NEWTONIAN/DOBSONIAN CARE AND UPGRADES

On this page, I aim to share some techniques that I have learnt during my few years of enjoying astronomy and I also wish to share the details of some modifications that I have carried out on my current 14 inch Dobsonian telescope. 

COLLIMATING A NEWTONIAN REFLECTOR

Collimation! A much debated topic! Read on for my experiences in learning how to effectively collimate my f4.65 Skywatcher reflector.

I use a laser collimator and home made `collimation cap' to align the position of my secondary mirror correctly and for my primary mirror, I use the laser in combination with a Barlow lens, the method that has thankfully been invented by Nils Olof Carlin.

I made my collimating cap from an old 35mm film cannister, which fits perfectly into a 1.25" focuser adapter. I made a small viewing hole in the centre of the lid of the cannister. This ensures that the eye view is kept central when checking the mirror alignment.

Collimating the Collimator

There ARE certain accuracy dangers involved with using a laser collimator. 

Before collimating, it is essential to ensure that the laser collimator itself is correctly collimated, i.e., that it sends a beam forward that is at 90 degrees to the face of the collimator that abuts the telescope focusser. It seems that many of the laser collimators that can be purchased are NOT necessarily collimated precisely at the factory, or they suffer in transit!

The method that I use for this is to mount the collimator upon a home made wooden jig and shine the laser dot at a target that is mounted on a wall that is at least 11 feet away from the collimator (twice the focal length of my telescope).

Wooden Jig

Using this jig, I am then able to carefully rotate the outer body of the collimator through 360 degrees, checking for any position`wander' of the laser dot that it projected on the wall. If there is any wander, the three adjusting screws that are located in the outer body of the collimator should be adjusted so that the dot stays stationary when the laser is rotated.

Collimating the Collimator

Simple Secondary Mirror Adjustment

CAUTION: Never perform secondary collimation with the telescope at or anywhere near a vertical position. If the telescope is tilted at such an angle, there is a danger of dropping tools down the tube onto the primary mirror.  

CAUTION: This method assumes that the laser collimator is a good, slack free fit into the focuser tube. This can be a problem. Centrality of the laser position in the focuser needs to be confirmed by checking repeatability of the projected dot position in Step 1 of the procedure below, when removing and replacing the laser collimator several times. Sometimes, the desired repeatability can be achieved by ensuring that the front face of the collimator is pressed firmly against the eyepiece holder each time the retaining screws are tightened. To ensure good centrality, I have fitted my telescope with an Orion Precision Centering Adapter (see below), which ensures that the collimator is centrally positioned in the focuser at all times.

Orion Precision Centering Adapter

Secondary mirror adjustment is usually the most difficult part of the collimation process. It is facilitated by a three grub screws and a central screw, all of which are mounted upon the secondary mirror housing. These should never be adjusted if not absolutely necessary - caution is the key word. This applies in particular to the central screw, which requires special care.

Usually, with luck, the only adjustment required for the secondary mirror is a slight tweak of the grub screws to correct a slight misalignment. It is when there is a more significant misalignment that things get much more difficult...but, with experience, I think that I've found a good method!

Please read on...

The simple adjustment for a slight misalignment is easily achieved as follows:

1.    Fit the laser collimator into the eyepiece holder and adjust the three grub screws until the dot projected by the laser is positioned within the red circle at the centre of the primary mirror. 

2.    Remove the laser collimator from the focuser.

3.    Insert the collimating cap into the focuser. 

4.    Check that a circular reflection of the complete surface area of the primary mirror is seen.

Collimating Cap

This completes the adjustment of the secondary mirror.

However...

...if this simple procedure is insufficient to centre the red dot in the circle, or a circular image of the entire primary mirror is not seen, then proceed as follows.

The centre screw of the mirror housing must be adjusted carefully. Turning this screw moves the position of the secondary mirror towards or away from the primary mirror, allowing the secondary mirror to be centered beneath the focuser tube.

The procedure below details what I have found to be my best method of achieving this.

Further Secondary Mirror Adjustment

The centre screw should always be adjusted in conjunction with the three grub screws in this procedure. It should be borne in mind that when adjusting the centre screw in the manner described below, the mirror can also be easily rotated axially - this is another adjustment that must be corrected whilst performing Step 4, below.

CAUTION: This adjustment can result in the mirror flopping down at an angle towards the underside of the optical tube. 

For these reasons, my method is based upon loosening all of the screws a little and carefully holding the mirror in the correct position by hand and tightening the screws so that they then hold the mirror in this position.

Proceed as follows:

1.    Remove the collimating cap from the focuser, so that the focuser tube is empty.

2.    Loosen all three grub screws several turns.

CAUTION: The next step involves holding the secondary mirror in position. This must always be done with great care not to touch the surface of the mirror and after washing your hands.

4.    Holding the secondary mirror with one hand, whilst looking through the empty focuser tube, adjust the centre screw to position the mirror so that it appears to be centrally mounted underneath the focuser tube, being neither too near to the primary mirror nor too far from it. It should also be axially rotated to be face-on to the focuser tube, as shown in the diagrams below. This involves rotating the mirror in two planes, simultaneously:

(i)    So that the left hand end of the mirror appears to be level with the right hand side of the mirror, as shown in the diagram below. 

(ii)   So that it is `face on' to the focuser.

