Using a Howie Glatter Laser for Collimation of the Altair Deepfield RC

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Phillip Lucas
5 years ago
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1 Using a Howie Glatter Laser for Collimation of the Altair Deepfield RC Background Collimating a Richey Cretien scope is fundamentally different to collimating a Newtonian or SCT. For optimum performance, three optical axes (focuser, secondary mirror and primary mirror) need to be accurately optically aligned. As none of these can be considered as fixed, there is no real reference axis to commence from. PRIMARY SECONDARY Primary axis Focuser axis Secondary axis There are many varied methods around for carrying out RC collimation, many of them involving iterative adjustments, not easily done with such a large instrument. The method below uses an accurate laser for aligning the focuser and secondary axes and as such can be carried out in daylight. Laser Collimation By far the best tool for aligning the focuser and secondary axes is a 2 Howie Glatter Laser Collimator with 1mm stop attachment (included) [ Image 1]. The Howie Glatter laser gives a well-defined laser spot in a wellengineered package. The 2 body is a snug fit in the focuser body and though they seem to be well collimated themselves from production, it is worth checking laser collimation by sitting the laser body in a v-block and whilst rotating the laser body, checking that the spot remains fixed in position on a distant spot. There are collimation adjustment screws if the laser does need adjustment.

As such, the alignment of the focuser and secondary axes is best carried out with the scope pointing vertically downwards [Image2].

2 Image 1 2 Howie Glatter Laser Collimation Tool. Step 1 - Aligning the Focuser Axis The Howie Glatter laser has an accurately machined stop collar but even so, locking this into a focuser sleeve can pull it very slightly off axis. As such, the alignment of the focuser and secondary axes is best carried out with the scope pointing vertically downwards [Image2]. This allows the focuser locking screws to be left undone and rely on gravity alone to seat the laser against the back of the focuser. It also has the advantages that the laser is loose to rotate and it negates any mechanical flex that may lead to different collimation results with the scope on either side of the mount. Image 2 Scope in ideal position for collimation

3 Most secondary s have a centre spot marking, though this can t be wholly relied on. Firstly the spot may not be accurately positioned on the secondary and secondly, truss geometry needs to be only slightly out of true to pull the secondary slightly off centre. Instead, if the focuser is not perfectly optically aligned, rotating the laser body will transcribe a circular arc from the laser beam onto the secondary. As such, it is a matter of adjusting the focuser tilt adjustments until rotation of the laser body leaves the laser spot in the same place or at least until the circle transcribed is as small as possible, certainly no larger than the size of secondary spot. Note that the necessary fine adjustments of the tilt screws can be as little as 1/16 th of a turn or less. Note also that the optimum position may not be in the centre of the secondary spot [Figure 3] though any major discrepancy may point towards an issue with the truss rod geometry. Finally the position of the focuser and any focuser tension adjustment can have an impact on this step so it is worthwhile having the tension adjustment set and the focus point where it would be with the imaging train in place. Image 3 Laser spot within centre mark on secondary Step 2 - Aligning the Secondary Mirror Optical Axis Now that we have one axis aligned as a reference we can make the secondary optical axis coincident with it. Looking upwards the reflection of the return beam can be seen on the face of the 1mm laser stop attachment [Image 4].

When this is achieved there is a visible brightening of the beam and a degree of interference fringing.

4 Image 4 Return beam non-coincident with source adjust secondary mirror. The aim is to fold the laser beam back on itself by adjusting the secondary mirror adjustment screws until the return beam is reflected back on the source. When this is achieved there is a visible brightening of the beam and a degree of interference fringing. Again the adjustments needed to get from say Image 4 to Image 5 are very small, 1/16 th of a turn or so. Once completed gently snug up the secondary mirror locking screws and then recheck. Image 5 The sign of a good secondary optical alignment. Laser beam folded back on itself. Step 3 Aligning the Primary Mirror Optical Axis The Howie Glatter laser can be purchased with a circular laser pattern disk, which replaces the 1mm stop attachment. This projects a series of circular interference rings onto a flat surface some distance in front of the scope. This needs to be done in reasonably darkness! The aim is to adjust the primary mirror collimation knobs so that the space between the shadow cast by the primary mirror holder and the first ring is equal around the

Step 4 Star Test The best method of achieving excellent primary collimation is on a star with a CCD installed as it would be for imaging.

5 circumference of the shadow. The adjustments needed can be quite fine and can be aided by employing a scale rule. Image 6 - Projected circular laser pattern The scope should now be reasonably well collimated and be a good starting point for a star test. Step 4 Star Test The best method of achieving excellent primary collimation is on a star with a CCD installed as it would be for imaging. Star images need to be bright enough to show clearly in a short exposure (2-3 seconds) but not too bright as to flare in the image. A mag 6-9 star, well above the horizon to minimise ground thermal effects, is a good starting point and that star should be placed in the centre 1/3 of the CCD image. Any shroud should be removed to minimise any thermal currents that can impact on the ring pattern and the scope should be cooled down if it has been moved. The classic image of Airy patterns [Image 7] are rarely to be seen in the UK, particularly the in focus rings in the centre of Image 7 which would only be barely visible on a UK night of exceptional seeing. Image 7 Classic collimation airy patterns The aim then is to refine the intra and extra focus patterns to be as symmetrical as possible. If the rings are constantly broken by turbulence, conditions are not ideal to try and refine primary mirror collimation further. Trying to refine collimation under such circumstances is a fruitless exercise. It is worth marking the starting

6 point on each of the collimation adjusters with a small piece of tape, cut with a razor blade after application. It is very easy to turn one in the wrong direction or even turn the wrong one. The tape allows you to undo any adjustment. Any adjustment to the primary mirror adjustment knobs will rapidly slew the target star off-centre so small adjustments (<1/8 th of a turn) are best followed by a slew of the mount to re-centre the target star. Getting good to excellent primary collimation can take a few sessions as it is largely seeing dependent. It is however worth waiting for a still night to make those fine adjustments which may only need a fractional turn of an adjuster. On such a night it is worth slewing the target star into the corners of the image to check for off axis coma. Any minor coma should be symmetrical with any non-symmetry suggesting that very minor adjustments of the secondary are needed.

Hubble Optics CDK 17 Collimation Instructions 03/27/2012 Hubble Optics

Hubble Optics CDK 17 Collimation Instructions 03/27/2012 Hubble Optics 1: CDK17 Specification: System Effective Focal Length: 2894.7 mm, (this might be slightly different for different set of optics) Figure