Gueri & Fred's astronomy
The main goal of the balcony observatory is to keep observing when the weather is nice, but we are not at country side. Therefore, it should be as low cost as possible. This mean that we need to reuse our equipment if possible.
This is the first critical point. At first the idea was to use the GP-DX mount that was the warhorse of the Traveler for more that 10 year
On the camera side, they are few choices available. Since we already have a HX516, a MX716 and a MX7c. As with the light pollution, it would be impossible to get faint galaxies, the HX516 is probably the best choice. The MX716 has bigger pixels and therefore has a lower resolving power than HX516. Moreover with its square pixel it can be useful for astrometry.
On the OTA side, the Traveler is too heavy to be used on smaller mount. and can't be used.
The FS-60C is a good OTA, it has a finder scope, which is good for no-GOTO mount. With the HX516 it has a resolution of 4.3"/pixel, and a 47'27"x34'52" field with is roughly the same as the traveler,MX7 and focal reducer. With its f/d 5.8 it's fast enough for short exposure.
The MTO, is also a good OTA for this purpose, Its main advantage is that it is relatively lightweight and has a 100mm diameter which is good compare to the FS 60. Its main disadvantage is that it has no finder scope in standard. And with a non-GOTO mount finding object is not easy. With this OTA, the resolution is 1.52"/pixel, which allow double star splitting.
Comparative field for FS60 and MTO and M65/M66 galaxies.
This ETX is a other option, but its f/d is around 14.
On one night, we tested with several OTA and several exposure the light pollution.
The background value was obtained with statistics panel given by astro-art. This value is the sum of the camera noise of which is mostly constant at a given air temperature for limited range of exposure. It seems to be around 2950+/- 100 And the most important part is linked to the sky illumination.
3 series of exposure were made.
The first one was made with the MTO lens at f/d 11. The curve is linear. And due to the low speed of the lens, light pollution is still acceptable for a 30sec exposure.
The second set was made with FS-60C, as the speed of this lens is faster (about 4x) than the MTO. The curve is also linear, but light pollution is appearing quickly. After less that 15sec, background is 10% saturation value. this mean that exposure should not last more that 10sec.
The last set was made with the FS-60C using an UHC filter from Astronomik. This filter is suppose the reduce the pollution ray while letting the useful ray passing thought it. With this filter the light pollution is reduced, by about 2 stop.