alignment
1. Introduction
A basic understanding of these telescopes is assumed:
· How a Newtonian telescope works,
· How the telescope is constructed,
· The scope has adjustments for the corrector cell, the secondary mirror cell, and the primary cell, and the focuser; and
· How to safely handle the optics without damaging them, notably the secondary and corrector as these will be removed and/or cleaned at the end.
Warning
During this procedure (depending on the scope) you may have to remove the corrector with the secondary and re-install it a few times to get the secondary mirror alignment right.
In particular the coating on the secondary mirror MUST NOT BE TOUCHED by anything – fingers, tools in particular as the coating is very fragile and easily scratched. Similarly the corrector coatings are fragile. If you are not confident that you could do this correctly without damaging the optics, please do not attempt this procedure, find someone who can.
2. Equipment and tools
This procedure assumes you have the following:
· A Howie-Glatter laser collimator, with holographic filters for (i) the centre dot and (ii) concentric circles pattern;
· A ruler, long enough to reach across the aperture of the scope, and a pencil;
· Tools to adjust the telescope, eg. Allen-keys, and screwdrivers that properly fit the collimation screws;
· A clean dust-free workspace with a soft surface so that if you drop your optics, it won’t be damaged;
· A small dish to collect small screws (Russian maks);
· Materials to wash and clean the corrector - a tub of clean water with dishwashing detergent, a bottle of distilled water for a final rinse, some clean gloves, and a clean lint-free cotton cloth (well-washed singlet or T-shirt) to dab the remaining drops.
3. Mechanical concepts
This basic configuration is shown in figure 1, where everything has been collimated.
Primary mirror tilt – virtually all maksutov-newtonians have a primary mirror cell with 3-point adjustments at the rear.
Primary mirror centre-spot – ideally your primary mirror should have a centre-spot, but if not, there is a workaround discussed later.
Figure 1. Mechanical adjustments
Focuser - it assumed your scope has adjustments to shift the focuser laterally along the OTA (original Russian (Intes) version shown). If your scope does not provide this adjustment, then you must alter the spacing of secondary mirror from the corrector, using the 3-point cell to align the optical centre of the secondary mirror with that of the focuser.
Secondary mirror – it is assumed the secondary mirror cell has a 3-point cell (Intes
version shown). The Russian versions have tiny brass screws which are fragile
and the heads are easily broken so please make sure you use a jewellers
screwdriver of the appropriate size and do not use force ! The central screw
pulls the secondary cell back onto the 3 collimation screws; clockwise will
loosen, anti-clockwise will tighten the secondary against the collimation
screws. More about this, later. The outer knurled retaining ring secures the
whole cell in the corrector, do not undo it !
Secondary cell movements – for most of the collimation sequence it is essential to rely on tilting the secondary using one screw and/or rotating it axially – either by rotating the corrector and the secondary with it, or rotating the cell axially, if it permits that. Only the final adjustment will be made at the end using all three collimation screws. To facilitate this you must loosen the corrector in its cell so you can wiggle it with your fingers.
Note:
There are variations in the way the corrector is held in its cell. In some maks (APM, or the Intes-Micro Alter versions) the corrector is inserted into the cell from the front of the scope, and is easily rotated without covering the whole thing in fingerprints.
On some maks – Intes (not Intes-Micro) and Skywatcher/Orion, the corrector is installed on the rear of the cell, secured from the inside, and the whole cell must be unbolted from the OTA to loosen it, and your corrector will need cleaning afterwards as the final step. If the scope is one where the corrector cell must be removed from the OTA, leave the corrector in the cell – do not separate them, as the cell should go back on the OTA in the same orientation (ie the bolt holes will line up in the original orientation).
4. Secondary mirror cell
You may have to remove the corrector, disassemble and rotate the cell accordingly. Put a pencil mark on the rim of the corrector facing the focuser, so you know where the screw should be. Reassemble into the OTA with the secondary mirror facing the focuser.
Rotate the secondary cell so that one of the 3-point adjustment screws is nearest the focuser. To do this, rotate the corrector by jiggling it with your fingers with a slight twisting motion.
We will use this screw – and only this one – to adjust the secondary tilt.
Insert the collimator with the centre spot filter, then look through the front of the scope to observe wherever the spot lands (probably somewhere on the inside of the OTA). Jiggle the corrector and rotate it a little, the spot will move around the OTA by 4X the rotation of the corrector. Now adjust the screw nearest the focuser – the spot moves longitudinally. We will use these two movements to collimate the secondary in the subsequent sections.
