Mounting the Nexstar

In the first photo on the right, the wedge with the center peg removed has been mounted on the Nexstar SE tripod. The center peg on the tripod fits well as it is much smaller than the space left by the removed wedge peg. The tripod bolts hold everything in alignment and there is little to no lateral movement of the wedge in relation to the tripod. The bolts are 5/16″ x 24 x 1.5″ cap screw bolts from the local hardware store. The azimuth adjusters work perfectly with this setup as long as you don’t tighten down the cap screw bolts too much.

One rather major item of caution is the length of the bolts that attach the Nexstar drive base to the wedge. Inside the drive base is a gear (right under the fork arm) that rides very close to the opening for the mounting bolts and is completely unprotected. (Seen in the 2nd photo.) If a bolt is too long and protrudes very far inside the telescope, this gear will bind against the mounting bolt probably causing damage to the drive base. These bolts are 3/8 x 16 — I got lucky and had an extra set from my old Losmandy GM-8 that were the right size but much too long. I cut and sanded them with a Dremel and then added some thick washers just to be extra careful. The final image on the right shows underside of the wedge with the Nexstar mounted.

Another modification I made was to remove two of the rubber feet from the Nexstar base to allow for more latitude range before the Nexstar base impacts with the side of the wedge as the drive base is slightly wider than the wedge. With the rubber feet removed I can get the latitude all the way to the 14 degree mark before the base connects with the wedge — with the rubber feet on, this was more like 30 degrees. The rubber feet peel off without too much difficulty and if I ever what them back on, they’re easy to reattach with some rubber cement glue. (I left one of the rubber feet on to designate the ‘top’ of the mount.)

Adding a Latitude Adjuster

My very first telescope back in 1999 was a Meade LX50 on which one of the first modifications I made to it was upgrading the rather flimsy wedge latitude adjuster that came with that scope. Since then I’ve had the old adjuster kicking around from box to box mainly because I’m a pack-rat but now I’m glad I never threw it away! The length of the bar turned out to be too long for the Celestron wedge by only about 3mm. I cut and sanded the bar down to size with Dremel cutting and grinding disks (at one point I had to wear work gloves because of how hot the bar was getting!) until it was a snug fit and then ‘painted’ any visible scuffs with a black Sharpie. The end result almost looks like it was meant to be there. 😉

Once down to size, I fixed the bar in place to the lowest point on the latitude guide groove on the wedge with a couple of bolts and washers to hold it in place. The long latitude adjustment screw fits well to the underside of the wedge and since this setup is not going to take a bunch of weight (certainly much less than the LX50 it was originally designed for!) it works very nicely. This modification also added a great deal of stability to this wedge. I was lucky to have saved the LX50 latitude adjuster and I think that even if I didn’t have it, I would have eventually wound up trying to make something similar out of wood.

One thing I’d eventually like to add is a pair of knobbed bolts in place of the current cap-screw bolts I have for the latitude adjustment on each side of the wedge so I can loose the allen wrench I currently have to use to loosen them.

Celestron 9×50 Finderscope Autoguider

I found a few pages and forum posts (http://bit.ly/N5GJ7D, http://bit.ly/QoZl9f and http://bit.ly/MUgGDD) with instructions on making an adapter for Orion’s autoguider using PVC pipe fittings but unfortunately all of them required drilling a hole in the finder which I didn’t want to do. I wanted a solution which would require no modifications to the finder and would be easy to undo.

Luckily, while fooling around with the finderscope one afternoon I realized that the threads on the back of Celestron’s 9×50 Model # 51611 finderscope that the cross hair eyepiece attaches to are common 2″ SCT threads! Given that the 1.25″ adapter on the Starshoot autoguider attaches to a standard T-thread on the autoguider body all I needed to find was a male SCT to male T-thread adapter to fit everything together.

I had no clue if such a beast existed and after a little bit of googling I found what I needed at http://agenaastro.com — their Blue Fireball T / T2 Male Thread to SCT Male & M48 was exactly what was needed to make this work. (Seen in the first photo on the right.) This adapter is very low profile and easily allows the finderscope to achieve infinity focus with the Starshoot autoguider.

As shown in the middle and bottom photos on the right, the whole thing comes together in a nice, compact and sturdy assembly and it fits perfectly on the standard Celestron finderscope mount with no modifications needed.

Side-by-Side Camera/Autoguider Mount

Next up was finding a sturdy way to mount the finderscope and camera on the Nexstar.

From a previous project I had a mounting bar with a center hole tapped for a standard 1/4-20 tripod stud with 4 untapped holes on each side. A couple of bolts and washers from the local hardware store worked to mount the Celestron 9×50 finder mount securely to the bar.

To attach the whole thing to the Nexstar, I used the mounting block from the Orion EON 72mm f/6. (Which normally rides piggyback on my CPC800.)

