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1. Hydrogen Alpha and Beta filters

There is something like this too, only that it is used to find absolute temperature instead of its gradient. It is called B-V index and relies on the UBV filters on the Johnson scale. It's just another set of filters which happens to be the standard for scientific measurements :))). You measure the magnitude of a star in the B filter and in the V one, you subtract the V magnitude form the B one and some math gives you the absolute temperature on the surface of the star.( I don't know how to use this for other objects :)) ). So.. yeah... B-V index. Adrian
2. Hydrogen Alpha and Beta filters

The idea behind narrowband filters is that they are centered one a specific wavelength and basicly allow only the photons with that wavelength to pass through. Of course, this would be for an ideal filter. Even the best filters have a bandpass of a few nanometers. You may find their bandpass to vary from 2 to 32 nanometers depending on the filter. This is not an issue, is just an engeneering problem, I guess. Anwway, the images given by the Hydrogen alpha filter will be different than the Hydrogen beta filter because they correspond to different emission lines. The H alpha is around 656.28 nanometers and the H beta is around 486.1 nanometers. As to why these to emission lines are different, well this is due to the way bodies emit light. If you aproximate a body to a Black Body than you can say that the flux of light it emits is described by Planck's law for Black Body Radiation. Since stars are a good aproximation to a Black Body we may very well take them as one. Depending on their temperature they emit more light at some wavelenghts that at onthers. Many of them emit far more light in the Hydrogen alpha line than in the Hydrogen beta. For the same time expusore, the Hydrogen alpha image would be brighter the the Hydrogen beta one. That's why they are different Adrian
3. HEQ5 Synscan DEC drifting problem?

I guess that the mount not being aligned with the Earth's rotation axis together with the One star alignment cause your mount to point at different coordinates that the ones you want. It is a good chance that it wasn't tracking the center of the Milky Way... Once the mount is set on a target it should only move the RA ( this is actualy the Hour Angle) since the DEC is constant. It is not clear to me why the DEC moves in the first place, maybe the mount is smarter than I thought and knows whether or not it has been polar aliged well. I've also got an HEQ5 and I think that a good polar alignment will make these issues vanish.

Hi Tim, I'm sorry if I've disturbed you, but I see you use a modded Canon 100D and the point is that I also have one and I'm thinking to mod it, but I don't know what mod to choose and how to actually do it, so I would be thrilled if you could share your thoughts about it. Thanks!