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Conceived to fill the need for a compact, rigid, and easily adjustable guidescope mount. Centering a guidestar is made simple by using micrometer dials to precisely adjust where the guidescope is pointing. The dewshield is none-standard, machined from alloy tube with internal baffles.

The controls are labeled on this photo. Each axis has both an adjustment dial and a locking knob.

This ETX 90 has been kitted out with a Vixen GA4 illuminated guider in the 90-degree eyepiece port, the straight-through visual back has the Meade extention tube plus a custom 1-1/4" adapter ready to accept an ST4 autoguider.

The eyepiece can be selected using the ETX's built-in flip mirror to select the guidestar. The flip mirror is also useful for taking dark frames with the ST4.

Unscrewing the Azimuth lock-knob a couple of extra turns allows the guidescope mount (plus ETX in this case) to be quickly released.

The base remains permanently bolted to the LX200 mirror cell. The tangent arm drive pin can be seen.

The tapered stainless steel spigot (seen sticking out below the bottom of the mount in this photo) provides the pivot for the azimuth movement, it also supplies the means to lock the mount to the base. The spigot is tapped through 1/4"x20 so it can also be mounted on a camera tripod.

A view of the underside of the mount showing (far R.H. side of the mount) the tangent arm drive for the Azimuth movement. Incidentally, the scope is seen resting on the dewcap which has drilled holes (and covers) so it doubles as a Hartmann Mask to aid focussing a CCD camera.

By using a 1/8" locating pin (attached to a parfocalising ring) and a corresponding hole in the 1-14" adapter, the ST4 can be removed and replaced without recallibrating.

The whole thing - LX200 (slightly modified...) 416XT and ST4/ETX guider.

Description:

I obtained an ETX optical tube (sans fork mount), for use as a high-magnification finder and also to see if it could be made to work as a guidescope on my 10" LX200. There are several very attractive features that would make the ETX suitable for this particular job, and also one glaringly obvious disadvantage:

PROs:

• 1. It has a long focal length (1250mm) in a compact optical tube.
• 2. Compared to an equivalent sized refractor it's very light in weight.
• 3. The autoguider can be firmly clamped to the rear of the ETX so no flexing focusser tube to worry about.
• 4. It has a built-in flip mirror to help select a guidestar visually without having to remove the autoguider. The eyepiece can be made parfocal with the guider making for rapid focussing.
• 5. It offers very sharp star images.
• 6. It has electric focus! (well, mine does anyway)
• 7. It has a convenient mounting lug (with two 1/4" x 20tpi threaded holes) on the bottom of the tube.
• 8. Older ETX's can be purchased cheaply - especially if you can find an RA model with a broken drive.

CONs:

• 1. The ETX uses a moving-mirror design to focus - a potential problem therefore in that it could give rise to differential movement of the two optical axes due to mirror flop (and that's exactly what you DON'T want from a guidescope!). Life's tough enough dealing with the mirror flop inherant in the classic LX200. I have no personal experience whether the mirror lock improvement in the new GPS models is effective in combating this but I have my own solution for the old classic LX200.

I had thought of using a pair of normal 3-screw rings to hold the ETX but having become accustomed to the convenience of using an ALT-AZ guidescope mount (with my 90mm Vixen refractor) I simply couldn't go back to that method! However, the ETX tube was too short to easily adapt to my current guidescope mount. So I sat down and thought how a completely new mount might be designed and came up with the following criteria:

• First and foremost, to work at all the mount has to be rigid.
• The pointing adjustments should be made using micrometer screw dials, and quickly locked in any desired position.
• Need about +/- 5 degrees angular movement to get a good guidestar in just about any position in the sky.
• The guidscope has to be quickly detachable without tools (because I have to setup everything in the back garden every time).
• The less weight the better.

