The scope that I use is TAL-250K, manufactured in Novosibirsk, Russia. Main parameters are: This is all spheric surfaces design that uses Mangin mirror with a menisk lens in front of it. It is easier in manufacturing than Schmidt-Cassegrain as there is no aspheric surfaces, and it does not have full aperture size refractive elements as any Maksutov desugn. It also has open tube that is very useful for fast cooling of the scope, cold air directly contacts with the primary mirror. The drawback is contamination of the optics inside the tube, but the coatings are strong enough to have a cleaning once a year. Another benefit of the tube design is a very high stability of the collimation. I have sent the scope from Novosibirsk to Moscow by train (it is more than 3000km) and it arrived with the perfect factory collimation. I have collimated it only once just by slight touching of the adjustment screw, and it keeps collimated since than. I still don't have any permanent dome or a pavillion with the rolling roof, so I have to keep the scope at home at a room temperature. Usually temperature during imaging is 10C - 35C lower than room temperature, so the cooling system is a must for a fast preparation of the scope for the session. Open tube is easier to vent, but active ventillation is bringing a dust to optics, so filtering of the air is desirable. |
| Here is a ventilation assembly, I have made it of car air filter of suitable size. 12V ventilator for computer and a piece of plastic tube that fits 2" focuser. Design is obvious and very useful: |
|
The mount that I use is rather unique and was developed by Mihail Teleshkov from Kaluga, town that is 100km from Moscow, well known for it's industrial cluster. The mount was designed at the beginning of 2000's and the main idea was to make a high quality mount for astrophotography. Special attention was made to reduce the periodic error and avoid backlash in the worm gear. The kinematic scheme consists of 2 worm gears, so the number of the components is minimized. Backlash in the main worm gear is removed completely, as the worm is not mounted permanently, it can move so that the special spring pushes it towards the gear. This is a view of the dec axis: |
| The results of this design is good, periodic error is small, this graph shows 2 periods of worm and the PTV is about 6". It is much smaller that on the typical EQ6 mount. |
| I use a popular camera DMK 31AU03.AS from TIS. You can find more information about it at the manufacturer website |
The most often used filter is Astronomik Ir Pro 742, that gives me enough light for imaging in infrared. Influence of the seeing is smaller in this spectral area, and I can get sharper images with this filter, although it is increasing the necessary exposition time or gain. Here is a filter transmission curve, I had a possibility to measure it by high quality spectrophotometer: |
