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Hints & tips for using a SkyWatcher telescope

 

Here are some illustrated notes relating to how I use my SkyWatcher 254mm Newtonian telescope. I suspect most of this is valid for all SkyWatcher Newtonians. I hope it will help people, compensating for the lack of any such documentation supplied with the instruments. I will include some really obvious things, just to make sure you know about them.

NB: I believe the advice I give here to be accurate and that following it could not cause any damage to your telescope or camera. However, you must assess the risks yourself.

 

 Holes in the lid (how many?)

As received, the lid had one hole and a cap for it. Strangely, an identical raised rim existed for a second hole but it was not cut through. I cut it myself. The cap from the finder telescope is identical to the one existing lid cap so I have used that to cap the second hole and I have improvised a new lid for the finder.

So why did I want the second hole? Just for aesthetic reasons? No, it enables the lid to be used very effectively for focussing on bright stars. When such a star is out of focus the 2 holes cause the image to be double but the two parts merge as the star is focussed. It is currently my best method for focussing my SLR camera on the night sky. I focus while using the live view feature of the camera, with the live image on the LCD display enlarged 10 times (Canon EOS 5D MkII can do that, probably other cameras can too). Once focussed I remove the lid of course.

It is important to keep the telescope completely covered when not in use, to keep dust off the mirrors. So the caps are then put over the holes. It is also important to cover both the main tube and the finder in case the instrument gets pointed at the sun.

 

 Focussing with the 2-hole lid

This sequence of images shows what the 10x magnified live view on the camera's LCD screen is like as the focusser wheel is turned towards focus. These are all 1s exposures at ISO 25600, averaging the seething bubbling spots that are really seen (due to atmospheric turbulence). The star is Vega, one of the brightest in the sky. This technique cannot be used on faint stars.

Then when the lid is removed from the telescope a shot with the same exposure looks like this:

(Click on that image for an even better view of Vega.)

Note: it is important to set the camera for single shot photography rather than videos, otherwise the live view sets the exposure time to 1/30s and you see nothing. At least, that is the case for my Canon camera and I assume it will be similar for others.

I bought a mains power supply for my camera so that I can use live view most of the time, avoiding the mirror flicking up and down whenever photos are taken. This is better than locking the mirror up via the camera's menu because that would require a double trigger for every shot and my Canon multi-shot cable release, that I can leave to take up to 99 shots automatically, cannot do a double trigger.

 Bahtinov mask

In early February 2011 I obtained a Bahtinov mask which makes focussing easier and more accurate. Like the 2-hole technique the mask produces a diffraction pattern, as in this photo of Capella:

The central line of the three moves across the other two as focus is adjusted. All three meet in a point when the focus is exact.

 

 Identifying some parts

Looking down inside the tube of the telescope we can see the primary parabolic mirror at the bottom. Too small to be visible in this photo, the primary mirror has a deliberate ring mark in its centre which is useful for collimating (aligning) the system - described below. At the top of the tube is the plane secondary mirror. This has an elliptical outline and it is angled at 45 degrees so that the converging light returning from the primary mirror is reflected out of the side of the tube, where we see the eyepiece focusser. The secondary mirror is suspended in the tube by 4 metal vanes. The vanes are as thin as possible, to avoid blocking light on its way in towards the primary mirror. The vanes do cause the spikes seen in photos of bright stars because they diffract the light slightly.

The focusser, like many these days, is called a Crayford focusser. However, I do not believe it bears much relation to John Wall's innovative design (which can be seen here - I was present when John first showed his prototype at Crayford Manor House in the late 1960's).

 

Eyepieces cannot be put directly into this SkyWatcher focusser. Adapters are provided, as described next. The adapters fit into the black collar seen on the focusser and they are gripped by tightening two screws.

There are 2 standard diameters for amateur astronomical eyepiece tubes: 1.25 inch and 2 inch.

 1.25 inch eyepieces

Here is the adapter for 1.25 inch eyepieces, and a view of an eyepiece in the adapter on the focusser.

 2 inch eyepieces

Similarly, here is the 2 inch adapter and an eyepiece sitting in it on the focusser.

