Equatorial conversion of an Alt-AZ Celestron Nexstar SLT mount

My main imaging equipment is an Atik 460EX mono, cooled, CCD, RC 8” OTA and a Celestron Cgem mount. This equipment is bulky and cannot easily be set up at remote locations.

I have for some time been searching for a ‘travel mount’ on which I could mount my modified Canon 1000D and prime lenses for the purpose of doing wide field long exposure Astrophotography away from home.

The costs of these mounts, for example the AstroTrac, is substantial and so are the downscaled equatorial mounts made for this purpose. I also considered the Vixen Polari and rejected that out of hand as it cannot be auto-guided. Apart from being expensive, most of these mounts have one motor tracking in the Right Ascension axis only and some have tracking duration limitations having to be reset.

I then had a bright moment, thinking that maybe my old Celestron SLT, my first telescope, gathering dust for years, might be converted into a light-weight DSLR mount that can be auto-guided.  

I must admit that I embarked on this project with little hope of ending up with a functioning mount. These ‘cheapie’ mounts have sloppy gears and the bushes are not machined to be able to track accurately, right? Whilst this is true to a certain extent I was pleasantly surprised that I was able to after the conversion, run exposures of up to 240 seconds, ten light frames and more at a time, using a 200 mm fixed lens on my modified Canon 1000D DSLR auto-guided through a Imaging Source cam and a cheap GSO finder scope.

This set-up produces remarkable images for a modest outlay and makes an ideal travel mount. Power consumption is low and a 12 volt battery pack combined with a fully charged laptop lasts for more that 5 hours. More than sufficient time to do some serious imaging. 

Construction notes:

The heart of the conversion is an equatorial wedge that can be adjusted both in Latitude and Azimuth.

Equatorial Wedge

The second most important part is a mounting plate for the DSLR and the guide scope. I have also attached a red- dot finder to the mounting plate to make it easier to know where you are when slewing as it is not easy to do this through the finder scope and impossible through the camera view finder or live view.

Camera/Finder/Guidescope/Red Dot Finder mounting plate

The DSLR, finder scope and red dot finder are adjusted so that they all have the same aiming point. In other words, if the object is in the finder scope it will also be on the red dot and on the DSLR chip.

I manufactured the wedge and the DSLR mounting plate from Plastic(Perspex). This material is easily glued, easy to machine, light and can be built up three dimensionally by gluing pieces together using Chloroform (use in well ventilated place, you do not want to go napping whilst doing the gluing) and so forming any shape and size you require. You can use wood as a substitute the only requirement is that what you end up with must be light so as not to strain the mount as there is no way to counter-balance a SLT mount.

My setup, DSLR Camera, 200mm Takumar Prime lens, Finder Scope and Mounting Plate weighs in at just under 3kg. I believe the maximum should be limited to 3.5Kg so as not to overload the mount. The lighter the better the auto-guiding will be!

Equatorial Mode: How

To allow the SLT Alt/AZ mount to function in equatorial mode you have to refresh your SLT hand control with Celestron NXS vs 4.18 and MC 5.14 software downloaded from:

 www.celestron.com/c3/downloads.php

The SLT hand control readily accepts the software and I have not experienced any problems in downloading. 

Auto Guiding: What you will need

I use PHD, free download from:

 www.stark-labs.com

 In order for the mount to accept instructions from PHD you will also need to download the ASCOM platform for Celestron mounts from:

 www.ascom-standards.org

In addition you will need a cable to download NXS vs 4.18 hand control software and to connect your hand control to your PC. You can build your own cable following instructions from:

 www.nexstarsite.com/PCControl/RS232Cable.htm

You can buy this cable from Celestron. It is a simple build but if you do decide to build your own, follow the instructions to the letter, test and test again that you have connected the pins correctly. A mistake here may damage the electronics of the mount beyond repair.

You will also need a Serial to USB device, freely available at low cost but buy the best you can, some cheap serial converters are prone to failure. 

 

Polar aligning your creation:

Bear in mind that you are not dealing with a high-end mount machined to fine tolerances therefor this mount is sensitive to accurate polar alignment. This mount was not originally designed for this purpose so it is a somewhat fiddly to set-up the first time but once mastered a joy to use and I still marvel at its ability to be auto-guided producing subs with no star-trailing.

How to EQ Align and Polar align. (The instructions apply to the Southern Hemisphere.)

1 Adjust the wedge to your latitude using a clinometer held against the wedge, a smart phone clinometer app. works brilliantly.

2 Set tripod level (bubble on tripod) and align the tripod due south using a compass; a smart phone compass app. works great for this as well.

3 Power up the mount and use the direction buttons on the hand control to level the camera and guide scope in both axes using the bubble levels you have glued to the camera mounting plate.

4 Power off and on again. This zeros the optical encoders on both axes. The SLT mount has no index marks and this is the only way I could come up with for zeroing the encoders.

Encoders set to zero bubbles on mounting plate level

5 Under Utilities on the hand control, Get Axis Position, set the camera platform to your latitude using the up and down direction buttons turning the declination motor. This is the same angle you have set your wedge to. The shaft of the main motor of the mount is now pointing at the sky at the same angle as your latitude and so is your DSLR and finder scope. 