5. Focuser lateral offset
Figure 2. The effect of focuser lateral offset
Ensuring the focuser is in the correct position can be an issue, especially if there are signs someone has attempted to collimate the scope before, the focuser has been replaced, or the secondary mirror has been replaced or fiddled with. It’s not obvious how to deal with this, and the usual consequence in a Newtonian is as follows. NB we assume the optical axis of the focuser is accurately perpendicular to the OTA - as most are made by precision machining this should be true.
Assuming the focuser has an unknown (small) offset along the OTA by a small amount Dƒ, and we then insert a laser and adjust the secondary mirror so the laser spot is centred on the primary, the result is shown in figure 2. Because the laser spot is displaced from the optical centre of the secondary mirror, tilting that mirror to centre the spot on the primary introduces a tilt error that cannot be corrected by adjusting only the secondary and primary mirrors, and it also results in a tilted focal plane. While this might not bother visual observers it may cause trouble for imaging.
6. Collimating
6.1 Align laser on the centre of the primary – with a centre spot
If your primary has a centre spot, put the centre spot filter on the collimator, install in the focuser and observe where the spot strikes the primary mirror. Centre the spot on the primary using only the single screw nearest the focuser to adjust the laser in one direction (say left-right), and rotating the corrector to move perpendicularly (ie up-down). Now go to step 6.3.
6.2 Align laser on the centre of the primary – without a centre spot
If your primary has no centre spot, put the concentric circles filter on the collimator, install in the focuser. Looking into the front of the scope through the corrector, observe the outermost laser ring around the primary mirror. Centre the outermost laser ring around the perimeter of the primary mirror using only the single screw nearest the focuser to adjust the laser in one direction (say left-right), and rotating the corrector to move perpendicularly (ie up-down).
6.3 Align primary to secondary mirror – first attempt
Put the concentric circles filter on the collimator,
install in the focuser and point the scope at a wall, so that you see the
concentric circles pattern above, with the shadow of the secondary mirror
visible. Centre the laser circles with respect to the secondary mirror shadow using
the 3 adjustments of the primary mirror cell.
6.4 Secondary mirror offset
Note the secondary mirror in a fast scope should be
slightly decentred to the side away from the focuser to ensure it
intercepts the whole converging (conical) light beam as it converges on the
focus. If we put a laser in the focuser of a perfectly-collimated scope, the
laser should strike the secondary mirror slightly to one side of the centre of
the mirror:
where d is the diameter of the secondary, and ƒ is
the focal ratio of the scope. In a slower scope (say ƒ/6 or more, typical of
the Russian scopes) the secondary can be concentric with no offset. Check the
offset using the ruler across the pattern projected on the wall, by measuring
the gaps between the innermost laser circle vs the shadow of the secondary on
both sides (o1 and o2) - the actual offset is then:
Adjust the primary mirror tilt to achieve the desired offset.
At this point the scope is as shown in figure 2, where it seems collimated, but there is still a tilt error in both mirrors due to the lateral offset of the focuser.
6.5 Correct the focuser offset
Now look closely at the outer rings on the wall (see
below). The rings are probably not concentric with the corrector cell, or
perhaps the outermost ring is dimmed or truncated by the edge of the cell.
Use a ruler across the corrector to measure the gap between the corrector and the outer ring on one side, and again for the opposite side, the correct value is the average of the two. The movement required to centre the rings equals the distance to move the focuser.
If the ring truncated or dimmed is on the side
opposite to the focuser, the focuser must be shifted AWAY from the primary
mirror. If the truncation occurs on the side closest to the focuser, the
focuser must be shifted TOWARDS from the primary mirror.
Shift the focuser accordingly and repeat 6.1-6.5 so the circles are concentric with the edge of the corrector cell or OTA. The result should look like this:
6.6 Secondary mirror – final alignment
Put the centre spot filter on the collimator,
install in the focuser and look through the corrector so you can see the face
of the filter on the collimator (see photo). Collimation should be close enough
that the laser spot appears on or close to the face of the collimator filter
disk.
Adjust the secondary mirror to centre the laser spot on the face of the collimator.
If the collimation was close enough to put the laser spot on the face of the collimator filter, at this point use the screw nearest the focuser to tilt it in one direction (say left-right) or the other two screws of the secondary cell to adjust it up-down.
If it is still well off the face of the collimator filter you will need to rotate the corrector by jiggling it – careful – rotation has a DRASTIC effect.
7. Finishing off
Now make sure the secondary cell is secure – in the Russian maks, the centre bolt should be turned ANTICLOCKWISE till it’s firm, but do not apply force (the brass is easily broken).
By now your corrector will be covered in grubby figure-marks, and lint. With a pencil, put an index mark on the edge of the corrector or cell so you can replace it again in exactly the correct orientation.
· Remove the corrector and secondary as one piece (possibly including the cell), wash it in cool water with lots of detergent to get your prints and lint off. Never use hot water as this may cause the coatings to craze, which will ruin your scope.