I use a 2 inch eyepiece so that I can swap between a camera using the 1.25 inch adapter and the eyepiece without moving the focus. The 2-inch eyepiece is held in its adapter at a suitable position so I can view things in focus without upsetting the focus position for the camera.

   

 Prime focus photography

I had been photographing through the telescope for several weeks using the negative projection method (see below), in the belief that that was the simplest feasible option. Then one day I discovered that the 1.25 inch eyepiece adapter unscrews into two parts. Furthermore the thread fits a T-mount adapter ring.

The 2 inch adapter does not come apart.

So it is possible to screw together a simple T-mount adapter ring and the part of the 1.25 inch eyepiece adapter thats fits into the focusser.

So then it was possible to mount my camera very close to the telescope tube. Would it be close enough to focus sharply though? To my delight, it was. This photo shows the camera in focus, at the prime focus and therefore working at f/4.8 with focal length 1200mm. This arrangement maximises the light available in the camera.

Some photos I have taken with this configuration can be seen here (configuration [A] eg, the M31 and M45 photos).

It would be really useful to have more than one 1.25 inch eyepiece adapter so that I could keep one attached to a T-mount adapter for prime focus work, another to a Barlow lens (see below) etc. It is not good to keep unscrewing and reattaching threaded items because of the risk of creating swarf that might get into the camera.

   

 Negative projection photography

A Barlow lens is a concave (diverging) lens that throws the telescope's focal plane further out. For photography this is called negative projection. The Barlow lens supplied with my SkyWatcher instrument usefully has a T-mount thread at the outer end, so a T-mount adapter ring can be put on it for attaching an SLR camera. Here are exploded and assembled views of this arrangment.

A = 1.25 inch eyepiece adapter

B = Barlow lens

C = T-mount adapter ring (Canon fitting)

Some photos I have taken with this configuration can be seen here (configuration [B] eg, the M27 and M51 photos).

The Barlow lens has the effect of doubling the focal length and therefore of halving the f/number. The top of its tube has a 1.25 inch inner diameter so that other eyepieces will fit into it. So it is a common way of doubling the range of magnifications available with any set of eyepieces.

Shorter focal length eyepieces fitted into the Barlow sleeve allow a T-mount-threaded tube to be fitted around them so the camera can still be added too, for more magnified photos. I have not yet tried photographing with that combination. It will be suitable for lunar and planetary photos, where there is plenty of light, but not for faint objects.

 

 Photography with filters

Telescope House supplies a useful little adapter which has a short 1.25 inch diameter tube with an internal thread for filters at one end and a T-mount-threaded flange at the other. The left photo above shows it.

The middle photo is an exploded view of a negative projection arrangement that uses this filter adapter to hold a filter in the Barlow lens tube while also being able to attach to a camera. The right hand photo is of the whole assembly fitted on the camera, ready to fit on the focusser of the telescope.

A = 1.25 inch eyepiece adapter

B = Barlow lens

C = 1.25 inch filter

D = filter-to-T adapter

E = T-mount adapter ring (Canon fitting)

I have not yet devised a way of fitting a filter for prime focus photography.

   

 Coma correction

An inevitable problem with short focal length Newtonian telescopes is a type of optical aberration called coma. It is apparent at the edges of the field of view, especially if using a large detector, such as in my 36 x 24mm full-frame EOS camera. SkyWatcher have recently released a corrector specifically designed for their instruments, so I have obtained one.

The first thing to say is that it certainly does work, as demonstrated in the following before and after examples (it should be obvious which is which).

Those are both the same corner of a 35mm frame centred on Alpha Cassiopeiae, at 50% scale (2 x 2 pixels in the camera become 1 pixel here).

Here is a photo of the corrector in front of my camera. This forms configuration [C] for my photos. The adapter ring for connecting the corrector to the camera is similar to T-mount but with a larger aperture: it is called an M48x0.75 adapter. Versions of this custom adapter are available for Canon, Nikon and Sony.

Two new problems arise when using the corrector. One is that the extra glass surfaces cause a purple ghost image, very noticeable for bright stars.

