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.

1.     Introduction. 2

2.     Equipment and tools. 2

3.     Mechanical concepts. 2

4.     Secondary mirror cell 4

5.     Focuser lateral offset 5

6.     Collimating. 6

6.1       Align laser on the centre of the primary – with a centre spot 6

6.2       Align laser on the centre of the primary – without a centre spot 6

6.3       Align primary to secondary mirror – first attempt 6

6.4       Secondary mirror offset 7

6.5       Correct the focuser offset 7

6.6       Secondary mirror – final alignment 8

7.     Finishing off 8

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.