SCT f/1.8 Super-Reducer?

[Stratis]

Any thoughts on this (seemingly absurd) focal reducer for Celestron/Meade f/10 SCTs?

The site lists the product as 'Coming Soon', optimised for 1/3" CCD chips and capable of reducing an f/10 SCT by 35 times down to f/1.8...

Now sure, they are quoting a tiny CCD size so you wouldn't get much joy from a 35mm sensor, but could it work with the smaller-format SBIG/Atik cameras in use by many members here?

If so, what sort of tradeoffs would there be with something like this? I like the idea of imaging wide-field when I pick up another SCT 

~Paul

[riklaunim]

Among DS cameras with 1/3" diagonal chips you have: Atik Titan and 16 IC-HS, QHY6, Moravian G1-0300 and  G1-0800, QHY IMG0X, QHY IMG0S, SXVF-M5, SXVF-H5 and SBIG ST-i if you want a SBIG here. Plus many planetary cameras with 1/3" and 1/4" diagonals. Very fast f-ratio is essential for DS imaging with video and planetary cameras (many short exposed frames), plus it helps putting things within the small field of view. Their wrote that it won't be easy to use their reducer with other cameras due to reducer to sensor distance. This also means that it may be impossible to put filters in the path (and many bandpass filters or even light pollution filters may not work optimally at f/1.8). f/3.3 SCT reducers allow 1/2" diagonal cameras and some backfocus for a filter wheel or drawer.

For Atik Titan and C14 FOV calculator says 23.82' x 17.86' at 2.17 "/pixel resolution and it frames Triffid, Dumbbell and many other not so big DS objects.

For C8 it will be 45.84' x 34.36' at 4.17 "/pixel. For a DSLR at f/6.6 56.34' x 37.56' 0.87 "/pixel - slightly more, but on much higher resolution (a bit of binning would be handy). Quite nice.

[Macavity]

Hard to comment specifically on the product? I just use the reducer formula:

R = 1 - D / Fr 

R is the reduction factor

D is the distance between the reducer and the CCD

Fr is the focal length of the reducer.

I happen to know the focal length of a few commercial focal reducers: The bog-standard Atik 1.25" "0.5x" is about (minus) 80mm etc. To get 0.5x it needs to sit 40mm from the chip and the in-focus is also 40mm. The width of the unvignetted image circle is about 12mm. (OK for a 1/2" video chip with diagonal 8mm!)  (Sadly I FORGET how to calculate the size of the image circle!  (Anyone know / remember?)

To get greater reductions I move the reducer away from the chip... the in-focus increases. I can get to 0.3x.... 0.25x even, with the Atik, but the in-focus is at the limit of my MAK150's focus travel. The vignetting increases too. What looks like coma is significantly greater! Focal reducers perform best at nominal values: 0.7x, 0.5x, 0.33x etc.

I imagine to get their greater (unvignetted) reductions here, it's a 2" reducer, with a short focal length. f10 to f5 is a factor 0.5x; to f2.5 is a factor 0.25x; to f1.0 is... 0.18x. It is "just maths" after all! How well the aberrations are controlled might be the real question?

[pete_l]

It would be interesting to see how easily this held focus. The calculations give a depth of focus of about 12 micron (c.f. 360 u for an F/10 SCT). So not only would the stock focuser on an SCT be exceedingly difficult to hit the sweet spot with, it wouldn't take much (thermal expansion, slip) to drift out of focus.

[Peter Drew]

I think thermal expansion might be its Achilles Heel for long exposure photography but it could make a great video unit. 

[Mav359]

it'll also depend on the sale price as too whether its worth it.

My Hyperstar reduces my 9.25 down to F2.3, i have no issues with focus and they have adapters to fit just about any camera, large CCD included but they are £850 a piece. I would suggest that the difference between F1.8 & F2.3 is negligible (F2 for the C8/C11) so if these units are £600 a pop then I'd say its not worth it especially if you still have to buy the CCD but if they come out around the £200 - £400 mark and you already have one of those cameras then it would be we'll worth it.

Knowing the Astronomy market as it is though I'd be surprised if it comes cheap

[Tim]

With all the manipulation of the light path going on, I'd be very sceptical about these. I'd imagine the image will be soft, almost impossible depth of focus, and a collimation nightmare.

As suggested above, the best way to get speed out of an SCT is to remove the secondary and use the primary at its given f2 or so, ie, FASTAR etc

[ollypenrice]

It would be interesting to see how easily this held focus. The calculations give a depth of focus of about 12 micron (c.f. 360 u for an F/10 SCT). So not only would the stock focuser on an SCT be exceedingly difficult to hit the sweet spot with, it wouldn't take much (thermal expansion, slip) to drift out of focus.

Very much the point I would make. Surely impossible to focus.

Why would you want to buy such an expensive scope (and probably reducer) and then put a webcam equivalent in the back for deep sky? Why not just use an ED80 and a DSLR or CCD?

Olly

[alan potts]

Someone will buy it I am sure.  I believe even the 3.3 reducer was poor I can't see this being any better.

Alan

[Peter Drew]

My F3.3 focal reducer worked very well for DSO video in conjunction with a 12" SCT, I'd much sooner screw one of those on to the back of the scope than the risky business of swapping out the secondary. 

[ollypenrice]

The 3.3 reducer wasn't poor, I don't think. It was simply very limited in terms of image circle, covering just the early webcam chips and certainly not the kind of chips in use today for CCD or DSLR imaging.

Olly