LX200 - Miscellaneous Accessories and Modifications

TELRAD Dew-Shield

Simple but effective dew-shield for the Telrad finder.

Couldn't be more simple, just take an A4 sheet of acetate film (the stuff used on overhead projectors) and trim about 1.5 inches off the end. Place it in position (as shown in the pic) and mark the position for the switch, then use a pair of scissors or a sharp blade to cut a 1/4" hole. Tape into position with two strips of double-sided tape. You're done!

The shield is all but invisible viewed from behind, it's tough yet flexible, and it's cheap to replace.

Corrector Plate Heater

Corrector plate 'dew busting' heating element.
Not wanting to pay silly prices for what is basically a very simple item I decided to make my own. Only very gentle heat is required to keep dew at bay, excessive heat will only degrade the image through increased air turbulence. My design simply fits on the inside rim of the front of the scope, it remains permanently in place there as the front cover and dew shield can still be used with it fitted.

It's construction is very simple: A length of 3/8" O.D. rubber tubing is formed into a ring of such a size that it fits inside the front of the scope. Nothing more is required to hold it in place. Eight 5 Ohm resistors are wired together and threaded into the tube, evenly distributed along it's length. The ends of the leads are connected to the power supply, and in my example these leads run back through the tubing shown and terminate in a socket at the rear end of the OTA. Another plug connects (via an on/off switch and power indicator LED) to a 12V DC supply voltage.

Only about 3W of power is being used - not very much - yet I have found it more than sufficient to keep dew off the corrector plate under the common conditions I experience here in the UK. Very high humidity will require more power, and you would be as well to increase the wattage (by reducing the resistor values). I always use my dew-shield in combination with the heater and have not found any deterioration in seeing with the heater operating.

Field Tripod: Modification to leg hinges

Field tripod hinges replaced with 5/16" SS rods to provide more stability.
This modification was made because I felt the mechanical arrangement of having the tripod legs hinging on threaded screws was unsatisfactory. The hard steel threads would tend to cut into soft aluminium casting, and further, on my tripod the holes were already oversized. The solution was to remove the original screws, bore and ream 5/16" right through the lugs, and fit the 5/16" stainless steel pivot pins shown in the above picture. These were nutted each end to hold them in place (about 25 thou end-float was allowed). Now the legs no longer flap about side-side on loose hinges, and I'm convinced the tripod is more stable when locked up.

An Eyepiece Rack

An eyepiece rack.
A simply but very useful accessory. It's made from 1/2" alloy plate, then black anodised. This version only holds 1-1/4" eyepieces as I don't have any 2" eyepieces at the moment. A future modification to this rack (when I get time that is!) is to install a heater element within the rack to keep the eyepieces clear of dew. Well, these little luxuries make life easier don't you think? :)

The 'DriCap' Moisture-absorbing Dustcap

DriCap - a moisture-absorbing dustcap.
The construction of the DriCap, the tube is filled self-indicating silica gel which is blue when dry.
Colour change as the DriCap absorbs moisture from within the OTA
Also available in 2" barrel fitting.
This item fits 8", 10", and 12" Meade LX-series SCTs, it will also fit Celestron SCTs. It's purpose to remove water vapour from the inside of the OTA during long-term storage or intermittent use. The device achieves this using a self-indicating dessicant (silica gel) contained within a perforated tube which forms part of the dustcap. The cap is made from alloy, black anodised, and a lens in the rear enables instant monitoring of the condition of the dessicant by reference to it's colour.

This is a very practical accessory which I've found actually works (!) in day-to-day use. It is NOT designed to remove huge quantities of water, but rather to absorb the condensation which unavoidably occurs with changes in temperature as the scope is moved (for example) from the cold night air into a warmer room. It is equally valuable for maintaining a dry enviroment within the OTA when the scope is kept outside in an un-heated observatory.