· Rinse liberally under tap water to wash off the detergent.
· Final rinse liberally with distilled water, on both sides, and shake gently to remove as much water as possible. Using a well-washed soft cotton cloth (an old singlet or T-shirt is ideal), use a corner to touch any droplets – this will pull the water off by surface tension. You can dab, but DO NOT WIPE the secondary mirror. It’s a good idea to wear gloves at this point so your fingers won’t leave marks; cotton, neoprene of even clean kitchen rubber gloves will do.
· If there are still marks, repeat until it comes out clean.
· With the laser with central spot still in the focuser, replace the corrector and secondary and secure in place. Check the collimation with the laser and a final tweak of the secondary screws will centre the spot on the face of the collimator.
Collimation finished.
- Bob Campbell, TG and Hockeyman65 like this
24 Comments
Excellent article. While I now only have SCTs and refractors, I do enjoy (reading about, not doing
) the meticulous process of collimating a Newtonian class scope. Reminds me of my time with a TAL-150P and a Cave 10" F6 Astrola.
One question though. You (The author) were last active 2014. How did this get posted, anyway?
Edit: someone by the name of bondiblue posted it, apparently, however a scan for that member comes up empty. Odd, but nothing to bother oneself about.
Regards,
Bob
I've long suspected my mak-newt's meniscus wasn't square to the primary mirror; something that wouldn't show up when collimating such a scope like a regular newtonian (as explained in Figure 2.) What's the best way to make sure the corrector is square to the primary mirror?
It takes some time to realize that Mak-Newtonian collimation is not the same as Newtonian collimation. In a plain vanilla Newtonian, the aperture stop is at the primary mirror. One can use whatever means of collimation that might also require tilting the focuser, resulting in the optical axis not being parallel to the tube axis. This results in no optical effects but if digital encoders are being used, you might have to account for this in the pointing software. In contrast, the aperture stop in the Mak-Newt is at the corrector and the optical axis is required to be square to the corrector's axis otherwise you get astigmatism and/or coma. While the primary is spherical with no preferred axis, due to a lack of lateral adjustments, it also needs to be physically square to the corrector axis. Any collimation method must preserve this fact. This is the crux of the problem with Mak Newt collimation.
If thats the case the focuser is misplaced along the OTA as in figure 2. Fix that and there should be no need to tilt the focuser.
More likely your diagonal is tilted. If the corrector was tilted that will introduce a whole class of issues that collimating the mirrors cannot fix.
In 25 years none of the MN's I've seen suffer from this... but admittedly they were all Russian with machined tubes.
Aaaaagh…that makes sense……I never considered that before. Thanks for sharing. I just ordered the Skywatcher 190mn this week. I only have a Farpoint laser so I guess I’m going to need the Glatter setup if this is comes outta whack out of the box.
I hate to say this, but can you even get these H-G Collimator lasers? Even on their website, they indicate very extended lead times, and I have not seen one in stock or heard of anyone getting one for a very long time. Years in fact (and I have looked regularly and even send emails to the company, never returned). So many Youtube and other collimation resources list them as the ones to purchase, yet there are none.
Will the procedure not work if a laser collimator other than the HG is used?
If it is the holographic circles, there can be an addon for other laser collimators.
Quick search yields:
https://www.starligh...hic-collimator/
Bob
Bob great find that’s the holographic attachment.
These have a threaded stud which screws into the Glatter collimator but there’s nothing wrong with using other laser collimators like the Farpoint one - though you have to find a way to attach it to the collimator and not lose it inside the scope. A little Blutack would do.
NB having experimented with mine I can confirm there is nothing critical about centring or tilt, you can put it on backwards… the hologram showed no shift or other effects. What DOES matter though is making sure the laser is accurately coaxial with the body of the collimator and adjusting accordingly if not.
Corrector "square on" to the optical axis -- yep I've been discussing this on this thread about SCTs and MAKs collimation
https://www.cloudyni...age-2?hl= aline
"The wifi-Aline is very helpful in checking that the focus tube, secondary and primary are not grossly misaligned causing vignetting. Given the nature of spherical mirrors used in SCTs and MAKs the secondary can be tilted and still seen as mechanically centered in the primary.
For SCTs and MAKs collimation requires the refractive component (The aspheric corrector plate in an SCT or the spheric corrector plate in a MAK) also be “square on” (not tilted with respect to the spherical primary and secondary optical axis) otherwise one side of the corrector plate will “correct” short and the other side “correct “long, producing aberration in the focused the star image.
As a consequence, collimating the corrector plate to the primary-secondary optical axis in SCTs and MAKs is best done by star testing. It is quick, simple, and requires no additional gear beyond a tool to adjust the secondary screws on an SCT or primary screws on a MAK."