The second photo shows the system all set up with the finderscope, Starshoot autoguider and an IR modified Canon 300D. One of the lenses I plan to use with this setup is a Zenitar 16mm fisheye but unfortunately, the finderscope is mounted so far forward that it projects into the Zenitar’s 180 degree field of view. So I’ll either need to crop the resulting images or figure out some way of mounting the camera further forward or the finderscope further back.

The last image shows everything attached to the Nexstar SE mount and ready to go. Note that I did have to remove the plastic altitude gear covering for the side-by-side mounting to fit which detracts a bit from the looks but everything still works fine.

I love how compact and multi-functional this setup is and can’t wait to get some time out in the field with it. Running with rechargeable batteries for the mount and with spare batteries for the camera and my netbook I think I can get a solid evening of imaging with this setup without ever needing a power plug.

The next step was to adapt a cheap, old wedge for the Nexstar

First lets take a look at opening the camera.

The picture to the right is confusing but there is method to the madness.

The view is of the rear of the camera. The 2 screws circled in green only hold the tripod adapter in place.

The screws circled in white attach the back plate to 2 long bolts that run the length of the camera that hold the optical window housing in place. These must be removed. (Be careful not to over tighten these when you put the camera back together!)

The hexagonal nuts circled in red hold the back plate of the camera to the 15 pin plug that is soldered to the circuit board. These must be also be removed.

Once the back plate and the camera housing have been removed you will notice a long brass bolt on each side of the camera.

These bolts hold the front part of the camera in place – the metal housing for the CCD to which the optical window is attached.

Be careful when removing these as the whole front of the camera will come off and the CCD enclosure is made airtight with

thermal heat sink grease as you can see in the picture on the bottom right. If you are going to do this it would be a good idea to get some computer silicone heat sink paste (a.k.a. thermal grease). at RadioShack or on-line.

The grease under the chip on my camera was completely dry and I had to clean it and reapply some. Remember though that a little goes a long way with this stuff!

While you’re at Radio Shack pick up an antistatic bracelet. CCD chips (like most electronics) can be easily destroyed by a static discharge and it is always good practice to keep yourself grounded.

Here is a close up of the chip. The easiest way to remove it is with a thin flathead screwdriver like in the image or prying it up with an x-acto blade. Alternatively, places like RadioShack have specific tools for inserting and removing chips from sockets without damaging the pins.

The trick is to gently lift one end and then the other off of the cold finger until the chip comes loose. It is very easy to bend the pins if you aren’t careful.

Note the notch on the left side of the chip. This is not pin 1. (It is in fact close to pin 10) It is there, I assume, for leverage to make removing the chip from a socket easier. Pin 1 on these chips is marked by a circular indentation on the underside of the CCD.

Since the notch is easily visible I used it to mark the orientation of the chip in the socket. This is very important to take note of. If the CCD chip is incorrectly inserted you could destroy the CCD and possibly the camera. That said though, the Sony CCD chips are very similar and pin 1 (or 10) should be easy to identify.

I used a drop of liquid paper to mark the position of pin 10 on the cold finger for future reference.

In the bottom picture you see the cold finger with the CCD removed and dried grease I had to replace. The grease around the edge of the camera was thick but holding out so I left it. It is there to make the enclosure airtight and prevent moisture condensing and then freezing on the CCD chip as it cools down. This was a major problem I had with my CB245.

Installation of the new chip is easy – correctly orient the CCD in the socket, check that all the pins are entering the sockets correctly and press down gently and evenly putting pressure over the pins on each side. Its should just slide in…

This image shows the CCD mounting as seen edge on with the new chip installed. Under the CCD is the cold finger and under that is the TEC or Thermoelectric Cooling Device.

Once the new chip is installed replace the CCD housing and secure it with the long brass bolts. Do yourself a favor and make sure that the glass is very clean both on the housing and on the CCD window before closing it. Any dust specs on these surfaces will show up as circles or doughnuts (depending on your telescope) when you try to image forcing you to take flat field frames.

Don’t close the rest of the camera yet as we now have to calibrate it for the new chip. For calibration we will run a few tests with the camera open and running so adjustments can be made to 2 variable resistors located on opposite sides of the printed circuit board.

VR1 is located close to the center of the PCB and is pictured on the right. Click on the image for a close up.

First of all fire up the camera and take a test image through a pinhole or with a camera lens attached. Hopefully you’ll get a normal image and we can move on to dark frame adjustment.

If there is a problem, power off the camera and double check the obvious. If the problem persists try reinstalling the color chip and taking an image with that.

For the dark frame test we need to let the camera cool down for about 5 minutes. The cover the camera so no light can make it to the CCD chip (Remember how sensitive these chips are to light! I actually put my camera in a black cloth bag as well as covering the CCD.) and take a 1 second exposure and look at the histogram of the dark frame you just took.

The histogram of a properly calibrated camera should look a lot like the one on the above. (Note: I used the MX716 version of the software – StarlightXpress Star_MX7 v2.0e (04/06/2002)) There should no more than 1 or 2 entries in the VAL field and the values should range between 006 and 016. If the values are greater than 16, use a small flathead screwdriver to turn VR1 a few degrees anti-clockwise. Then take another 1 second dark frame and examine the histogram values to see if further adjustment is needed. If the values are lower than 6, use a small flathead screwdriver to turn VR1 a few degrees clockwise and repeat the dark frame histogram process until the values are correct.