Why should you bother about moisture in the OTA? - well, it's not so much the water (which is harmless) but rather the nasty substances that are dissolved in it. Oxides of nitrogen and sulphur are present in the air (abundant in urban environments) and these dissolve in the water to form corrosive solutions. It's these that will attack your mirror coatings over time. If you remove the water then these gasses have nothing to dissolve in and so can't exert their corrosive effects - you coatings will therefore last longer.

The perforated tube is lined with fine filter paper which prevents any dust from the dessicant finding it's way into the OTA, it's then filled with dry (blue) silica gel. As water is absorbed the gel turns from blue to pink - whereupon it is saturated. The gel can be regenerated by placing it in a warm oven, or on a hot radiator to drive off the water (it will turn blue again) and it can be re-used. Sufficient silica gel is supplied to refill the tube twice so the cap will never be out of action.

The colour of the gel can be viewed through the lens on the back of the dustcap, so you can quickly assess the condition of the air inside the OTA at a glance without removing it.

I am working on a design to fit refractor tubes. Whilst these do not have mirror surfaces Fluorite elements are very sensitve to condensation. Even if the glass elements are all hard coated, removing the water vapour will reduce the accumulation of stains and prolong the interval before cleaning is necessary. The only problem with refractors is that with 1-1/4" draw-tubes there is limited diamter for the silica gel tube - this necessitates a rather longer dustcap than I would have liked. 2" draw-tubes are no problem.

Aids to Collimating the LX SCT

Simple replacement adjusting dial to replace the wrench.
Replacements for the 3 screws with integral dials
I guess those who have recently purchased a Meade SCT are a little reluctant to start fiddling with the collimation, I know I was - I resisted the temptation for at least a week :) Seriously though, there must be many SCTs in current use with optical performance seriously undermined by poor collimation of the secondary mirror. Put a high power eyepiece in (about 5mm f/l) and focus on a bright star - do you see a tight airy disk with a nice set of diffraction rings? No? - Well get that wrench out because achieving good collimation is really easy.

This is not a technical description of collimation, just a simple DIY method you can use. All collimation needs is a night of good steady seeing, and a mag 3 or 4 star at a high altitude giving a good stable image clear of atmospheric crud, and also a high magnification eyepiece (or barlow). Center the image in the FOV (always collimate with the image in the center of the field of view) and de-focus slightly to produce diffraction rings about 1/8 the FOV in diameter. Is the central bright point in the middle of the rings? - and more importantly, are the rings symmetric and concentric with it? Look carefully, take your time, follow each ring around the star image, the rings are delicate and can be tricky to follow with precision. Look for narrowing gaps between adjacent rings, look also for incomplete rings or rings that are brighter on one side than the opposite side. If all looks symmetrical, then you need do no more. More likely, the rings will be slightly offset in one direction indicating a small degree of mis-collimation.

Whilst viewing through the eyepiece put your finger in front of the corrector plate and you will see it's effect on the de-focussed star image, usually an indentation or darkening on one side. Move your finger around the periphery of the corrector plate until the indentation is level with the segement of rings that is narrowest. Go look where your finger is pointing on the corrector plate. The screw closest to where you are pointing is the one to adjust, or if pointing between 2 screws, the one opposite. Insert the wrench and turn it *very slightly* - about 1/16th of a turn - anti-clockwise, then go look in the eyepiece again. If the image has moved off-centre then bring back to center again. Has the image improved? Are the rings more symmetric than before? If the image is worse, then you will need to turn the screw the other way. And that's about all there is to it. The de-focussed star image can tell you an awful lot about the quality of the optics but you'll need a book on Star Testing (Suiter's) to interpret the results.

A couple of additional notes: It's best to collimate 'straight-through' without using a diagonal. If you do use a diagonal then check the final star image without it when you have finished (if there is any difference with the diagonal in place consider exchanging it for a better quality one). If you have not collimated your scope before it's as well to check the positioning of the adjustment screws. The ideal position is for all three screws to be within 2-3 turns of 'all-the-way-in', you definitely do NOT want any of the screws to be 'bottomed-out' in it's hole. You can ensure this is the case in the first instance by screwing them all in as far as they will go (don't over-tighten), then back each one out 2 full turns. This will be a good starting point.

