What compromise in Focal Reducing an SCT?

[Alan White]

All the recent chat on SCT's being made of Gold, set me off pondering things, as it does.

If you had say a Celestron 9.25 for visual only use and fitted a focal reducer, 

A What benefits would you see?
B What is the compromise you are striking, theres always a compromise after all?

And other than a narrower field of view, how would it compare to a f5/6 Newtian of similar size visually.

 

 

[johninderby]

Was reading on CN recently that adding a focal reducer to a Celestron C8 turned it into a C7.5 so slight reduction in effective aperture but the reducer did work well for visual. 

Edited November 14, 2021 by johninderby

[newbie alert]

9 minutes ago, johninderby said:

Was reading on CN recently that adding a focal reducer to a Celestron C8 turned it into a C7.5 so slight reduction in effective aperture but the reducer did work well for visual. 

How does it reduce the aperture?

[Deadlake]

You might see some "dimming" around the edge when using EP's with large fields stops. I believe the CN thread you referring to is here:

https://www.cloudynights.com/topic/499161-c11-edge-07x-reducer-visual/

[NGC 1502]

6 minutes ago, newbie alert said:

How does it reduce the aperture?

Although I’ve never owned an SCT the second post does say “effective” aperture. I’ll be back later to see what others have to say. An interesting thread….

Ed.

[newbie alert]

10 minutes ago, Deadlake said:

You might see some "dimming" around the edge when using EP's with large fields stops. I believe the CN thread you referring to is here:

https://www.cloudynights.com/topic/499161-c11-edge-07x-reducer-visual/

I've just read that through, It never says anything about aperture reduction  it touches on the subject  about vignette and darkening of the edges with some ep... that's to do with the image circle  and touches on the subject of the baffle tube and size of the ep being used...  reducer/ flattener  reduce the light cone at the back end of the scope..aperture is at the front

[johninderby]

No that’s not the CN thread I was reading. Now if only I can find the right one. 

[newbie alert]

 

1 hour ago, Alan White said:

All the recent chat on SCT's being made of Gold, set me off pondering things, as it does.

If you had say a Celestron 9.25 for visual only use and fitted a focal reducer, 

A What benefits would you see?
B What is the compromise you are striking, theres always a compromise after all?

And other than a narrower field of view, how would it compare to a f5/6 Newtian of similar size visually.

 

 

Hi Alan, hope that you're keeping well?

As you know I'm predominantly photography minded,  I know nothing about exit pupils etc but have experience with sct's 

The c8 has a native fl of 2032mm and with the .63 reducer it's 1280mm

The 9.25 has a native fl of 2350mm and with reducer 1480mm

When you add a reducer it also reduces the effective imaging circle

A 9.25 has the smallest secondary so this will help with contrast 

To keep things simple for comparison with a Newtonian ,a 200mm f5 will have a 1000mm fl

And to confuse you some more you can add extra spacing to a c8 to achieve a faster f ratio.. a friend who's also a imager had his system working at 11.20mm, so that's f5.5 ish. . But he was using quite a small chip compared to today's standards..

Just to clarify this is the non HD version above

Edited November 14, 2021 by newbie alert
Adjusted

[vlaiv]

I guess it is down to illumination / diameter of illuminated circle.

C9.25 is in some sense comparable with stock 8" Newtonian

I often wondered how I would feel owning long focal length telescope and if I would get "boxed in" feeling.

With stock 8" newtonian you get 25mm 50°ish eyepiece - let's say that it is similar to 25mm plossl and hence has about 22mm field stop. 8" Newtonian in F/6 variety has 1200mm of focal length. C9.25 has almost double that at 2350mm.

If it can illuminate 44mm circle - then all you need is 50mm plossl eyepiece and you'll get almost the same field of view (exit pupils will be a bit different - but that can be sorted if one wishes by using different AFOV EP).

According to this:

https://groups.google.com/g/sci.astro.amateur/c/QfLsV_roCQY

C9.25 has baffle wide enough to let 47mm of light thru - so it will fully illuminate complete field stop of 2" Eyepieces. I think it would be better to use it without reducer unless reducer is also corrector. SCTs suffer from coma.

According to:

https://www.telescope-optics.net/SCT_off_axis_aberrations.htm

Quote

The standard commercial SCT has both mirrors spherical, thus K1=K2=0. Obviously, coma in such arrangement is not corrected; for the linear field, it is approximately at the level of an ƒ/6 paraboloid, with the coma increasing to the diffraction-limited level (0.80 Strehl) at about 2.5mm off-axis.

That is not that bad. I'm never bothered by coma in my F/6 8" newtonian, but I don't have EPs with very large field stop so I can't be sure.

In the end - reducer will "compress" things down into smaller circle. If C9.25 can illuminate 47mm - adding x0.67 reducer will turn that into ~32mm illuminated circle and using EPs with larger field stop will lead to vignetting. We need about 10% drop off in illumination to start noticing vignetting.

[RobertI]

I did actually have a fiddle with my 0.63 reducer and C8 recently to see if I could realistically use my 70 degree 38mm eyepiece and 2” diagonal (theory say it’s doable but vignetting and light loss occurs). From memory it did work and I could get a pretty wide FOV  but the stars became non-sharp fairly quickly off axis. And again from memory it didn’t look like normal coma-type aberrations, I suspect there are some complicated optical effects taking place involving both the cheapish eyepiece and reducer. However there is a review on the FLO website from somebody saying they were getting stars sharp to the edges with a 9.25” plus the reducer and a 40 mm eyepiece and no obvious vignetting. I believe the C8 has a relatively narrow baffle (38 mm diameter?) compared to the larger SCTs so vignetting more likely to occur. The reducer did seem to work quite nicely with my 1.25” eyepieces though. 