When I took this picture of Vega I first thought I had discovered a very unusual purple nova. But I very soon realised that the purple spot moves around, depending where the bright star is in the frame. That means it is less of a problem when a series of poorly guided images are combined - the very thing that GRIP was designed for - because the purple ghost is in a different position on each frame and so is watered down in the final result.

The second problem is that the mounting completely changes the way eyepieces are mounted on the instrument. That is because the corrector comes with a different version of the black threaded ring that forms the exit of the focusser, marked as R in the photo on the right.

The previous version, already described above, needed an adapter to hold 2 inch eyepieces because its diameter was a fraction of a millimetre too small for eyepieces to slide in directly. The new version is the right size to take a 2 inch eyepiece without another adapter. However there is still a problem because I want to keep the focusser clamped in one position while I switch between camera and eyepiece. That is not possible as it stands because the eyepiece has to be significantly further out than the camera when both are in focus - far enough that there is a 0.5cm gap between exit ring R and the end of the eyepiece.

SkyWatcher's solution is to supply a 2in eyepiece holder to go in place of the M48 adapter. That does enable the corrector to be used with eyepieces BUT the threads on the adapters involve dozens of turns, so it is not practical for quick switching (and I have concerns about swarf entering the camera).

I only need the corrector for photos, not when I am using an eyepiece for locating targets, so I sought a different solution.

Fortunately I already had a 2 inch Barlow lens. That has a 2 inch external diameter tube to go into the focusser and a 2 inch internal diameter tube to hold eyepieces. Furthermore the Barlow lens itself unscrews, leaving me with just the adapter I need. The photo on the left here shows two views of the Barlow, with its lens in place. The other photo shows an eyepiece held in it (40mm wide view) and, though not obvious at the far end, the Barlow lens itself has been removed. This assembly I can very easily swap for camera plus coma corrector and keep both accurately focussed.

Furthermore my 2 inch Barlow (which I got from Telescope House) also has another adapter for its eyepiece end to take 1.25 inch eyepieces. So perhaps I no longer need the SkyWatcher 1.25 and 2 inch adapters described further up this page. Time will tell.

 

 Collimation

The telescope tube flexes slightly as it is moved around so at some stage the optics may go out of alignment. It is necessary to be able to check the alignment and correct it. There are several techniques but the simplest is to use a laser collimator. That may sound expensive but Telescope House supplies its own brand one for less than 40 pounds. If you ask you can also get a simple instruction sheet. The device has a 1.25 inch outer diameter so it fits in the relevant adapter as an eyepiece. It has a surface visible through a hole in its side, angled at 45 degrees. In the centre of the surface is a small hole from which a parallel laser beam emerges, aimed into the telescope.

 Aligning the secondary mirror

First we check that the secondary mirror is angled so that the beam hits the primary mirror right in the centre. There is a small ring marked on the primary mirror for this purpose. If the red spot is not in the ring we have to adjust the supports of the secondary mirror. That is done by slackening and tightening opposite silver screw heads holding the vanes supporting the secondary mirror. The opposite screws must be adjusted together as a pair so that they are both tight, working against each other.

 Aligning the primary mirror

Then if the primary mirror is aligned the laser beam will reflect back exactly into the hole from which it emerged in the collimator. If that is not the case we have to adjust the screws underneath the telescope. These are in three pairs, 120 degrees from each other. In each pair of screws one is the position adjuster and the other locks the adjuster. You will soon discover which is which. The tricky bit is that locking each adjuster moves it slightly, so it is very much an iterative process to get it right.

The photo of the collimator above does show it in a state of correct alignment. I think the splashes of red light are spurious reflections off dust, support vanes, etc. If the telescope were out of alignment we would see a very much brighter red spot in the collimator.

 Hanging the controller

Last but by no means least: I opened the hand controller for the motors (be careful: the square buttons pop off and can fly a long way if you are not expecting that) and drilled 2 small holes in the top of the case. I threaded some decent cord through and knotted it inside. The result is that I can now hang the controller on any convenient protrusion on the telescope or tripod when I am not using it. Previously I had to put it down on the ground!