Last updated: 6 July 2005
Subject: Optimising the ETX for CCD Photography Sent: Sunday, July 3, 2005 12:36:38 From: Stephen Bird (email@example.com) Optimising the ETX for CCD Photography The ETX is great little scope for visual astronomy, but it is also good for astrophotograpy as well, but it takes a little work to get good results. In my quest to get good pictures from the ETX, I decided to try the Meade LPI and DSI (original and Pro versions), I guess the price was right and they ought to compliment the scope being made by Meade. Having got some average snaps I wanted to move into the next league, so my mind turned to the issues of "autoguiding", and I had an exchange of e-mail with Dick Seymour, resulting in the following first thoughts. some of this needs refinement, and may be a little wide of the mark, but hidden in here are a few pointers for would be CCD astronomers. Well I guess where I am coming from is I just wanted to know the limitations of the "autoguiding" using an ETX. Because effectively the ETX via Autostar Suite and the Autostar can autoguide using a single LPI or DSI. I think it achieves this be interleaving your image capture frames, with an alignment frame which you don't see. This extra exposure between frames, uses the centroid box on screen to check for a change of position of the chosen object, and send some guidance commands via the Autostar to the ETX, to correct any drift in the image. I have at the moment an LPI and a DSI Pro, I returned the DSI standard as it was a "lemon" in my opinion but judging from the DSI Pro that I now have, I was probably unlucky, so don't dispair if you have the DSI original (standard model). So here is my weighty tome on how I see it all. If any of it is wrong, it is because it is not in the manual and I had to deduce it myself, so I deny any responsibility, blah, blah, blah! TIP 1 As an aside, the manuals being on the CD or on Meade's website is not an issue. It is certainly a simple matter, especially in Windows XP, to copy them to the LPI / DSI Documents folder, then create a shortcut for each manual, and add the shortcuts to the start group (or create a start group and add the manuals to it) for the manuals. Then they are easily accessible via Start Programs - Meade - Manuals. I have added the LPI, DSI Quickstart, ETX AT, Autostar 497, and ETX Premier manuals to that start group, so the information in each is instantly accessible. CD's are not convenient in the dark! What I have found for the Autostar Suite Autoguiding using the ETX is that first of all you need to reduce all sources of drive error to an absolute minimum. Here are the 10 worst error sources I have indentified, I'm sure there are more: 1. Reset / training / calibration of the scope - you need to redo this minimum every 3 months (as things just change for the hell of it anyway!). 2. Change of power source - (new batteries, but don't use batteries, as the voltage drops, old batteries cause errors to build in my opinion), best thing is a big AC mains power pack with a good regulator to produce a stable constant voltage supply or second best, one of those hefty 12 volt power packs, I use a 17 Amp Hour supply fully charged each evening, and fed via a regulator (one of those things from the car accessory store that has options on the voltage, it gives a good regulated 12 volt output). Any unusual voltage spikes will be a source of drive errors. 3. Alignment - Accurate polar alignment is essential. If you are using a permanent pier and wedge set acccurately, then you only have the RA drive causing drive errors. If you use Alt / Az alignment, then you multiply your errors many fold, as every time you "Go To", the DEC errors accumulate. (You can see this by going back to Polaris over the course of a nights viewing and watching the orbital distance from dead centre (celestial pole) grow, it is larger for Alt / Az than Polar alignment). 4. Field Tripod - Using the field tripod also adds to errors, but they are small if it is set level, fastened firm and not kicked or tripped over! Also don't add a heavy piggy back camera, or significantly change the set up of the scope over the night, or what happens is as the scope moves and the centre of gravity shifts, the tripod "settles" and the alignment goes out. Using the photo port at the back or putting the DSI up on a Barlow lens can make a difference if the tripod is not rock solid. TIP 2 Mother-in-laws blinkies - Buy 3 Duracell 9 volt batteries, 3 flashing red LED's from Radioshack / Maplins, 3 reusable tywraps and 3 lengths of velcro. Use the tywraps to hold the wires of the LED's to the battery connectors and the velcro to hold the battery to the legs of the tripod. that way when the mother-in-law comes to visit, the old duchess won't trip over the tripod legs in the dark! 5. Cords and cables - My favourite bugbear! Hanging off the scope, dragging around every time you need RA movement (all night!) and DEC movement are cables. They drag, they snag, they tangle, and each time a clutch slips a bit. I have one USB cable for the DSI or LPI, one focusser cable for the electric focusser and one power cord for dew heaters. I put all my "eggs in one basket". Strip the insulation from the USB cable where it will drag against the scope in any way, (when the plastic cools, it gets less flexible, increasing drag and snag), thread the USB cable through the coils of the focusser cable. For the length of power cord for the dew heaters that will drag against the scope, use single insulated wire of the minimum power rating (one red, one black wire rated 240v 0.5A is fine for 35W (at max setting) of heaters running at 12v). thread them inside the coils of the focusser cable. Now you only have one cable route to contend with. TIP 3 To keep my cables in check, I use a cable tray between the ETX base and the tripod wedge. It is a black large size plant pot tray. The scope power cord and focusser cable from the ports on the ETX switch / control panel exit through a hole in the cable tray immediately below the switch / control panel. This presents as few snagging opportunities as possible in this area. The focusser, dew heater and USB cable bundle pass up the right hand fork to the OTA, fastened to it with the same velcro strap that holds my 2 channel dew heater controller to the fork, at which point they split into the component cables. Each of which is routed to avoid snagging or dragging as the scope makes DEC movements. At the other end the cable bundle exits the cable tray at the side of the tray approximately in the "due South position", because in Polar mount this is where the cables naturally fall. The trick is to only have sufficient loop of this cable bundle laying in the tray to allow the scope to go from RA lock to RA lock. On an ETX, this is plus and minus 0.875 rotations from the centre of the run lock to lock, this figure will make sense when you realise, it is the minimum cable length needed in the tray. Not yet added to my tray is a PTFE lining (sad, yes I know, but I don't want snagging and dragging!). 6. Alignment Method - Forget One Star, and any variant of Easy alignment. Waste of time! Get out your planisphere and choose two stars that are not at the zenith and not at the horizon, and are at least 90 degrees of arc apart. I have found, and my reasoning is too lengthy even here to go into, that one due East and one due West at an Alt of 45 deg, or as near as possible is best. 7. Drive Percentages - Spend some time getting the Alt / Az percentages optimum. If your scope doesn't move for 30 seconds on speed 1 or 2 when you press the button, then you are never going to get good autoguide tracking. 8. Drive Training Optomisation - I use the spreadsheet tool and patches from the ETX site to optomise drive training. Every little bit of accuracy helps! 9. RA drive mechanics - The RA drive is without doubt the weakest link on the ETX.It suffers from either too much lube or too little. I open it up once a year and check the lube situation. The wires to the DEC drive pass through the key way and RA clutch / drive, and are prone to wrapping and snagging. At the least sign of trouble, they need checking, and again once a year servicing checks. The mountings and settings for the whole RA drive mechanism need checking, though with a little Locktite, once set, even after a year mine seems fine. Amazingly I have never had DEC problems with this scope, yet! 10. Too hot / too cold - Well in the UK, these are the two temperature extremes that dictate our whole weather pattern! But due to lubrication and I suspect tolerances, (and probably shivering / sweaty hands), drive errors are larger below -5 deg C and above 25 deg C. They just are!! Now having reduced the above 10 items, you can make a start on the problems of autoguiding. First off, connections. If you don't have the RS232 port on the Autostar handbox connected to the serial port on the PC, and the Autostar via comm port or Autostar via network setting on, nothing is going to happen, yes been there, seen, done it! Next the tracking centroid. Choice of object is crucial. If you choose something bright, make sure it is the brightest object in a big area surrounded by either a complete dark area or objects that are far less bright and / or prominent. The size of this area is related to the drive error and exposure time per frame you will use. My reasoning for this is as follows: When you autoguide, you shoot one alignment frame, followed by one image capture frame. The alignment frame is short. So if you choose a dim or low contrast object, it might be visible in the live image exposure and the image capture exposure, but not in the alignment frame exposure, which is based upon a longer exposure than alignment frames. So what I am saying is Meade runs an algorithm, that makes the alignment frame a