[Mr Spock]

The focal reducer has a narrow aperture so you would be losing some FOV. Also the extra elements will reduce brightness and contrast.

When I had a C9.25 it would just squeeze a 42mm LVW's FOV. If you want wide field, then a long focal length eyepiece is the way to go.

[vlaiv]

Yes, one of important aspects of using reducer that is also corrector is getting the distance right.

For example - SCT x0.67 reducer has 85mm working distance. Although it says +/-15%, if you don't get that right - level of reduction changes and so does correction.

This means that pretty much all 2" diagonals are out of the picture as most have 100mm+ optical path.

Longer working distance means higher reduction factor.

Tech sheet also says 41mm clear aperture - so even if SCT is not vignetting - reducer will, and also only 24mm of corrected image field.

[Alan White]

Thanks so far folks

Some reading later when not just on my phone.

Most glad I asked as good and interesting information coming out.

 

[RT65CB-SWL]

I have on the odd occasion used the Celestron FF/FR with my ‘re-modded’ ETX105.

Not sure whether I gain anything in doing so, as I am purely visual at time of writing.

[nfotis]

I suppose that the Starizona reducer-corrector is a quite different animal, but it's quite pricey from what I remember.

Better to get a refractor instead, which is quite faster (if you don't want to go the Hyperstar route).

N.F.

[GavStar]

I use the 0.7x edge reducer with my c11 edge. The required distance is 146mm so it’s straightforward to use a 2?inch diagonal. This is for visual only with my nv monoculars. Works very nicely in conjunction with my 55mm plossl. I also use a standard c11 with AP 0.75x photo visual reducer and 55mm plossl. Visual views are shown in the phone images in the linked thread (about 1 degree fov)

 

Edited November 15, 2021 by GavStar

[Alan White]

12 hours ago, nfotis said:

I suppose that the Starizona reducer-corrector is a quite different animal, but it's quite pricey from what I remember.

Better to get a refractor instead, which is quite faster (if you don't want to go the Hyperstar route).

N.F.

How many can afford a 8-10" Refractor though?

 

[ollypenrice]

My understanding is that the focal reducer makes the telescope's baffle tube the source of vignetting/ reduction in effective aperture. For this reason you can open up the view to the widest extent possible by using a 2 inch visual back and widefield, long focal length, 2 inch EPs. The only reason for using the reducer visually would be achieve a wider view at lower cost with less expensive EPs. The more expensive solution is preferable if you don't mind it.

If we move on to imaging we enter the dreaded minefield known as the F ratio myth.  It's a minefield for several reasons but...

1) A focally reduced scope does not take the same photograph as a native scope. If you're imaging an object which fits nicely on the chip at native, introducing a reducer brings in precisely no new light from that object. It brings in new light only from the region, now fitting on the chip, around that object.  Starizona would do well to make this clear in their advertizing. The reducer puts the same amount of galaxy light onto fewer pixels, 'filling' them faster but giving a smaller image. The other way to do that is to bin the pixels 2x2 or 3x3 etc. Which brings us to...

2) F ratio, of itself,  does not matter. Exposure time is determined by the area of the objective relative to the area of the pixel. The professionals often use huge apertures of fairly slow F ratio and, critically, huge pixels.

Olly

[vlaiv]

2 hours ago, ollypenrice said:

The only reason for using the reducer visually would be achieve a wider view at lower cost with less expensive EPs. The more expensive solution is preferable if you don't mind it.

I agree with going without focal reducer, but does that really mean more expensive eyepieces?

Some of cheapest 2" long focal length EPs - actually work better at F/10 then at F/6.3.

We often go for wider field shorter FL eyepiece to obtain smaller exit pupil and darken the background - but that can be achieved with longer FL EPs in F/10 configuration.

If I want to maximize FOV and get say 3mm exit pupil on my F/6 dob - I need very expensive EP - I need around 18mm FL and that would be say - ES92 17mm or perhaps 100° eyepieces in 18-20mm range. These don't come cheap and need to have good correction.

On the other hand - with F/10 scope I can get the same for say 100euro in form of APM 30mm 80° or maybe 180ish Omegon Oberon 32mm 82° EP.

[ollypenrice]

1 hour ago, vlaiv said:

I agree with going without focal reducer, but does that really mean more expensive eyepieces?

Some of cheapest 2" long focal length EPs - actually work better at F/10 then at F/6.3.

We often go for wider field shorter FL eyepiece to obtain smaller exit pupil and darken the background - but that can be achieved with longer FL EPs in F/10 configuration.

If I want to maximize FOV and get say 3mm exit pupil on my F/6 dob - I need very expensive EP - I need around 18mm FL and that would be say - ES92 17mm or perhaps 100° eyepieces in 18-20mm range. These don't come cheap and need to have good correction.

On the other hand - with F/10 scope I can get the same for say 100euro in form of APM 30mm 80° or maybe 180ish Omegon Oberon 32mm 82° EP.

I didn't know about these budget ultra-wide EPs and take your point.

Olly