So using a laser will help put the optical axis into collimation (focuser, secondary, primary) but not put the optical axis "square on" to the corrector plate.
So I'm going to hypothesize, after roughing in the finder, secondary, primary optical axis to minimize vignetting, star testing for "square on" to the corrector plate just requires adjusting just the primary tilt as in a classical MAK? Will be interesting to see how one adjusts a Mak Newt optical axis to be "square on" to the corrector :-)
No it wouldn’t. What it would do though is introduce a noticeable spherical aberration which on one side of the focal plane appears overcorrected and on the other under-corrected, resembling astigmatism.
What would cause the scope to appear collimated on one side of focus but not on the other? I have this issue right now with a Mak and I'm unsure how to fix it.
I think this is the closest answer, the primary (or corrector plate) is over/under corrected. I found these kind words.
"Spherical Abberations
Another problem frequently encountered is spherical abberation. This is when the outer parts of the lens or mirror don't quite have the same focus as the center parts. Cheap telescopes often suffer from spherical abberation and it makes viewing faint objects quite difficult and can make pin sharp views impossible.There are two types of spherical aberration, under-corrected and over-corrected.
If you have over-correction the concentric rings inside focus will not be sharp, yet the outside focus will be. If the inside focus is sharp and the outside focus is blurry then you have under-correction.
If the outermost ring has spikes or streaks it indicates that the mirror may have a turned down edge. The methods for fixing these abberations, or if they are even possible to fix, depends on the type of telescope you have any the amount of adjustability it offers. You should check your handbook that came with the 'scope."
Might be interesting to try…
I've always looked at the alignment of the Mak-Newt corrector surface to the surface of the primary the same way you would look at the alignment of the surfaces of an air-spaced achromat, where thin shims are often used to keep the surfaces in the correct alignment (obviously, the air-space is significantly larger in the Mak-Newt). This is the critical alignment for Mak-Newt optical performance, so these axes must be maintained.
On first read, your procedure seems to be valid, and provides correction for each element (given a fixed meniscus). If it works for imaging scopes with large sensors and small pixels, that should be enough to keep your readers happy! If it doesn't work, it probably means a meniscus/primary error that will require some sort of bench autocollimation, probably with a large optical flat.
I'm not sure of your math re: secondary offset and actual offset of the laser rings relative to the secondary shadow.
For secondary offset, you suggest (1.41 X d)/(4 X f), which represents the offset on the surface of the secondary mirror (the hypotenuse). The lateral offset away from the focuser side of the tube assembly is d/(4 X f). For a 48mm minor axis secondary mirror and an f/6 primary mirror, the offset is 2mm.
For the actual offset (laser rings relative to the secondary shadow), you suggest averaging the opposing inner gaps (o1 + o2)/2. This provides the correction for a centered secondary mirror shadow (o1 = o2). For an o1 of 6mm and an o2 of 2mm, the average is 4mm. For the actual offset, subtract the average from the measurement opposite the focuser side of the tube assembly, in this case, o1 is 6mm, the average is 4mm, and the actual offset is 2mm.
For about the past 3 years. I've been checking on it.
Given that Howie passed away some time ago, who's making them now?
Attached is Messier 106
I own the Sl-LC2-635-NV with the concentric circle attachment . Best purchase I have made to date. This unit can get you very close to a final adjustment. Used in conjunction with PHD2 and a Bahtinov Mask . You can reach collimation very easily and quickly.
The photo attached was taken after only one collimation session of about an half hour the first night.
Oh yes Bobs Knobs were involved too
.
Here is the equipment and process used:
Meade 12" GPS mounted on a JTW Astronomy Trident Friction mount. Using a Nikon D5100 on an Optec TSFs focuser . About 14 hours of integration time and over 1700 30 second images.
Sequenced in NINA and processed in Siril
So this collimation tool is a must in anyone's repertoire.
So for a rebuilt older Meade I think it came out ok.
Scott
How do you achieve good collimation if your Mak-Newt scope doesn't have any alignment screws?
On mine, the secondary is fixed, the primary is fixed. All I can do is rotate the whole corrector (and consequently the secondary/diagonal attached to it) by a small amount.
-Christoph
Gets you close then you can find tune it with PHD2.
It says "Collimating a Maksutov-Newtonian..." (Mak-Newt) right in the title. All is well if your scope has adjustment screws on the primary mirror cell, screws for tilt on the secondary and also allows for proper rotation of the secondary holder over the drawtube or even allows to adjust the focuser. My Ceravolo HD216 has none of this. I can only slightly rotate the whole corrector with secondary mirror attached but everything else is pretty much permanently fixed. I was wondering if anyone had good tips how to achieve good collimation in this case. It's for purely visual use.