Images
Finally, all the parts could be assembled and the telescope was ready to deliver first light. It was achieved in extreme unfavorable conditions with the telescope inside a room, through opened windows and at 5 meters from a strong street lamp. Once the optical alignment made, this is the first 10 sec 2x2 binned full frame exposure :
The star sharpness over the field looks uniform, since a field corrector has been used. Due to imaging through a window, large turbulence prevents me from getting sharper images.
The background is 6000Adus (in 10sec !) meaning that I will saturate the CCd is the exposure time is larger than 30sec. This issue comes from the fact that I was pointing toward a street light city, and that most of the parts inside the tube are not yet black painted properly. For sure, I will improve both points in the future....
A
3 sec exposure, bin1x1, central area...
Uniformity/Vignetting
On the 22nd of May, the tube was completely repainted/recoated. The optical alignment was loss. This is not an issue, and it allows me to measure the field uniformity (1sec exposure):
Flat
field with a shifted optical axis, cross section has been done :
The field of view is 108x108 arcmin , the semi-diagonal axis is 76 arcmin. This measurement shows a Flat field with a shifted optical axis, a cross section has been achieved : The field is uniform better than 10% across 120 arcmin, the large 85 mm secondary mirror paid off !
Shutter Uniformity/Patterns
For short exposure during flat field acquisition, the IRIS shutter needs time to open and to close, resulting to a non uniform exposure time across the field. Hereafter, a 15 msec exposure has been performed, showing the famous IRIS pattern shutter.
Flat field with the IRIS shutter pattern.
If one divides the IRIS shutter pattern by the flat field, and plots a diagonal cross section, it yields to :
It means that the opening PRONTOR Iris shutter time is roughly 5 msec and to have this effect neglected on a flat field frame, we'd better use an exposure time of 1 or 0.5 sec.
Tracking
errors
The sidereal speed has been adjusted to 130 1/128microsteps per seconds, the original value, 125, was making an error of -12 pixels per minute (no binning, no mirror, CCD readout register toward the DEC drive)
Our PRISM software allowed me to perform, online, the following plot:
This plot made by the focus function, each point is based on 2 seconds exposure time, binning 1x1. The pixel size is 3.16 arcsec. Unfortunately, this plot shows large periodic errors every 210sec (the rotation rate of the main screw), amplitude is +/- 10arcsec. This is too large compared to the specs (+/-5arcsec max), and not very nice for a mount of this class. Nevertheless, best efforts will be engaged to overcome this issue. The mount manufacturer told me that this is the larger error ever seen, and I will receive a new tested-on-sky main screw.
So far, the CCD camera worked well, optics are OK, and I will more likely use the bin2x2 mode to reduce the amount of data.