18.08.2021 Big screen H-alpha and camera crop factor.

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Wednesday 18th 62F, overcast with rain forecast. 

I have just tried viewing my captured H-alpha videos on our 4K 55" TV as an experiment. It only needed an HDMI cable plugged into the laptop and then "Duplicate screens" under W10 Settings: Display. Seeing the huge prominence "live" at 18" [50cm] tall is quite an experience! The prom appeared equally as sharp as it does on my observatory monitor. A 27" AOC medium high resolution. [2560x1440.] The laptop has a native resolution of 3840x2560.

The ZWOASI174MM camera has a sensor 11.34mm x 7.13mm = 13mm diagonal.
Crop factor = 3.23x. 1500mm = native focal length of my 150mm at f/10.

1500mm x 3.23 = 4845mm equivalent focal length!
 

Then consider the various GPCs I add to the camera nose. None of them amplifies as much as a 2x Barlow. The quoted figures of 1.6x, 2x and 2.6x are for binoviewer fitting. Which add a lots of focal distance to these Glass Path Correctors. That is compared with the short distance from the camera sensor as I normally use them. I also have a 1.25x Baader GPC fitted just behind the etalon. Which, though weakly amplifying, in itself, is quite a long way from the sensor. 175mm from the focal plane?

I just tried my triple thickness, internal zenith brace. It needed to be relieved slightly. To allow for the dome's gentle roof slope. I was going to clad over the two, zenith boards with 10mm aluminium sheet to stiffen them. 

Though I will probably use galvanized, perforated, roofing plates. I have plenty of 8"x4" plates but could go bigger. A much larger plate could be fixed in landscape mode for increased strength and stiffness. 

The pulley, OTA, lift system, eye-bolt will pass through this plate. So it wants to be really sturdy. There are some difficult clearance issues between the shutter ribs and this area. The shutter, drawer slides are also mounted on the exterior of the zenith board. These must not be obstructed by the eye bolt. On the plywood dome I mounted the duplicated, eye bolts in the lower corners of the one piece, zenith board. Duplicated as insurance against a single failure. "Belt & braces" style but probably, completely unnecessary.

I could run long, narrow, coach bolts vertically through the thickness of both zenith boards for mutual support. This would sandwich the dome and aid load spreading. Then run the lifting eye bolt through the lower, internal, bracing board. I had better study the situation more thoroughly. To see what can be done. 

The eye bolt nut[s] could easily be lost inside the 40mm thickness of the upper zenith board. Provided the reinforcing plate was large enough and stiff enough. That would safely avoid penetration of the drawer slide area by the eye bolt. While giving a much freer choice over eye bolt placement. Provided, of course, it misses the dome thickness itself. Lifting quality, eye-bolts are readily available. They offer safe loads of large fractions of a ton.

The maximum load I am applying to the eye bolt should not exceed a 50-60kg. This would be from a pair of OTAs mounted on parallel bars. So we are not remotely into serious lifting territory. I use ladders and a chain hoist for lifting the present mounting. Though this will not apply to the intended Cross-axis mounting, it might still be very useful during assembly. 

I have two pieces of 5mm thick, aluminium plate. One 25x25cm and the other is 20x40cm. [10"x10" and 8" x 16"] The 10mm plate is only 150mm wide but can be any length.

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