Autoguiding with a webcam for astrophotography and astronomy
Autoguiding guide Page 1
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Autoguiding guide Page 4
Step by step autoguiding instructions
I use an SC1 modified toucam webcam with a black and white ICX098 CCD chip and a separate guidescope to accomplish autoguiding. The steps are as follows:
- Mount guidescope on your telescope mount pointing in roughly the same direction as the imaging scope. Either piggybacked on the main telescope, or mounted on the counterweight bar. This second approach can save you some counterweights! The mounting must allow some adjustment to help you locate a guidestar. The field of view of a webcam is very small, and the process of finding a guidestar must be made as painless as possible. I devised my own computer controlled guidescope mount to help here. Having the guidescope pointing three or four degrees away from the main scope is not a problem.
- Mount the guide camera webcam on the guidescope. Either at prime focus or by using a barlow lens to increase the effective focal length of the system. The focal length of the guiding system and the pixel size of the CCD sensor combine to give you an image scale (resolution) in arc seconds per pixel. The better the resolution of the guiding system, the more effective it will be. However, as focal length increases, the field of view decreases, and the challenge of finding a decent guidestar increases! Like many things in astronomy, a compromise must be sought. I have guided happily at 400mm focal length and imaged at 1000mm. These days I now guide at roughly 1200mm.
- Find some autoguiding software. The best free offerings are GuideDog autoguiding software and K3ccdtools. I use GuideDog myself. The rest of these notes utilise GuideDog.
- Plug your camera into the computer and make sure you have an image in the software. Using the correct settings in Guidedog is important, but requires experimentation.
- Make sure the camera is rotated so that RA is left and right and DEC is up and down. This point is VERY important. Guidedog lets you flip the axis, but the camera must be square. I hope that the forthcoming new version of GuideDog will have a feature to take into account camera alignment.
- Interface your computer to the telescope mount. With Meade Autostar scopes this may involve an RS232 cable from the handbox to your computer's COM port. Other mounts will need a relaybox interface. You may need to Google for your mount to find the exact details.
- If your camera is a long exposure modified toucam/webcam, then you will also need to plug in the parallel cable to the computer so it can control the long exposure system.
- The setup I use with my Meade LXD55 mount for autoguiding is roughly as above. The webcam is mounted on the 80mm/400mm guidescope at F15, its usb cable is plugged into the computer. The parallel cable from the guidecamera is also plugged into the computer. The computer is connected to autostar with an RS232 cable. The autostar is connected to the mount with its own cable. Please try not to trip over this lot in the dark.
- Now check the balance of the scope as mentioned above in the mechanical section.
- The next stage is to point your imaging system at something you wish to record. First get the object roughly on the CCD chip.
- Now frame the shot and focus the imaging system. At the same time look at the output from the guide camera. Put the guidecamera at the slowest shutter speed (for non modified webcams, for modded cameras use 2 sec exposures) and max gain. When framing most DSOs in a typical imaging system, there is often a certain amount of leeway - move the scope around a bit incase there is a guidestar handy. As soon as a star is visible in the guidecamera preview stop moving!
- If this does not give you a guide star, you will have to adjust the pointing of the guidescope independent of the main scope until you do find one. Either use an eyepiece to centre a suitable guidestar in the guidescope, or, as I do, just move it about randomly until you find one. This sounds easy - believe you me, it can be a very frustrating process. Sometimes, in the act of adjusting the guidescope, you inadvertantly move the main scope a bit, loose the target, and have to start all over again. Time invested in perfecting your guidescope mounting and adjusting system is time well spent.
- The brighter the guidestar you find, the better the guiding will be. So its worth looking for a good one before starting the imaging and getting that cup of coffee/bottle of beer/cigarette.
- Once you are happy with your guidestar, lock Guidedog onto the star. It should remain fairly stationary, drifting slowly in DEC and RA with the errors in your mount's tracking. Get GuideDog connected to your scope, set the aggressiveness at 100 and press guide. Watch the star.
- In guidedog there are two tick boxes for reversing the EW and NS. If Guidedog starts sending adjustments to your mount that cause it to whizz off in one direction, you need to change these tickboxes.
- Watch the guiding for at least a couple of minutes. If you feel its undercorrecting or over correcting, then tweak the aggressiveness.
- The guiding interval is another factor to twiddle with. Longer exposures give you easier access to those elusive guidestars. On the other hand, if the guiding interval is too long, it will not be quick enough to correct your problems. You guessed it: Time to experiment again.