The mount consists of a baseplate and spigot to provide the azimuth movement, controled using a tangent arm. The spigot has a 10-degree taper, the end of the locking screw has a matching 10 degree conical end. When tightened, the spigot is pressed against the opposite side of the bore, whilst at the same time the baseplate is drawn downwards onto the top face of the bracket (which is permanently attached to the main telescope). The altitude movement is accomplished by adding a very close-fitting tilting plate, the locking system for which clamps two side plates together slightly (only a couple of thou movement is needed) thus forming a rigid box-section. When both clamps are engaged the whole assembly very rigid indeed. A slight drawback is the limited 'footprint' afforded by the radiused base bolted to the rim of the LX200's mirror cell. with no support forward of this there is potential for a lot of leverage to be applied with shifting weight. My impression is that it will not flex in normal use, but care needs be taken to ensure the CCD autoguider cables are not left hanging loose.

It remains to be seen how well the ETX performs, some modification may be necessary to fix the mirror in place, although I'm rather hoping that it's light weight will mean it's less likely to flop around than that of the 10" LX200. Even if the ETX proves to be a failure the guidescope mount itself will be equally happy carrying a small refractor (Ranger, Proto etc.,) so the effort making the mount will not be in vain.

12th October, not a bad night, reasonably dark but lots of mist around. The guidescope was easy to set up but getting the eyepiece in the ETX's 90 degree port parfocal with the ST4 guider proved impossible, there was not enough back-focus when using the extra diagonal of the GA4. Trying to use it without the GA4 was no good either - too much back-focus! I'll need to make an adjustable connector for the ST4 so I can figure the best position (distance) for it. I ended up repeatedly removing the ST4 and replacing it with an eyepiece to aquire a guidestar, no real hassle thanks to the locating pin.

Other than that the mount seems to work reasonably OK. A bit of movement when the clamps are firmly tightened, to be expected I suppose with the camming arrangement of the azimuth lock. I'll see if I can modify that (maybe change the spigot taper from 10 degree to 30 or 45). Also, the view through the 90 degree port does not match that seen straight-through. The selected guidestar is visible in both but offset. Although this can be compensated for using the GA4's movable reticle I'll probably correct the positioning of the mirror. Lastly, there is a worringly significant mirror-shift when focussing, indicative of the mirror slider being too loose on the primary baffle tube. Although this did not appear to affect the guiding capabilities once focussing was completed it is still a possible source of misguiding. If I can think of an easy fix for the problem (locking the ETX's primary mirror) then I'll go ahead and do it, if the problem seems unresolvable without a lot of effort I'll abandon the ETX in favour of a small refractor.

These images were taken using the ETX to guide the 10" LX200:

	M27, 10" LX200 @ f/6.3. Meade 416XT 4 x 5min exposures.
	Part of the Veil Nebula, 10" LX200 @ f/6.3. Meade 416XT 4 x 5min exposures.
	Here I've cropped out parts of the four 5 minute exposures (unprocessed) used to make the M27 image. That's a total of 20 minutes guiding. From what I can see, any overt tracking errors are less significant than the general noise in the image.
	This photo of M31 (Losmandy G11 / Genesis SDF f/5.4 @ 45 minutes) was autoguided using the ETX. No evidence of mistracking (but the focus might leave a bit to be desired!).

As mentioned above, to get the ST4 parfocal with the eyepiece in the GA4 I found it necessary to have an adjustable connector between the ST4 and ETX. The one I made a couple of days later is shown below. How to set it up? -