         Mirror Incorrectly Positioned                                                               Mirror correctly Positioned

5.    With your hand still holding the mirror in position, carefully tighten the three grubscrews (not very tight) so that they just make contact with the rear of the mirror to hold it gently in position.

6.    Insert the collimating cap into the focuser tube and check that you can see a nice reflection of the whole of the primary mirror in the secondary mirror image. If not, slightly loosen the grub screws and adjust the mirror until you can see a nice circular image of the entire primary mirror reflected back, as shown below.  If the image is elliptical, rotate the secondary mirror axially until it is circular.

Once the view is as shown in the diagram below, this means that the secondary mirror is positioned directly below the focuser drawtube.

(Note: the image of the mirror is slightly offset from centre in the picture below, as this picture depicts how things look in a typical fast focal ratio Newtonian. In a f6 or f8 scope, the primary mirror will appear central in the field of view). 

Correct View Through Collimating Cap

        NOTE: At this stage THE ONLY CONCERN is that the whoIe of the primary mirror (blue circle) is visible. It may be the case that the reflection of the spider arms and secondary mirror housing is asymetrical (see diagram below). This is NOT a concern, at this stage, but is indicative that the PRIMARY mirror requires adjustment. This will be performed later in this procedure.

Primary Mirror Misaligned, but Secondary Mirror Correctly Aligned

The next stage is to adjust the tilt of the secondary mirror so that the focuser axis is aligned with the centre of the primary mirror. 

7.    Remove the collimating cap from the focuser.

8.    Insert the laser collimator into the focuser.

CAUTION: The next step involves tightening the grub screws. Never overtighten the grub screws, as this can cause damage.

9(a).    If the dot projected by the laser is positioned within the red circle at the centre of the primary mirror, carefully tighten the grub screws just a little, maintaining the dot as close to the centre of the red circle as possible. This successfully completes the adjustment of the secondary mirror.

9(b).       If the dot is outside of the red circle (far more likely!), carefully adjust the grub screws to bring the dot a little nearer the red circle and then remove the laser collimator from the focuser.

10.    Unfortunately, this adjustment will usually upset the previous secondary mirror/focuser drawtube alignment that was performed with the collimation cap, so now it is necessary to keep repeating steps 6, 7, 8 and 9(b) until the laser dot is centralised within the red circle and the view through the collimation cap shows the entire primary mirror reflected. You may well have to slightly readjust the centre screw and rotation of the mirror when repeating step 9(b). The two processes tend to be antagonistic to each other, so several repeats of the process may well be necessary.

This successfully completes the adjustment of the secondary mirror.

How Secondary Mirror Miscollimation Affects Telescope Performance 

There are two main concerns with secondary mirror adjustment:

1.    That the mirror is positioned correctly underneath the focuser tube. Failure to do this can result in the reflection from the primary mirror being partially blocked (vignetting) by the side of the focuser tube and reduction of the light reaching the eyepiece.....effectively your telescope aperture is reduced! As the centre of the primary mirror image is now offset relative to the eyepiece, on-axis coma can occur, as the normally coma free centre of the field of view is shifted to one side.

2.    That the tilt angle of the mirror is adjusted correctly. This can also cause the centre of the primary mirror image to be offset relative to the eyepiece, resulting in on-axis coma as the normally coma free centre of the field of view is shifted to one side.

Primary Mirror Adjustment

This can be done by simply inserting the laser collimator into the focuser, viewing the return beam in the 45 degree side window of the collimator and adjusting the primary mirror bolts until the mirror tilt is such that the return beam is centralised in the collimator window. However, this method is easily subject to error as a laser light path length of twice the focal length of the telescope is involved and the laser collimator can be a poor fit into the focuser tube.

Fortunately, an ingenious method has been invented by Nils Olof Carlson.....a much better method! 

Fit the laser collimator into a Barlow lens and then fit the Barlow plus collimator into the telescope. This will then transmit a divergent laser beam which spreads out over the centre region of the primary mirror. The return beam then contains a shadow of the centre ring of the primary mirror. I observe this in the 45 degree window of the collimator and then adjust the three primary mirror tilt knobs until the centre ring shadow overlays the centre of the 45 degree collimator window.

I believe that the primary mirror adjustment is the most critical part of collimation for image sharpness and detail resolution, especially if planets are to be observed.

SECONDARY MIRROR COVER FOR A TRUSS-DOBSONIAN

For those of us who prefer our truss/flex-tube Dobs permanently deployed as ready-to-go, the only unsettling thing is that we find that the secondary mirror is exposed in all its vulnerability to the outside world and all of its insects and snails and things like that! 

A SIMPLE ELECTRONIC LOCATOR FOR ELUSIVE ASTRONOMICAL TARGETS

As a typical Dobsonian owner, I like to learn the sky so that I can quickly just slew my telescope to point at the desired object. However, there has been the odd occasion when I cannot find what I am looking for.....maybe it's a target that isn't near to any convenient `landmarks' (like the globular cluster M3 or a comet), or maybe it's Uranus, which can be difficult to discern from nearby stars when initially searching at low magnification.

As I don't have `GoTo', I have fitted this telescope with two bevel boxes (tilt monitors), see the red and blue boxes in the picture.

The red box is connected to an encoder which is mounted at the centre of the mount base and measures azimuth bearing.