SCT, Mak or frac?

[HollyHound]

2 hours ago, JeremyS said:

2-inches bigger 😊

And significantly heavier too, I suspect 😬

[johninderby]

3 minutes ago, HollyHound said:

And significantly heavier too, I suspect 😬

I did ask Steve about a 10” solid tube CC. With the carbon tube wouldn’t be much heavier than the 8”.

He said it would be possible if there was enough demand. 🤔

Edited February 15, 2022 by johninderby

[cloudsweeper]

Interesting thread!  For a while I've wanted an 8" aperture (or more), GoTo, and sharp stars (at higher mag).  My 8SE fails somewhat for the latter criterion, and there is no easy solution except for a 9.25 Edge HD tube (or CPC set-up), but that is prohibitively expensive!

So I've come back to the idea of a Celestron f/6.3 reducer/corrector as a fairly cheap improvement to the 8SE performance.  Visual only.

Observers have reported that the reducer performs poorly at low power, and with 2" EPs, but I don't want it for getting more FOV, just for sharper stars at high mag, and EPs are 1.25" then anyway.

So, has anyone used a reducer with an 8SE?  Would it be worthwhile for my needs?

Doug.

[Dixie]

Got to say I can't agree that SCTs nearly always produce sub standard views. Between an 8 inch Edge and a four inch refractor under anything but the poorest conditions the former will provide  more detail.  The example I bought second hand provided some absolutely stunning views of Jupiter at Kelling last year, as did two C11s nearby.  

[Louis D]

3 hours ago, Dixie said:

Between an 8 inch Edge and a four inch refractor under anything but the poorest conditions the former will provide  more detail.

I already pointed out that the 8" EdgeHD displays stunning planetary views.  To reiterate, in my experience, it's standard 8" SCTs that always seem to show mushy, low contrast views of planets in comparison to 8" Dobs with hand figured mirrors and 20% or smaller secondary obstructions.

[Mr Spock]

4 hours ago, Dixie said:

Got to say I can't agree that SCTs nearly always produce sub standard views.

Agreed. My former C9.25 was capable of spanking any refractor out there. Providing the seeing conditions were right of course and it was properly collimated. 
SCTs have to be collimated spot on. Only slightly out and you get mush. I've had an easy separation of an 0.7" double with that scope.

[johninderby]

Reminds me of the old joke about guitar players. They spend half their time playing out of tune and the other half tuning. 😁

That’s the greatest weakness of the SCT design at least in the medium and bigger apertures. Keeping one in perfect collimation all the time is the problem.

[Mr Spock]

40 minutes ago, johninderby said:

Keeping one in perfect collimation all the time is the problem

Yep. Damian Peach says he collimates his before every shot and spends 20 minutes doing it 

[Louis D]

2 hours ago, Mr Spock said:

Agreed. My former C9.25 was capable of spanking any refractor out there. Providing the seeing conditions were right of course and it was properly collimated. 
SCTs have to be collimated spot on. Only slightly out and you get mush. I've had an easy separation of an 0.7" double with that scope.

I'm jealous you got to compare views against a 9.25" refractor.

[Andrew_B]

On 16/02/2022 at 17:33, johninderby said:

Reminds me of the old joke about guitar players. They spend half their time playing out of tune and the other half tuning. 😁

That’s the greatest weakness of the SCT design at least in the medium and bigger apertures. Keeping one in perfect collimation all the time is the problem.

It seems to be that when you make an optical system better by adding more lenses or mirrors, it also becomes more sensitive to being out of collimation on top of there being more elements to become misaligned or out of place.

Reminds me of an article I read about how they make the very high performance optics for photolithography stepper-scanner machines that are used to produce microchips. The tolerances are so fine that individual lens elements (and there might be 25 of them in a purely refractive system) have their supporting rings attached to actuators which alter their position to account for changes in the refractive index of the air caused by variations in barometric pressure! On top of that, the structures being created are so tiny and precise alignment of the lens and the silicon wafer is so critical that engineers have to account for the time it takes light to pass through the optics even though it's no more than a few nanoseconds, and they use adaptive optics to correct waveform deformations introduced by absorption-induced zonal heating of individual elements.

Thank goodness amateurs don't have to deal with any of that, but I liked the idea of the corrective adaptive optics - could be a great way to get around the problem of cool down time!