LX200 - CCD/Photo/Visual

LX200 - CCD/Photo/Visual Accessories

These items fall under the general catagory of accessories for CCD, Photographic, and Visual use. A fairly mixed bag of items but most attach to the rear cell of the LX200 either as custom made camera connectors, filter holders, slide prisms etc.

Slide-Mirror

Prototype Slide-Mirror

This was my first attempt at making a slide-mirror. Shortly after aquiring my 416XT CCD camera I realised that it would be of great benefit to be able to see exactly what the CCD camera is about to image. Additionally, an eyepiece can be made parfocal with the CCD chip via the viewing port, thus it also acts as an aid to achieving approximate focus.
The mirror was a front-surface aluminised mirror taken from a broken SLR camera. It obviously doesn't provide image quality as good as a normal diagonal does, but then this in no way affects it's functionality as a focussing/framing aid. I used it successfully for a while until it was replaced with the multi-purpose unit described later.

Slide Filter Box

Prototype slide filter box

This accessory was made to hold filters for visual use. I quickly became tired of screwing my 1-1/4" threaded filters on and off various eyepieces, in the dark, with cold hands, and figured it would make sense to have my more frequently used filters held permanently in a slider box. Such an arrangement offers many advantages:

Eyepieces can be quickly exchanged so different magnifications can be used with the filters without having to screw the filter on and off (the device's primary function).
The filters are well protected inside the slide box from finger marks, scratches, dewing etc.
I know exactly where each of the filters are - they don't get lost.
'Blinking' a filter in and out of the lightcone now becomes possible - especially useful with narrow-band filters (such as my OIII). Faint planetary nebulae are now a cinch to spot in a complex star background.
Filters can be 'stacked' - a combination of filters can be used together.

The prototype shown in the photo above is not anodised, and the top 1-1/4" port is redundant. The 3 rods on the left-hand side push the filters (which are the normal 1-1/4" type mounted on slides within the alloy housing) in and out of the light cone. This prototype proved the value of the idea, and again, it's functionality was incorporated into the combined unit described later on.

Combined sliding filter carrier and slide prism.

	Combined sliding filter carrier/slide prism, disassembled
	Filter assembly
	Prism assembly
	Collimating screws for prism

This device combines the functionality of both prototypes described above. By doing this the back-focus requirement was reduced to only 1-1/2" total when fitted to the rear of the telescope, and the weight was also reduced. I made a few modifications based on experience with the prototypes:

The filter slider rods have neoprene 'O' rings fitted to provide smooth slip-free movement. The rods on the prototype would tend to move of their own volition when the telescope was pointed at certain angles.
A prism was used instead of the front-surface mirror. The prism provides superior image quality.
Provision was made to adjust the collimation of the prism through the use of 3 small screws fitted to rear of the prism carrier. This enabled synchronising the views seen by the camera and eyepiece via the viewing port. Framing the image on film camera or CCD is now more accurate.

Colour Filter Wheel.

	The idexing wheel holds 4 filters - The coloured pins show in the indicator window on outside
	Construction of the rotator knob - a neoprene O-ring runs in a groove on wheel periphery
	A spring-loaded ball detent positively locates the filter positions.
	Overall depth when attached to the filter carrier/slide prism unit is only 2-1/2"
	The filter indicator window is just visible below the rotator
	Unit shown with camera attached
	Camera uses a screwed connector for rigidity

A colour filter wheel is needed for tri-colour CCD imaging with B&W CCD cameras. Fully automatic motorised filter wheels are available from most CCD camera makers, but they are very expensive. The only drawback of a manual filter wheel is that it cannot be used via remote operation, other than that it's no real hardship to turn the wheel by hand for each exposure. To be successful the filters need to be precisely aligned (to maintain exact focus) and the wheel free turning, and it's these features I concentrated on getting right.

As shown above, my filter wheel can hold up to 4 filters. In fact, one position is always left empty for un-filtered exposures, the other 3 are occupied by the red, green and blue filters. I have an infra-red filter permanently installed within the screwed camera connector so this does not require a position of it's own. The filters I use are ordinary wratten filters, not the more expensive interference filters. Perhaps when I gain more experience I can justify spending the extra cash, but for now I will make do with the ones I have. One problem with using wratten filters is that they are not designed with tri-colour work in mind. It's critical that the thicknesses of filters be the same in order to bring the focused image to the exact same position. To get around this I spent a little time measuring the focus off-set requirements for each of the filters so that I don't have to re-focus for each exposure with red, green and blue filters - I simply dial-in the pre-determined offset from the un-filtered focus position on my callibrated focussing knob.

