Hello Pierre and Christian,
I believe the comparison is not fair.
Firstly, Alpy 600 does work down to 3700A (not 3800-3900A) with Newtons:
Note that I had a 10" Newton with an Orion G3 camera (8.6 micron pixel size) that time, and had wrong practice focusing the Alpy correctly especially in the blue.
But, the continuum till 3670A have always been had the right curvature (of course Alpy degrarded resolution, and need careful processing to avoid filtration here).
For Vega, I can detect signal down to 3550A... (signal cut seem to star at 3700A)
Also must take precautions at flat taking (I believe reflections are coming up below 3700A, and ref.star must also be at good quality in this region, and must be treated precisely).
Pierre,
- the C8 (and higher) to have optimal usage with Alpy, you most certainly have added a focal reducer. I see it for all Cxx instruments + Alpy users, they have a hard end only at 3800A or so (see database - newtons work down to 3700A with just more or less noise, depending on effort...)
Some of such reducers already reported problems below 4200A earlier.
- the 5 min exposure is low for no reason. For the Alpy (when the core element is firmed correctly), can maintain calibration for hours... (unlike LHires where 5 minute is reasonable) It is *ALLOWED* to let the H-alpha burn in, till you notify the database owner anyhow. I prefer zeroing H-alpha out (ISIS -> Edit tab) to really prevent its use. When weather allows, just go up to 20 minute when shooting faint object. However, reaching 3700A with the Alpy I saw working only with Newtons (see database).
Christian,
Congratulations for the UVEX2 !
Yes, I was also suggested by a professional to use Alt/Az for full spectrum use (and did start on yahoo such a discussion thread in the past already).
When I mentioned my setup has short focal length (300/1200 Newton: only 1200), I was also told I can try to operate in the zenith. This is essentially what I'm doing for ARAS symbiotics since then, to make sure ref.star calibration works, and for simplicity.
To operate at lower levels and get the ref.star calibration correct, maybe the wide slit is the right solution for EQ mounts, as already noted by others.
When you optimize the guiding for throughput with the narrow slit, I suspect the guiding won't be fully reproducible by practice (guide optics not errorless - random reflections come up when star is getting off the slit). It was nice to get the UVEX2 to work at f/4 rather than f/8 RC, then the athmo. aberration also gets halved, and yet useable when operating in Zenith.
- Peter