Lens and Teleconverter Versus Small Scope

Hi,
Apart from mount considerations and weight, in terms of pure optics, here is what I can say :

- lenses usually can’t be used wide open, due to too much aberrations (coma, chrimatic etc…, you have to stop them down, usually at least to f/4 to start getting nice stars, while a small refractor that has the same diameter is made to be used « wide open » (no diaphragm). I am of course taking about flat field corrected refractors made for astrophotography.

- if you start to increase the diameter, you can get much better resolution, due to diffraction phenomenon : punctual light sources at infinity are not points on your sensor; the image is an interference figure called Airy figure (concentric circles fading away). The higher the diameter, the best your scope will be able to distinguish two stars very close together (otherwise, their Airy figures will overlap too much to really see contrast between them). That’s also why it is preferable to get a bigger refractor (higher diameter) when you want higher focal length, than just using Barlow lens / 2x or 4x teleconverters. It is not just for FD ratio considerations.

- Teleconverters and Barlow lenses are used mainly for bright objects (planetary observation for example) when you already have a long focal length. You have FD ratios that are « ugly » for a classic photographer but OK to image ver small and bright objects. For faint deep sky objects well, most uses fast FD devices (except again for small bright deep sky object like planetary nebulas, using techniques of short exposures to limit guiding problems and seeing limitations).

That’s the general optical considerations. Now if you seek portable setups, many lenses are used for deep sky photography with GREAT success. But if you start seeking higher focal length with portable setup, well, I would personally avoir teleconverter or Barlow for deep sky images : I was a bit on your case with a Skyguider Pro and I chose a camera with a small sensor (ASI183MM Pro), because it is 2,7x smaller than a full frame sensor and thus the « equivalent focal length » I get is 2,7x higher. On top of that I invested in the lightest 70mm diameter reafractor I could get (TS Optics Imaging Star 71/347). It is a flat field corrected refractor of 347mm of focal length, so FD ratio of 4,9. With my small sensor it would be equivalent to a 936mm focal length lens on a full frame sensor (in terms of FOV).

But at this focal, with a small sensor, you need guiding, particularly on a small mount.

Last advantage of the refractor over lens : I find it easier (especially at this equivalent focal length) to focus, with or without electronic focuser. I also use some of my lenses (Mitakon Speedmaster 85mm, Sigma 105 EX Macro, Samyang 8mm…, and each time, focusing is a pain in… you get it :-)

Hope this feedback will help you a bit.

If the lens is well-corrected at full aperture then going for the lens would be the easy choice. Nothing beats aperture, if optical correction is (about) the same. 200mm is a good focal lens to work with too considering that you have a D750. Adding a teleconverter could be a good choice if said teleconverter is a good match optically, which there are few around. Stick with Nikon as this is best option in my experience. If you can test them and return of not happy that is the way to go. I suspect that going at longer focal length would be too much a strain for you current mount so you might need considering getting a proper mount of you set up for a small APO instead (not my choice, to be clear).

Your might want to take a look at the Rainbow Astro RST-135 mount. Very small and light, no counter weights, but very capable.

http://www.rainbowastro.com/rst-135/

Tim Hutchison:
Your might want to take a look at the Rainbow Astro RST-135 mount. Very small and light, no counter weights, but very capable.

http://www.rainbowastro.com/rst-135/

Nearly 4 grands for THAT!? C'mon, the guy is just starting this is well over the top!

If anything get a traditional small mount, a CEM-28 maybe.

I have to say I had the same reaction as andrea tasselli had in regards to the recommendation of the very expensive tracker.  I certainly don't think there was anything nearing indecency in tasselli's response, it seemed like a natural repsonse.  The OP was asking opinions on using a zoom camera lens and extender along with a few other sub-$1000 options and got a recommendation to spend $3900 on a tracker.  

Tim Hutchison:

Any additional comments about the mount (or other recommendations) would be appreciated

I was replying to this request... But that's fine. I'll gladly remove myself from this conversation.  Good luck bluespeck.

Fair enough, but  a leap from a $350 tracker to a $3900 tracker is enough to raise one's eyebrows...no need to leave the conversation because someone voices their opinion.