In the construction details above you can see I have used a ball-bearing for the wheel central pivot, and indexing is taken care of by a spring-loaded ball detent. Moving the filters is achieved by turning the rotator knob, and the filter positions are shown by coloured disks in the indicator window (not a great idea in retrospect, a red torch doesn't show the colours too well - I think I will modifiy it to use simple numbers instead).

Initially, I used the LX200 visual back and the 1-1/4" eyepiece adaptor supplied with the camera to connect the 416XT to the rear of the colour wheel. Not a good idea really because not only is accurate orthagonal registration difficult, but the camera is less secure. I machined a tube which threads directly onto the front of the 416XT and uses the standard screw ring at the other end. This gives me exact positioning of the camera each time it's fitted, and it's very secure (i.e., it won't accidentally fall off!)

UPDATE:
I encountered some difficulties with the original drive mechanism used to turn the filter wheel as it tended to slip more than I would like. I spent some time trying making adjustments but was never really satisfied with it, so eventually I decided to abandon the friction drive and switch to an all gear-driven system. This did mean having to make a whole new casing with the drive gears in the back - but still using the original filter holder (the actual wheel). It was annoying me so the modification was worth the effort.

	The new gear drive system, I had to make a new casing to hold the gears (about 1/2" thicker than the old one)
	Photo of the rear with the cover in place, showing the manual drive knob. I'll be looking to motorise the wheel in the near future.

	This pic shows the position of the filter wheel's rotator knob (at lower left). I find it very quick and easy to select filters, only a light touch is required so no danger of moving the scope off target. Eyepiece is a reticle EP (non-illuminated) with interchangeable reticles.
	Viewed from the 'front' you can see the indicator window (showing red to correspond with the red filter being in the light-path). The cord you can see is a safety harness - attached to the main scope in case the CCD camera etc., should happen to fall off/drop off/whatever... Stops it hitting concrete in the worst case!
	Side view, still uses little backfocus despite the addition of the gearing. Whole unit is now 2.78" (not including screw connectors).

Wired LED Adapter for Guiding Eyepiece.

A wired LED adapter for wireless reticle eyepieces (this is the Meade 9mm)

Same job as above on the Vixen GA-4 guider

This simple item is an alloy turning containing an LED and resistor which fits the tapped hole of the Meade 9mm illuminated reticle eyepiece. It will most likely fit other reticle eyepieces (check the thread - 5/16" x 32 TPI). It's main use is to convert the illuminated eyepiece from battery powered to main panel powered. There are two reasons you might want to do this: Suddenly discovering the battery is dead in the field, or the need to make use of the 'blink' feature afforded by the LX200 handset. Additionally, I've found the handset offers a wider variation in illumination compared to the battery version - using the handset the reticle can be set to a much dimmer illumination (I find that even on it's lowest setting the battery version is way too bright). This is a sale item and the unit comes with a 6 ft lead and mini jack plug to fit the LX200 panel. The lower picture shows a slightly different version which fits the Vixen GA-4 guider, not screw-in this time.

35mm Camera Lens Adapter for 416XT CCD Camera.

An M42 threaded 35mm camera lens adapter for Pictor 416XT CCD Camera

The Pictor 416XT does not come with any provision for attaching standard camera lenses, and I'm not aware of any source of such adapters (though I'm sure they exist - and equally sure they are a silly price). 35mm camera lenses are an excellent method of achieving wide-field images with a CCD camera as the focal length is short, and the f/ratio is usually much faster than a telescope. My adapter shown above has provision for adjusting the distance between lens flange face to CCD chip surface, thus it's possible to give a correctly reading focus according to the distance scale on the lens. The adapter also has provision for installing a standard 1-1/4" threaded eyepiece filter between lens and CCD camera. I normally have the IR filter installed to reduce infra-red sensitivity because a camera lens (and refractor for that matter) will not focus IR wavelengths to the same point as visual wavelengths, thus the CCD image would never be well focussed without the filter fitted.

Cable clamp for the 416XT CCD Camera

It's only happened to me once - having the cable drop off the end of the camera during an imaging session. But that's once too often as if it goes un-noticed the 416 control box will still be trying it's best to cool the now disconnected camera. So this is what I came up with, a simple bracket which the cable hooks over. The cable cannot be pulled out, and the bracket takes the weight of the cable instead of the plug and socket.

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