Apologies for the delayed response:
2. Manual only, although has encoders for Nexus DSC fitted (also have AZ GTi and M2 mount, the latter with DSC encoders so have flexibility for different weight scopes)
3. Nebula the priority followed by galaxies, but believe the latter not best suited always to NV
5. Sorry no
6. Regularly with something genuinely portable (so unfortunately rules out something like the C11 although I’m sure the views through that would be immense!). Amazingly dark skies within 1hour drive but weather here very unpredictable so needs to be something with minimal cool down time.
Pretty sure I’ll stick to my original choice of the FC100 when they’re back in stock here in the U.K.
Thanks again for the feedback
OK, thanks. With your 18" you have globulars, planetary nebulae and galaxies well covered, so I understand your need for wider FoV for medium to large emission nebulae. With H-a, your need for speed becomes critically important, and although the Tak refractor will provide excellent views with glass eyepieces, you will be under-whelmed using it with NV. I have owned 9 Tak scopes and I would select none of the refractors for use with NV except the FSQ 85 with either the Tak reducer or preferably using it in afocal to reduce speed. It is possible to increase the speed of the FC 100 Tak refractor using an eyepiece in afocal, but there are caveats to this method. I did not like my f:9 FC 100 DL used with NV and subsequently sold it.
Because a fast focal ratio is paramount and because you want it to be portable, I would recommend one of two optical options... perhaps BOTH options. The first would be a small 6" f:2.8 reflector like the Boren-Simon which uses a .73x reducer in an f:4 native scope. It produces a flat FoV with no coma and provides full sensor illumination; an important issue for a bright image without vignetting. The advantage of the B-S system is that you could probably use the reducer in your 18" f:3.9 scope, making it a prime f:2.8 optical system with a wider FoV than you experience at f:3.9. I have used the TS (ASA) reducer for almost 3 years in 3 different scopes having native focal ratios of f:3.9 to f:4, and the reducer performed flawlessly, although I did need to shorten the light path on one scope by 15mm to use the reducer. The B-S option is a bit more expensive, but the reducer makes this scope quite excellent with NV for emission nebulae.
Sharpstar offers a similar, more expensive f:2.8 system, but I know nothing about the reducer they use to achieve the faster speed. IIRC, the Sharpstar uses a hyperbolic instead of a parabolic primary, so I doubt the reducer would be useable in your 18" Dobsonian. The B-S 6" scope is relatively small, lightweight (CF tube) and has a good record with NV for visual use.
If you choose to go afocal using a small astrograph, like the Askar 108mm or even the 73mm, you can reduce the focal ratio using their proprietary reducer, but it still only gets down to f:3.9. Using the TeleVue 55/67 Plossl in afocal (instead of the reducer) reduces the to about f:2.2, but makes a much longer stack from the diagonal.
The second optical option I can recommend was already listed above by bobhen. Nikon or Cannon camera lenses used in prime focus offer some excellent wide field images with very fast focal ratios. And they can be adapted for use with either 1.25" or 2" H-a filters which are probably already in your possession (PM me if you want to know more about using filters with camera lenses). I have stuck with Nikon lenses only because I used them early in life. For use with NV, you need only to create or purchase a C-mount adapter to use them with your OVNI B. Used visually, straight through and hand held, I prefer either a 105mm f:1.8 at 4x or 135mm f:2 lens at 5x using a 3.5nm filter in a LP red zone. Many others prefer the 180mm which I found too bulky and heavy. At a dark site I prefer a wider H-a filter of 6-8nm to see more stars. But a 3 or 3.5nm can be used under any conditions. Since your OVNI B does not have gain control, you will find that the aperture ring on a camera lens works in a similar fashion to control the brightness of the image. Take a folding lounge chair and your filtered OVNI for a low-power hand held view of the night sky.
You can also mount your OVNI on your AZ GTi with the camera lens attached, although the straight through method is a pain in the neck, literally.
You can also use a C-mount lens but they often result in vignetting because they are often designed to illuminate smaller sensors. And longer focal length C-mount lenses often cost more than available Nikon or Cannon camera lenses.
There is a lot of good information in the BEST of NV; click on "NV and Lenses." Many of the old Nikon AI or AiS lenses are found for a lower price than many astronomical eyepieces. I recommend sticking with older lenses with manual focus and having an aperture ring. Many of the auto focus lenses produced today are problematic in use because they require a special adapter that allows manual control of the aperture; many of them do not have an aperture ring on the lens barrel. Also keep in mind that H-a or long pass filtration must be placed behind most camera lenses, which requires an adapter for the lens, a way to insert a filter and an adapter for the NVD. But it's not complicated and all parts are available on-line.
Last thing... PEterW above recommended the Bracken Astrophotography atlas. If you do not have one of these you should seriously consider the $20 price. Nebulae are shown in different shades of red on the white pages so you can estimate the brightness and the size for optics with an adequate FoV. It's a valuable NV tool for emission nebulae and it has a very user friendly index.
NOTE: The recommendation for a camera lens is based on Joko's response that your objective lens is removable. If it is not removable, as you say, then please disregard the camera lens suggestion. Good luck. Ray
Edited by GeezerGazer, 18 February 2021 - 01:54 AM.