What sort of image might one expect from an f/30 6

Fellas,

Been quite a while since I posted at Astromart. I used to get into the scrums back on the old site, before the redesign. Six years ago or so.

I've been dreaming about a long focus refractor for a long time, and might build one next year. D & G has had long ratio doublets listed for several years now, at not unreasonable prices. I'm not ready to buy and haven't wasted their time asking about availability yet.

I have in mind a sloooowwww doublet. Depth of field for holding focus and natural magnification for planets. I've dreamt up a folded mount to hang onto a 6", but would love to go wild with an 8".

Of course, more length than necessary is only more headaches in the mounting.

What might I expect to find, image-wise, with these four choices, please:

1) 6" f/25
2) 6" f/30
3) 8" f/25
4) 8" f/30

And since we're dreaming, a 10" f/30. Too much residual color? Not in the budget but curiosity beckons.

By the way, do slow ratio lenses tend to be figured better? It seems to me there'd be more wiggle room for getting the shape and possibly smoothness worked out.

I thank you all for your time.

Regards,
Joel Elliott

Hi:

I imagine that you would get some pretty fun views with a 8 inch F/30 if you could get it together and working... At F/30 an 8 inch would have a "chromatic ratio" of 3.75, better'n the 3x rule but not up to 5x... Probably pretty fine views though.

It seems though to be that a single fold would still make for a 10 foot OTA and probably require at least a 5 inch flat...

Sounds like a fun project but gosh, with one good sized mirror already and a second as a diagonal if needed, it may not have too many advantages over an F/10 Newtonian.

Jon

Big heavy glass will need big strong tubes and at those F ratios, tubes would be long even if folded. You would need to take great care to ensure tube currents don't destroy the image.

I did work out the spot diagrams for a 4"F25 once and was tempted to build one. At this aperature and F ratio, all visible light shows up inside the airy disk and the field is free from abberations over a very large diameter. However, this changes quickly as the aperature increases.

The suggestion to get a Meade 178 ED is a good one. It would be easier to recoup any investment when it was time to sell.

have fun,
dan

While dreaming of 8" f/25 to f/30 folded refractors, other
designs come to mind like 3-mirror Schiefspieglers.
Probably more compact, unobstructed, and no color.

[QUOTED & G has had long ratio doublets listed for several years now, at not unreasonable prices. I'm not ready to buy and haven't wasted their time asking about availability yet. [/quote]
I do from time to time visit D&G website, just curiosity, and have always wondered about their F/25 and F/30 objectives, is there really a market for these? Who buys these and in what configuration are these used? A folded refractor IMHO is not easy or cheap to build and introducing two flats will compromise the end result compared to straight through system using the same objective.

That said, I am a big believer in slow F ratio telescopes and just like you I kept dreaming about assembling one but gave up once I factored in the practicality issues.

When it comes to color issues, size and weight, for an apo F/9 is the sweet spot, for an achro F/15 is the most reasonable ratio, making them longer will definitely improve things but the negatives pile up quickly making them not practical.

I never gave up on long focus instruments, currently I have two F/20 TEC Maksutovs, a 10” and a 8”, I use the 8” under less than perfect seeing conditions or when I am not in a shape to do heavy weight lifting, the 10” is reserved for stable seeing, I had it out for Jupiter last night, seeing was near perfect from Houston, the four moons were round discs without any spikes, the detail on Jupiter was amazing @ 260x & 312x and the contrast produced by these two TEC’s are almost as good as my AP refractor., in fact better than the refractor once I go over 240X where the 6” refractor will max out on Jupiter.

To me for anyone considering long focus refractors for planets, Maksutovs offer an attractive alternative, very compact, considerably less color than the finest apos. Cooling is an issue, but most imported Maksutovs do have cooling fans that do control thermal problems reasonably well.

Just something to think about.

Vahe