fraction of the exposure time that you set for the image capture frame. Now I'm not sure, but I suspect that what we actually have here are three frame timings, the live image refresh rate, the alignment frame rate and the exposure frame rate. When you lock that centroid to your object of choice, Autostar Suite is going to take an alignment picture frame, register and caluclate on it and follow it, if it can! So if the object is one that is the same or very similar to a number of objects around it, how can Autostar Suite decide which one was the original if it has to make a choice, next alignment frame? This gets more complex with the DSI Pro software, as you now can use two centroids to take out rotational misalignment (or is that easier as Autostar Suite can perform a checksum to see if the two followed objects match the original distance between centroids in the first alignment frame??). With me so far? Now if your drive errors are small, and atmospheric turbulence low, then between alignment frames, the centroid will still be on the alignment object, and Autostar Suite only has to apply a couple of guide pulses to the RA and DEC drives. But if your drive errors are larger, then this takes more work, all of which has to happen before the next image capture frame is shot. And, you lose alignment if the drive error moves the alignment object off screen or Autostar Suite cannot decide which of two or more objects (three or more with DSI Pro!) is the alignment object. So, with the LPI, when you are shooting image capture frames of a 1/10 of a second, and locking the centroid to a planet which has real angular size and is unarguably the brightest object within a country mile, as long as there is not a 30 second lag before your RA and DEC motors take up the drive slack, then you would be hard pushed to lose alignment. Move to the standard DSI and things become a little more complex, as you start to shoot open clusters and globular clusters, you are aligning on stars, and there are a lot more of them in the field of view. With exposures up to 10 seconds, you could get object confusion errors, whereby the centroid doesn't know which object to lock on to if you have drifted outside the box, again my reasoning is this, in laymans terms; When the box is drawn on screen, your object is inside it. The Autostar Suite software checks the object size and brightness and position in the box using a bit of math (checksum values) on the pixel values. Next alignment frame, if the same math is roughly in the box, Autostar Suite can send a few drive pulses to the ETX to centre the object in the box. If the math is way out, Autostar Suite looks for the group of pixels close by with matching math, and sends more drive pulses to the ETX to catch up. If you choose an object too close to another similar object, confusion could arise and you lose alignment! OK now move to the DSI Pro, and shoot Deep Sky Objects, with exposures of minutes, and L,R, G and B filter frames to take. Alignment can be lost unless you have a well prepared scope, with few drive errors. I'm sure that the two centroid solution is going to help (when I get that far). But my recommendation for the ETX has to be to stick to the shortest image capture exposure time possible and stack the images. Unfortunately Autostar IP is not great for checking pixel saturation levels quickly to adjust exposure times for DSO's, Maxim DL is better for that. Add in the time taken for Bias Frames (if you bother!), Flat field frames (do you bother?) and Dark Frames at regular intervals, and you have a long night ahead! And of course if you want more than pretty pictures, e.g., spectra, measuring asteroid light curves, calculating orbital periods of eclipsing binaries, then you need to shoot single exposure frames. Now some late thoughts: If the ETX drifts due to drive errors during the image capture frame, you get smeared images, i.e., star trails. So maybe all the autoguiding is doing primarily is providing a frame of reference for image stacking. Any significant drift due to drive error and you just stack smeared images to get a high resolution smeared image. The DSI Pro software with the dual trackboxes would help to remove rotational smearing to a degree, which is the point behind it if you read the small print, but in the end it has to be all down to reducing drive errors to a minimum and the use of edge enhancement and sharpening filters. Certainly by using the DSI Pro with its IR, Red, Green and Blue filters there is a chance of sharper focus per image component as with less bandwidth by definition you can achieve sharper focus. Again precise focus is hard to achieve using methods like FWHM with Autostar IP. Seems the only way to get decent long exposure guiding is with a second camera, perhaps a low weight guide scope and the LPI has some merit. Regards Stephen Bird
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