I collimate every night I use my scope - when the seeing is good enough to warrant it anyway. The second picture above shows that I eventually replaced the 3 collimation screws with 3 thumb screws so that I don't have to fish around for that pesky small wrench, and I can make adjustments by simply reaching around without taking my eye from the eyepiece.

Replacement Finder Bracket

Dovetail finder bracket.
Well I finally got fed up with setting the LX200 up on it's wedge, selecting Polar Align on the handset, aligning Polaris in the finder, looking through the main scope and seeing... Nothing. Yup, I forgot to re-align the finder again.

It was not too difficult a job to machine an accurate dovetail bracket which exactly relocates the finder when it's removed and refitted. Great pleasure in chucking the original in the bin, and now Polaris is always... there in the FOV :)

A Binocular Microscope Head as a Bino-viewer

My 'Bino-Viewer'.
I was curious to see what looking with 2 eyes was like through the 10" LX200, but not so curious I wanted to pay lots of money to find out (and it seems, no one I know locally is either). So I modified this microscope binocular head to fit the 1-1/4" visual back, and added clamp screws so the eyepieces don't fall out when pointed up. The eyepieces are rather good quality (Lietz Wetzler), and the image produced is very good on bright objects, particularly the moon and bright planets. It's fairly poor on dim objects though because of light loss through the beam splitter. The view of M42 was in some ways spectacular and more '3-dimensional', though the 5th and 6th Trapezium stars disappeared. I've tried it on my 90mm Vixen refractor too but there is insufficient back-focus without using a barlow.

A Star Diagonal

Home made star diagonal
I made use of a good quality coated prism by making a 1-1/4" diagonal housing for it. The prism is mounted on a seperate plate which can be collimated with the optical axis.

Adjustable Guiding Eyepiece Tube for Lumicon Giant Easy Guider

On the right is the original eyepiece tube, on the left the adjustable tube assembly, in the middle a long threaded extension which extends the reach of the adjustable tube.
The adjustable eyepiece tube assembly with short extension. Once achieved, parfocal position is locked in place with the locking ring seen at the bottom of the outer screwed barrel.
A one-off request this - and a useful modification to the Lumicon Giant Easy Guider. Lumicon sell extention tubes of differing length to set the distance of the guiding eyepiece away from the pick-off prism, but they are not screw adjustable, and the eyepiece position is lost when released from the tube. This modification is a complete replacement eyepiece barrel assembly consisting of base unit, 2 threaded inner barrels (of different length - they can be interchanged), and an outer barrel which holds the eyepiece. The guiding eyepiece is made parfocal with the camera and then the locking ring is used to secure the outer barrel in position. The eyepiece can be removed and replaced without losing it's parfocal position relative to the filmplane.

LASER Collimator

Pictures of a laser collimator made from a cheap laser pointer.

WARNING: ALWAYS KEEP LASER DEVICES AWAY FROM CHILDREN - IF A CHILD MANAGES TO TURN IT ON THE FIRST THING THEY WILL DO IS LOOK DIRECTLY INTO THE BEAM WITH POTENTIALLY DISASTEROUS RESULTS!

This aid to collimation was originally made to help collimate a fairly fast (f/5.3) Newtonian telescope, but with a bit forethought it was possible to design it to be useful for collimating Cassegrains and SCTs.

