What is it with focal reducers and SCTs?

[lukebl]

I’ll probably wind everyone up with this question, but it is a genuine puzzle to me. 

Why is it that when folk buy an 8 or 10” SCT, with a focal ratio of say f/8 or f/10, they immediately slap on a focal reducer to bring down the focal ratio and focal length for imaging? I know that it’s partly to reduce aberrations when imaging, but if the focal length is too much, why not just get something with a native lower focal ratio and length in the first place, like a Newtonian? Cheaper, too. I’ve owned SCTs myself, mainly to capture smaller DSOs and planetary detail so I’m not anti-SCT, but it always struck me that adding a focal reducer defeats the whole advantage of havIng the longer FL. Which is why I went back to Newtonians. Much simpler and more logical.

Now strike me down…
 

[Cosmic Geoff]

The SCTs are lighter and more compact. And if the SCT and the Newtonian are mounted on GoTo mounts of equivalent performance, the argument that the Newtonian is cheaper loses much of its force.

I have owned both types, and there is no way I would want to revert to a Newtonian.

I have reently acquired a f6.3 focal reducer BTW.

[Fegato]

I guess it's the flexibility.  Imaging DSOs with long exposures at F/10 requires a lot of sky time, and with modern CMOS cameras, the pixel resolution is such that you won't get benefit from it really. Thus the reducer - half the exposure time.  However - shooting the moon and planets with lucky imaging at the longer focal length is perfect.

[BlueAstra]

I think one of the major reasons is as a field flattener for imaging. Imaging Newtonians or edge variants have optics before the camera to flatten the image. These are not present in standard SCTs so the reducers give the flat field.

[cathalferris]

I got my very rough looking CPC800 secondhand, as scrap/spares/repair, and with a few hours, some HCl, and a few replacement circuit boards for less than €100, I had a working SCT for the first time.

I did notice the curved focal plane compared to the Newts and refractors I've been accustomed to. Combine that field curvature with Ethos eyepieces and I was having a bit of a hard time with the field edges. I figured "why not?" for the reducer, then I realised that the Celestron reducer also flattened the field - and that made things a fair bit easier for me.

For the moment, I do keep the reducer in place. It seals the tube from dust while I have the scope indoors, and I do like the extra field of view available with my 1.25" eyepieces. It's not really worth my time and effort to put a 2" back on the scope, and getting the extra FOV is more useful than getting the higher magnification.

For my balcony observing area, the effort in getting the SCT up and running of an evening is significantly less than getting e.g. my 8" Quattro on AVX working, and the SCT has an easier viewing angle and eyepiece location. When it's still of upper atmosphere then it's worth getting the 130m mapo out for the planet views. The ease of the SCT means I get more viewings than I otherwise would, and having the reducer on it is a low-cost (in time) benefit to me.

Edited April 16, 2022 by cathalferris
spelling mistakes fixed

[newbie alert]

As you said that you use it for planetary and DSO.. for planetary you want the f ratio to be F10+.. for DSO you don't necessarily..

[Simon128D]

I have a CPC1100 and only use it in F10 for planets and lunar, visual and imaging . If seeing is really bad I will drop it to F6.3 to help compensate for the poor conditions. 
My main reason for dropping to F6.3 would be for visual use on DSOs. As someone who is used to used to F4.7 with a 10" Newt, 14" Dob and one 16" UL Dob at F4.5 I find trying to look at galaxies, nebula and PN unbearable at F10. The image is so faint and resolving detail is usually not possible. Dropping to F6.3 really brightens up the image and helps provide a nicer range of medium to semi low magnifications.