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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.


  •  Mount

   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

  •  GP-DX

 This is a good mount it can handle about 10kg. But for this use, there are several issues. First, our flat is located on the southern side of the building. This is the best for stargazing. But the point is that we have no access to Polaris. So we have to set up  the mount, that can't be permanently installed, thanks to Bigourdan method, for each observing session. The second issue is that the mount has to be set up in the middle of the balcony, and due to the upper balcony, a large part of the sky is lost.

The main feature of this mount is that is can  be used on the parapet, and using a wedge, it can set up permanently. The main and obvious point is that the mount is mostly plastic made. And therefore as a poor tracking quality.

  • Ioptron mount

This mount seems to be a good candidate. It is GOTO and its tracking seems good according to several users. But the point is that this is an A/Z mount. And as with GP-DX, it should be set up in the centre of the balcony. 

The point with A-Z mount is field rotation. This field rotation depends on Its position in the sky (Altitude and Azimuth) and on observer latitude.

It is reduced when a direction variation is greater on one direction than the other. Unfortunately, as the flat is located in the 48th's, sin(48) and cos(48) are very closed. This mean that the rotation is maximised.

Rotation around HIP 12916 for 2 minutes exposure :

This mount is also a very interesting mount. Even if this is not a GOTO mount, it is suppose to handle up to 15kg.

  • Sky Patrol

This is probably the better mount for this purpose. It can handle up to 3 Kg,  a test made in sky and telescope explain that it can be overweighed. The point is this not a GOTO mount. And with the 3mag sky limit in town, finding targets is not an easy job.



  • OTA and camera

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.