After this process is complete it is a good idea to take a 5 minute dark frame to make sure the darks are free of hot spots bright streaks. There will probably be a brighter area in the upper left area of the frame. This is caused by the output amplifier and should be quite faint. If you have the chance it would be a good idea to compare your dark to the dark frame of a calibrated camera of the same type.

The method I used was to tape a high power eyepiece to a flash light as seen in the image on the right. The eyepiece will focus (or narrow) the beam of light to a finer point. I used a University Optics 12.5mm Orthoscopic. I found the beam to be too bright even with the flashlight on the lowest setting so I added a variable polarizing moon filter to dim the light even more.

VR2 is located on the opposite side of the PCB towards the edge and front of the camera as seen in the picture on the right. This variable resistor controls the ABG (Anti-Blooming Gate) bias on the CCD chip. Anti-blooming prevents streaking and bloating of bright stars but slightly and affects linearity. ABG can be effectively turned off by turning this VR about 20° anti-clockwise.

To calibrate VR2 we need to focus the camera on a light source (as previously described) in a dark room and take a 1 second integration. My camera’s ABG bias was set too low and my image of the light source looked a like the image on the right. Instead of being as close to a point of light as possible, I got an oval due to “bleeding” caused by anti-blooming being almost set to the off position.

To calibrate the camera take a 1 second picture of your light source. If the resulting image looks like the middle picture on the right and/or if there is a bright streak in the image then the ABG gate is set too low. Turn VR2 a few degrees clockwise and take another integration.

If the image is gray or grainy then the ABG is set too high. Turn VR2 a few degrees anti-clockwise and take another integration to test the result. If you overshoot too much in the clockwise direction the camera sensitivity will be decreased by too much anti-blooming. If you overshoot anti-clockwise sensitivity will be higher but stars will be bloated and oval and streaks will appear so proper calibration is critical. This is the hardest and most painstaking part of the calibration to get right. My suggestion is to turn VR2 anti-clockwise until you get a bloated star image like mine and then take 1 second integrations turning VR2 clockwise 2 or 3 degrees at a time until the blooming just disappears.

For the final step in calibrating the camera point it at a white target and take an integration that results in a mostly saturated image. Here I printed a 4 colored boxes and a thin crosshair for reference. The resulting histogram should have a major peak at a VAL of 255.

If the peak does not reach a VAL of 255 or if the image look gray or grainy then VR2 is set too far clockwise and you must go back and adjust it.

And that is it! You now have a MX7 with either the color or the mono chip installed, calibrated and ready to go!

Check my gallery to see images taken with both the MX7 and MX716:

http://astroturtle.com/imaging/

April 13 2003 update: Due to, among other things, time constraints and a brand-spanking-new MX7C, this autoguider is pretty much an abandoned project. Which isn’t to say that I won’t give folks a hand if they need it. I have some goodies that may be of help or at least a starting point for this project.

This is page will hopefully help some out there build the CCD Cookbook based autoguider circuit and provide some background on how to connect one to an LX50.

First of all I want to thank a good friend of mine, Michael McNeil over at http://www.caltel.com/~cno [on http://web.archive.org] for all the help and great advice.

This is really his baby as he put the board together for me leaving my limited knowledge of electronics to work out how to connect the thing to my LX50. Even then a good number of e-mails went back and forth reflecting on the pro’s and con’s of 74LS14’s… Sheesh!! :o)

This autoguider is an update of the version available in the CCD Cookbook which is based on the AY-3-1015D chip that is no longer in production and getting harder to find. The guider will work with the CB211 and CB245’s as well as with the old Connectix black and white parallel port quickcams thanks to Martin Niemi’s great autoguider software.

Here’s a link to Marty’s page [on http://web.archive.org]:

http://www.ameritech.net/users/mniemi000/auto.html

To the Cookbook autoguider page:

http://www.wvi.com/~rberry/cookbook/serial1.htm

This is the pinout for the LX50 Autoguider port. The orientation is as if you were looking at the LX50 panel straight on. The best thing to do is just to get a ready made cable that is wired straight through.

You need a 6 pin RJ-12 plug. The pin numbering on these plugs is read from left to right (duh) with the plug held ‘upside down’. That means the little plastic flange that locks the plug is pointed at the ground, the embedded contacts are pointing up and the side the cable goes in is pointing towards you.

Here’s a screen shot from Marty’s program in action. One thing that had me confused the first night out is that if the guide star is too bright the program won’t lock on it. I was using a bright star just to make life easier but I eventually chose a dimmer star and threw my Hartmann mask on the scope for good measure and everything went great from there.

In my case, the calibration routine threw the guide star right off the frame in the Y-axis the first couple of times. Try, try again and start the calibration with the guide star as close to center as possible!