• First, choose an eyepiece to use with the GA4. Best to use a low-power one, and in my case that means a 32mm Celestron Ultima which will give a nice bright, wide field.
• Focus the GA4's reticle using the screw adjustable connector (part of the GA4 itself) so the reticle is sharp viewed through the eyepiece. It looks fairly small in a 32mm eyepiece but that's OK.
• Focus the ETX so stars look sharp visually through the 32mm and GA4.
• Now the somewhat tricky bit, without adjusting the focus of the ETX, focus the ST4 using the screw-adjustable connector shown below. This means taking a continuous focus image, removing the ST4 and turning the connector, and checking the image again. Use a Hartmann mask to make things easier.
• Once everything is set tighten the locks on both eyepiece and ST4 adjusters, there will be no need to go through this again.
• Loosen the rear connector where it screws directly onto the ETX and turn the whole assembly (ST4 plus connector) until the ST4's chip is orientated orthogonally with the drive Axes. This makes it easier for the guider to callibrate star movements. The dowel pin allows the ST4 to be removed and replaced in exactly the same orientation so if you never remove the connector from the ETX you won't need to recallibrate at all. MaximDL actually compensates for changes in DEC so if you have a PC connection to your telescope everything is done automaically, otherwise you need to type in the DEC of your guidestar before commencing an imaging run.
• Finally, get a guidestar centered in the ST4's image, switch to the eyepiece and move the reticle so the star is centered within it. It's then easy to visually place any selected guidestar into the middle of the ST4's FOV.
• To set up each night all that's needed is to focus the ETX visually, then just check the focus of the ST4 - slight tweaking here won't make any significant difference to the view through the eyepiece. A slightly out-of-focus guidestar will not affect guiding accuracy but it will reduce the lowest magnitude of star that can be used.

	Here is the adjustable connector for fixing the ST4 to the rear port of the ETX. This is before it's anodised black. Notice the parfocalising ring on the ST4 has a pin which locates in the holder at far right, this enables the guider to be removed and replaced in exactly the same orientation (saves re-callibrating).
	This is an alternative base which can be bolted to a flat surface, such as a GEM saddle-plate. If the new G11/Gemini mount I ordered ever turns up I'll be bolting this to it.

A Pronto GuideScope

	The Pronto mounted on the AZ guidescope mount. The alloy adapter plate can be seen beneath the Pronto (as yet not anodised)
	Another image of the Pronto - note the ST4 guider is inserted into the Vixen GA-4.
	ST4 image of the Vixen GA-4 reticle. A useful way of selecting a guidestar is to use the GA-4, with the reticle aligned with the center of the ST4 FOV then it's easy to replace the ST4 with an eyepiece to scan around for a guidestar. The ST4 has a small chip and consequently small FOV, this method guarantees putting the guidestar in the middle of the chip.
	Here can be seen the ST4 cable clamp (to the rear of the 35mm camera - with the yellow plastic screw). As mentioned in the text, this is useful feature for preventing the cable pulling on the focusser.

As mentioned earlier, this AZ guidescope mount is suitable for small refractors in addition to the ETX90. I recently managed to obtain a TV Pronto which I thought would be a suitable guidescope for my TV Genesis SDF doing prime focus 35mm film work. It has a focal length of 450mm (it's an old Pronto!) compared to the 540mm of the SDF, so the guiding ratio is OK using the ST4.

I wanted to be able to swap between the different guidescopes so this required some sort of interchangeable mount system. The ETX90 was simply bolted directly to the AZ mount's tilt plate from the underside using a couple of 1/4"x20 screws, which means the whole unit has to be disassembled to release it. Adapter plates were therefore machined up, one for each guidescope, and by this method the guidescopes can be quickly exchanged by releasing four thumbscrews threaded into the top face of the tilt plate.

The pronto has a good 2" focusser which locks firmly with the clamp screw, and as you would expect from TeleVue the focusser tube is a close fit in the bore of the focusser resulting in very little lateral play. These are essential features for a refractor guidescope because even slight movement here would be just as fatal to obtaining sharp images as is mirror-flop in the ETX. As an added precaution the ST4's signal cable was secured to the mount head (see picture below) thus preventing the cable pulling on the focusser. The ST4's cable is exceptionally inflexible, especially when cold, and this is a very real problem. If you want to test it yourself then try inserting a high-power eyepiece (say, 5mm f/l) which will mimic the image scale of the ST4 CCD chip, and gently push laterally on the eyepiece. You might think your focusser was free of play but this test usually reveals that very little pressure is required to cause significant image shift. This shift translates to differential flexure in the final image, and elongated stars result.

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