R5 lunar photography with stacked EF extenders

Larry Rexley wrote:

I should clarify --- I wasn't referring to a loss of 50% light loss by adding teleconverters, I was referring to a 50% loss in the whole optical path (some lens reviews use T-stop vs f-stop). This is useful when comparing with telescopes dedicated for lunar or planetary use.

DXOmark measured the EF 600mm F/4L IS II USM at T/4.5, so that's (4/4.5)^2=79% transmission. I can't find measurements for the TC. If we use the guideline in your link below, the EF 1.4x III has 3 groups, and the EF 2x III has 5 groups, for a total of 8 groups, and 0.985^(8*2)=79%. So total light loss is 62%. But for lunar photography, this is really a non-issue because there is plenty of light.

https://www.dpreview.com/forums/thread/4608842

Thanks for the link. Lots of info there.

If the point of this discussion is to see what Moon photos can be gotten with telephotos and equipment already owned --- that's a very fun thing to do and I've done it too --- even bought cheap vintage lenses for $20 and $30 in the hopes I could get good Moon shots, made my own teleconverters, tried a wide variety of teleconverters to see which performed the best, etc. If I happen to be out shooting in the evening and see cool Moon or a conjunction, I'll still shoot it with my telephoto and teleconverter.

Well, yes, that's the point of a thread on shooting the moon with stacked TC.

I don't mean to spoil the party, but if the end goal is high quality lunar or planetary photography, I eventually realized that stacking teleconverters is not an economical way to do this and get the best results, for the reasons I mentioned above. One very high quality teleconverter does generally produce very good results with a high quality telephotos, but by the second I think you are at the point of diminishing returns already. Even if you still get some resolution benefit from another stack, you're still lagging further behind a good telescope's performance of similar aperture.

What do you see as the tradeoffs between a lens and telescope? Obviously, the extra TC increases CA, but that's easily removed in post now. You mentioned light transmission, but I don't think that really makes any practical difference since you can shoot a big stack in several seconds anyway, so even if it takes twice as many seconds, NBD.

My earlier post in this discussion above with a very portable Meade ETX-90 ($100 used or ~ $500 new) on a night with average 'seeing' did about as well as some $2000 - $3000 combinations posted here. For that kind of spend, $1000 - $2000 will get you a reflector or refractor of 6" or more aperture which will put any Moon photo posted here to shame.

Sure, aperture diameter is everything. But what about a 6" scope, other than price, would make it better than a 6" lens, e.g., 600mm f/4 for lunar photography?