Wedge set to site Latitude and Camera lens set to the same angle pointing at Celestial Pole

6 You are now ready to EQ align and Polar Align the mount.

6.1 Choose EQ South Align. Then choose the One Star Align option. Input the data for your position, date, time and coordinates as prompted by the hand control. Using the direction buttons slew to your chosen known alignment star when prompted by the hand control.  Once you have centered and aligned the alignment star, press enter. The mount now knows where it is and is ready to be Polar aligned. In the Southern Hemisphere Acrux and Hadar work well as Alignment and Polar Alignment stars.

7 Press align on the hand control then press Polar Align and follow the hand control instructions. Make the fine adjustments to the wedge in Latitude and Azimuth to accurately align the mount. If you have taken care with the rough polar alignment of the tripod and care in setting the wedge angle as well as the axis position angle (hand control) the adjustments required will be small.

Important: these adjustments are made to the wedge itself, adjusting the latitude and the RA axes, not touching the hand control, as per the instructions displayed on the hand control. Take care not to disturb the tripod whilst adjusting the wedge.

8 Once you are happy that your alignment star is dead center in the eyepiece, press enter and your mount should now be Polar Aligned.

The Go To accuracy of the mount, using the one star alignment (Acrux), will not be great. This will improve substantially if you do an EQ South two star alignment, especially if the second star is close to your intended imaging target.

Alternatively use the position of a known alignment star in the vicinity of your imaging target, for example Antares, near the Eagle, Triffid and the Lagoon Nebulae. Slew to Antares and replace your initial alignment star (Acrux) with Antares. This will result in sufficient Go To accuracy to get these Nebulae in the eyepiece of the finder scope. (Consult your SLT user manual for detailed instructions on the replacement of alignment stars)

9 Before slewing to your imaging target focus your DSLR on one of your alignments stars. I use a DIY Bathinov mask for this. Taking, say 1 second previews, adjusting in between to ensure 100% focus. Do not trust your infinity setting on the lens to give you perfect focus. 

10 Let the mount slew to your target by inputting its NGC designation and then enter. Use the hand control buttons to center the object in the eyepiece. If the DSLR and finder scope has accurately been set pointing at the same spot your target should now also be on the chip.

Auto Guiding: How

1 Once you are satisfied that your target is centered on the imaging camera chip by taking a few short exposures and adjusting with the hand control replace the eyepiece of the finder scope with your guide cam.  The finder scope now becomes your guide scope. Take care in doing so as not to disturb the mount when switching the eyepiece with the  guide cam. I manufactured an adapter in which the guide cam and eyepiece is an easy press fit in such a way that the eyepiece is par-focal with the guide cam.

Eyepiece and guide cam adapter

 

Guide Cam with adapter in Finder/Guide Scope

2 Launch the PHD guiding software.

3 Check the ASCOM box and follow the prompts to select the mount.

4 Check the Camera box and set the guide cam settings. I use .5 second to 1 second exposures and a low gain for the guide camera.

5 Select a guide star and let PHD do its thing with the guide camera orientation protocol. The calibration steps might be as many as 40 or more. If calibration fails, increase the calibration steps.

6 The following ‘aggressive’ PHD settings work for me. (Brain Icon in PHD)

6.1 RA aggressiveness 100

6.2 RA Hysteresis 10

6.3 Dec mode, sometimes Auto works but if not choose either South or North depending on which way the mount is tending to correct the most.(drift)

6.3 Max Dec duration 1800

6.4 Calibration steps 1500

Use these settings as a base and experiment to find the settings that work for you.

Once PHD has gone through its guide cam orientation routine (calibration) guiding should commence. Give PHD a few minutes to settle before imaging.

Some notes:

1 The PHD graph will probably show substantial peaks and troughs. With a 200mm focal length lens, I have found that it is of no consequence, as long as the cross hairs remain on the guide star.

2 Sometimes when PHD starts guiding the guide star goes off in a direction and eventually works its way out of the green alignment box. This can be due to poor polar alignment, due to play in the mount or poor calibration. If your polar alignment is not spot on redo the alignment. If you are sure that poor alignment is not the cause it may help to carefully nudge the star with the hand control into the green box again and in doing so take up the play. If this does not solve the problem force calibration. (Brain tick box)

 3 The above sounds more involved than what it actually is but after doing this a number of times the whole process now takes me about 15 minutes. I have been able to get it to work 9 times out of ten on the first try.

4 With the fast Guide/Finder Scope I have never had a problem not having a suitable guide star on the guide cam chip so it is not required for the guide/finder scope to be adjustable.

5 This conversion was aimed at long exposure imaging using my DSLR and I have easily managed 4 minute exposures. I am sure longer will be possible but have not found the need to go beyond 4 minutes.

I have not tested this with a telescope and doubt if it will work with focal lengths longer than 200mm. I believe using longer focal length optics will result in noticeable elongated stars due to the increased magnification. For the fixed lenses, 50mm, 135mm and 200mm I use I have been pretty pleased with the wide field imaging results.

A telescope instead of a camera lens will also present challenges for not fouling the EQ wedge when zeroing the optical encoders as well as adding weight the mount may not be able to handle.

The ancient fixed lenses I use are optically of surprising good quality and the whole purpose of this conversion is to do wide field photography. A telescope with a longer focal length will sort of defeat the object of the conversion. 

6 Depending on your latitude the power connection to the mount may foul the wedge. I resolved this by connecting the cable directly to the battery terminals in the battery enclosure and then ran the wire along the mount arm in such a way that it is does not foul anything.

7 Used SLT mounts can be had for song.

Check out the images produced by this set-up in my gallery under the Canon folder.

Contact me at This email address is being protected from spambots. You need JavaScript enabled to view it. if you need further clarification.