- Make sure your volume is turned up - if GuideDog loses the lock on the star (clouds etc) it will sound a deeply annoying alarm from the computer sound system. You can change this to something slightly less nerve jangling if you want.
- The best thing to do is measure your tracking with the guiding and experiment with different settings to see what it does.
For exposures of more than a few minutes, you need to guide in DEC to counteract drift due to poor polar alignment.
It is important to note that DEC Drfit is generally only in one direction. In theory it is not required for the mount to make corrections in the other direction! However, most autoguiding software does not take this into account.
Why is this a problem? Well, with RA guiding, the adjustments take effect instantly. If you change the speed of the RA motor, the effect is immediate because the sky is always moving. Which dec corrections, you must contend with backlash. The guiding software might make many small DEC adjustments to no effect because it is only "taking up" the backlash.
Then, suddenly, it will jerk in DEC as the gears mesh! One thing we can do is make sure the DEC worm gear is "wound up" or "loaded" in the direction of drift before starting to guide. After a while you will learn to mentally visualise whats going on in the telescope's gearbox. Almost as much fun as driving a car without syncromesh (something everyone should do at least once).
As mentioned above, DEC guiding also causes image rotation.
It is best to avoid DEC guiding at first. Start with only RA autoguiding, and progress to dec guiding when you have mastered the RA axis.
Autoguiding and Seeing
As we all know from planetary imaging, the stars wobble in the atmosphere. Adjust your autoguiding so that it does not try to follow this wobble: Chasing the seeing. One trick is to slightly defocus the guidescope so the effects of seeing are less apparent. Using longer guiding exposures also reduces the effect of bad seeing.
As you increase the focal length of the guiding system, seeing becomes more and more of a problem. That would be one of those compromises again.
Webcam RAW mode and Autoguiding
I always use RAW Mode when autoguiding. To be honest I am not sure how much of a difference it makes, but I still use it!
How much focal length do I need to autoguide with?
This a topic about which a great deal of rubbish is discussed. Some folks will tell you that a guiding system must have at least twice the image scale (focal length) of the imaging system. That is, quite frankly, bollocks.
It is true that increasing the image scale improves the tracking... although going too far results in your "chasing the seeing". However, my view is as follows: If you have a poorly tracking mount, then ANY guiding will help. Perhaps you have a 1000mm telescope with +-30 arc seconds periodic error. In a batch of 20second sub exposures you find yourself discarding three quarters of them. Now, even if you try autoguiding with a 200mm SLR lens (image scale on a standard Toucam: about 6 arc seconds per pixel) you will get an improvement: Perhaps you will increase the number of good frames. Its worth trying anything. It might not give you perfect tracking, but it might make things a little bit better. Improvements in astronomical kit must come step by step.
Another arguement against the Long Focal Length Brigade is the advent of modern autoguiding software. This is able to calculate the centroid of a star to sub pixel accuracy - and is far more sensitive than guiding by hand.
Autoguiding and Flexure
Once you move into the realms of minutes long exposures, the main gremlin of autoguiding catches up with you sooner or later. Flexure.
If you are using one camera to image and one camera to autoguide, you might suffer flexure problems. Mounts and telescopes are mechanical devices. As such, parts of them tend to bend slightly. As the mount tracks the sky, the degree of flexure between the parts causes some parts of the scope to move very slightly with respect to others. The autoguiding system will try its best to keep the guidestar locked in one position. In theory, this keeps the imaging camera target in one place on the imaging camera. However, if the guidescope should move very slightly with respect to the imaging camera, the autoguiding system will correct the guidescope, keeping the guidestar centred. The result of this is the imaging target moves slighly on the imaging camera.
If you images still have very slight star trailing, usually in a direction diagonal to the RA/DEC, but the autoguiding system reports no errors, then you are most likely suffering flexure in the scope components.
Examples of things that flex include:
- Camera with a 1.25" nosepiece held in the focuser with one set screw.
- The focuser drawtube moves inside the focuser.
- The mirror settles in its mirror cell.
- The guidecamera flexs in its attachment to the guidescope.
- The guidescope flexures inside the tube rings.
- The guidescope mount flexs with respect to the main telescope (normally the main culprit).
- There are many others!
It is your job to make the all these componants are rigid to avoid flexure effecting your autoguiding.
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Autoguiding guide Page 4