The pictures show the general construction, and also the device attached to the LX200's visual back. The collimated (with the barrel that is) laser beam exits a 1mm aperture as shown, passes through a viewing gap and then through a 1.5mm aperture to strike the secondary mirror. The 1mm aperture is necessary because the direct output from the pointer is a poorly-defined rectangular shape. The beam is reflected from the secondary back onto the rear of the translucent nylon screen. If the secondary is not correctly collimated the beam (diverging somewhat but still small) will be seen off-center through the nylon screen (it doesn't show up too well in the photo but there really is a small, red dot in the middle of that bright red patch). The secondary is adjusted until the reflected beam passes back through the 1.5mm hole. The construction of the holder is a bit more complicated than can be seen from the pictures. The spherical nose of the laser pointer sits in a shaped seating, and a spring and collar hold it down in place. The 3 screws were adjusted with the device held in the lathe chuck with the beam projecting through the spindle onto a 8ft distant wall. In this way, turning the lathe spindle indicates if the beam is concentric (if it's not it describes a circle on the wall). Does it work? I've no idea, not having had chance to compare it with a star test (a view of the stars is a rare event around here...). However, the adjustment of the secondary to get the beam to reflect back through the hole in the nylon screen is a delicate job. It was important (of course) to ensure the device is held truely in the visual back!

201XT Alignment Aid

The 1/8" dia pin facilitates replacing the 201XT in the guidescope in the correct orthagonal orientation.

A simple modification, but a very useful one. My 201XT is normally used with the 90mm refractor guidescope (setup shown elsewhere), and this requires first aquiring a suitable guidestar using a parfocal eyepiece, and then replacing the eyepiece with the 201XT. The problem is that the callibration is done with the 201XT in a particular orientation to the drive's axes (preferably orthagonal), and replacing it in exactly that orientation is difficult - especially with the scope pointing near the zenith. The alignment pin shown guarantees replacement in exactly the same orientation each time. The eyepiece adapter on the end of the focusser is a complete replacement, the pin is press-fit into a partially reamed hole in the original 201XT nose adaptor.

Finder for 12" Newtonian

Home-made finderscope and bracket for a 12" Newtonian reflector.

My new scope needed a finderscope, so I made one from a 42mm Zeiss objective (270mm f/l) I had spare. The dovetail bar on which it mounts is useful for attaching other accessories too.

LX200 Cooling Fan

A rosette of holes is drilled in the rear cell of the LX200.
A 12V CPU fan is fitted into custom casing and bolted in place.

For times when observing sessions are expected to be brief, or just to reduce the time necessary for the scope to cool down this auxilliary cooling fan is a useful addition. Situated at the lower-left on the rear cell, the fan sucks in air at ambient temperature, circulates it inside the OTA, and blows it out through the baffle tube. The main heat sink is the primary mirror and this receives the greatest cooling effect because of the placement of the fan (airflow strikes the rear of the mirror, deflects around it into the OTA). A filter can be added between fan and rear cell, I did try a piece of high-filtration material cut from a vacuum-cleaner bag, but this reduced airflow to about 5%! I'll have to try a more open-weave material.

Note: I've since replaced the filter with a circle cut from a Scotch Bright pad, airflow is hardly affected by this material but it's not fine enough to filter fine dust or pollen out. Still, it will prevent bugs being sucked into the OTA.

12" Newtonian Cooling Fans

3 computer PSU fans mounted on an aluminium plate used to cool the 12" Newtonian.

As with the LX200, cooling fans are useful for speeding up the cool-down of my 12" Newtonian. Three 12V fans salvaged from scrap computer PSUs were mounted onto an alloy plate, the original protective grilles and rubber mountings were also used. The plate itself is isolated from the primary cell carrier to which the plate is attached with twin rubber O-rings for each of the three bolts. Vibration does not appear to be a problem although I don't usually need to have them running whilst observing. Note that there is a gap around the edge of the plate (about 1/2") to allow free flow of air down the inside surfaces of the optical tube, this allows reasonable air circulation even when the fans are switched off. The power connection is made via a 2-pin socket and curly cable to a small 12V battery, the socket has a screw-on cover for protection when not in use. The three wingnuts used for collimation have been replaced with larger stainless steel knobs and are easier/more precise in use.

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