User Links for

(North America)
Mount Tips
Tip Title: 
JMI Encoders on Losmandy
I got a good deal on a set of encoders and an NGC Max computer from JMI.  However, the suggested installation scheme for the encoders borders on pathetic.  The problem is that the only place on the shafts where you can mount the encoders is between the pressure points of the Losmandy clutch system.  If you tighten the encoders onto the shaft you will not be able to tighten the clutch and if you leave the encoder loose (even with the set screw against the flat side of the shaft, as JMI suggests) it is certain to slip.  It takes only a very small amount of slippage to totally ruin your pointing accuracy.
I thought I came up with a brilliant solution to this problem, but it turns out that this is exactly how Losmandy does it with their encoder kit (I don't know if I had previously seen their solution and remembered it subconsciously, or what):  Acquire or fabricate a set of 6 or 8 steel pins about 1/8 inch in diameter and 1/8 inch longer than the thickness of the encoder mounting disc.  Then drill clearance holes around the rim of the mounting disc, evenly spaced.  Mount the  modified disc on the shaft and tighten it to the shaft with the set screw against the flat side (if available - some models don't have a flat side) and washers/thrust bearings on either side.  Lubricate the pins and put them in the holes in the disc, then replace the thrust bearings, washers and clutch knob (you will have to leave off the spacer as the encoder disc replaces it).  Once the clutch knob is in place so that the assembly is held in place you can loosen the encoder disc and reposition it such that with the clutch tightened there is a small gap between the disc and washers on either side then tighten it again.  Repeat for the other axis.
What is happening here is that the encoder disc is now firmly attached to the shaft, but the clutch pressure is relayed through the pins without affecting the disc.  This system has worked great for me and is fairly easy to implement, although I'd have to say that given the amount of work it might have been better to just buy the Losmandy encoders, even though they are much more expensive.

Tip Title: 
Losmandy Polar Alignment Scope
When installed as intended, the polar scope in a Losmandy GM-8 (and probably the G11 as well) will generally not provide accurate polar alignment.  The reason is that it is attached not to the RA axis shaft itself, but to the clutch.  The pressure from the clutch knob will almost never be even and the tube of the polar scope is significantly smaller than the inside of the RA shaft, so it can be quite off from straight in there.  The solution is to get some teflon tape (used to wrap pipe threads before joining them) around the far end of the polar scope until it will just barely fit inside the shaft.  Then when you thread the scope in make it just slightly tight so that you're not applying pressure that would push it to the side.  This made my digital setting circles actually useful and greatly improved my astro-photos.

Photography Tips
Tip Title: 
Curvature of Field
One of the most popular ways to do astro-photography these days is to put a DSLR (digital single-lens reflex) camera on a small refractor.  This provides a suitable magnification for many good targets and is fairly inexpensive and easy to manage.
However, very few small refractors (or telescopes of any type, really) have a sufficiently flat field for the relatively large image sensor in a DSLR.  The solution is to use a "field flattener" lens between the camera and telescope.  Most field flatteners also function as focal length reducers, which is usually a good thing.
The distance between the flattener/reducer and the focused image plane is very important both to get the correct reduction and to avoid aberrations.  In most cases, the reducer is attached to the camera using a "T" mount adapter, which sets the distance from the mounting flange at 55mm.  Don't mess with it!
Reducer/flatteners (FR/FF) are not all the same.  In the case of my Megrez 90 a FR/FF that was theoretically designed for the scope did not work well at all, but one made by Televue and designed for a slightly shorter focal length works beautifully and is only a little more expensive.  Experimentation may be required to find the right one for your telescope.

Tip Title: 
Drift Alignment using PHD
Drift alignment is generally considered the best way to achieve accurate polar alignment, but it also has a reputation for being a very time consuming process.  Neil Heacock and others have pointed out that if instead of visually watching for drift you use guiding software such as "PHD Guiding" the process can be much quicker because the software can detect drift much more quickly than you can see it.
A potential drawback to this approach is that PHD needs to be calibrated for a given region of the sky before it will run and the calibration process can be quite slow.  However, since drift alignment always uses stars in the same region of the sky (south near the celestial equator and either east or west near the horizon), the calibration parameters are essentially the same each time you do drift alignment.  The next time you do drift alignment, after doing calibration go to the "Tools>Enter calibration data" menu.  Don't change the values, but record each of the parameters as measured by the calibration for each region of the sky.  The next time you do drift alignment, after launching PHD go to the "brain" menu and uncheck the "force calibration" check box.  Then go to the "enter calibration data" menu and enter your recorded parameters for the first region of the sky (you can cut and paste them from a text file).  You are then ready to start drift alignment.  You will have to change the parameters for each region of the sky, but note that you don't have to use exactly the same coordinates each time because the parameters don't have to be precise.
Tip Title: 
Focal Length : Pain
I and astro-photography colleagues have defined the following rule for astro-photography:  Pain is proportional to the square of the focal length.
More precisely, everything gets more difficult as you increase the magnification.  Focal length and size of the image sensor determine the magnification for prime focus photography, which is the recommended technique for most astro-photography.
Both optics and mechanics (tracking/guiding accuracy and stability) are more critical as you increase magnification.  And to add insult to injury, higher magnification also usually means a larger focal ratio, requiring longer exposures, thus requiring even better mount performance.
So we always recommend that beginners start with very low magnification, such as mounting a camera with ordinary lens on top of your telescope ("piggyback").  Many people move on to prime focus photography through a small refractor.  Beyond that, the number of successful astro-photographers drops off dramatically!

Tip Title: 
T-Ring Wiggle
DSLRs are a great way to capture astro images without spending a fortune, but they do present some problems that derive from not being designed for this purpose.  One problem that's easy to fix is getting your T-ring to mate tightly with the camera body.  The rigidity of this connection is far more important in astrophotography than in conventional photography and the bayonet mounting system in most cameras is just not very good for a high magnification (long focal length) telescope.
The solution is to tap 3 holes in the T-ring body, 120 degrees apart, and put set screws in them to press against the flange of the camera.  To put the system together, first loosen the set screws and attach the ring to the camera.  Then tighten the set screws (not too tight!) and thread the camera on to the telescope.  It should be much more rigid this way, although a little less convenient.

© 2008 Triad Technology Systems |