Focal Reducers

[25585]

Not much written on these compared to Barlows. They seem ideal for long fl scopes to get wider fov and lower mags from an ep for Cassegrains and refractors.

Are there any that can be recommended for good optics that can take 2 inch eyepieces. For use with SCTs etc, would image quality for optical be much worse, and would an ep's limitations be more apparent?

[ollypenrice]

Their drawback in SCTs is that they don't allow for a wider field than can be obtained with a 2 inch visual back and widefiled 2 inch EP. The baffle tube is the impediment to going wider still. 

Olly

[25585]

5 minutes ago, ollypenrice said:

Their drawback in SCTs is that they don't allow for a wider field than can be obtained with a 2 inch visual back and widefiled 2 inch EP. The baffle tube is the impediment to going wider still. 

Olly

Thanks. Just bought a 2 inch visual back for my C8 so now unsure if its worth getting a reducer. Both my w/f eps are 2 inch - TV 55 Plossl and 35 Panoptic.

[ollypenrice]

Just now, 25585 said:

Thanks. Just bought a 2 inch visual back for my C8 so now unsure if its worth getting a reducer. Both my w/f eps are 2 inch - TV 55 Plossl and 35 Panoptic.

I did once try my 35 Panoptic in a 10 inch SCT reduced to F6.3 and found it rather unpleasant around the edges. I hadn't, at that time, sussed out that it wouldn't widen my FOV because of the baffle tube but it wasn't very nice optically anyway. I think you'll be about at the field limit without reducer. Someone will be able to give accurate figures, I'm sure.

Olly

[Luna-tic]

1 hour ago, ollypenrice said:

Their drawback in SCTs is that they don't allow for a wider field than can be obtained with a 2 inch visual back and widefiled 2 inch EP. The baffle tube is the impediment to going wider still. 

Olly

The greater advantage is that the reducer drops the effective f/ ratio, and can cut photographic exposure time by half or more. 

I'm not sure the statement you made is entirely accurate. I've viewed with and without the reducer in my C6, using both 1.25 and 2" back and EP's. The FOV is substantially larger with the reducer in place, especially when shooting prime. I am unable to get the entire disc of the Sun or full Moon in the camera's FOV without the  reducer in place.

I don't have any EP's with FOV larger than 63 degrees, so maybe there's some truth once you go with a widefield EP, but it does make a difference with "regular" EP's.

[knobby]

As Luna-tic said, makes a difference to a DSLR, I can vouch for it making the Moon fit on a D7100 chip with the reducer but there seems to be quite some argument about the reducing imaging time ...

Yes it makes a F10 scope in theory an f7 scope , but it's no bigger at the front so where does the extra light come from 

[The Admiral]

12 minutes ago, knobby said:

Yes it makes a F10 scope in theory an f7 scope , but it's no bigger at the front so where does the extra light come from 

There isn't any more, it's just that what light there is 'more concentrated' (i.e. more photons/sec per pixel). This comes at the 'expense' of a wider field of view, but if that's what you want , then all to the good.

I know Olly has some views on this!

Ian

[ollypenrice]

5 hours ago, Luna-tic said:

The greater advantage is that the reducer drops the effective f/ ratio, and can cut photographic exposure time by half or more. 

I'm not sure the statement you made is entirely accurate. I've viewed with and without the reducer in my C6, using both 1.25 and 2" back and EP's. The FOV is substantially larger with the reducer in place, especially when shooting prime. I am unable to get the entire disc of the Sun or full Moon in the camera's FOV without the  reducer in place.

I don't have any EP's with FOV larger than 63 degrees, so maybe there's some truth once you go with a widefield EP, but it does make a difference with "regular" EP's.

'The greater advantage is that the reducer drops the effective f/ ratio, and can cut photographic exposure time by half or more.' Be warned, this is a deadly subject and a matter of much dispute. Eek!! Very obviously, since the reducer does not increase the aperture, it cannot increase the number of photons from 'an area of interest' which lands on the chip anyway. We've talked this one to death on the forum so I won't rehearse it again here but try Stan Moore's famous article which also appears in Rob Gendler's AP book, Lessons from the Masters. http://www.stanmooreastro.com/f_ratio_myth.htm

' I've viewed with and without the reducer in my C6, using both 1.25 and 2" back and EP's. The FOV is substantially larger with the reducer in place, especially when shooting prime. I am unable to get the entire disc of the Sun or full Moon in the camera's FOV without the  reducer in place.' In this sentence you're doing two things, 'viewing' and 'shooting,' which are not at all equivalent. The FOV when 'shooting' can be, and probably will be, limited by the size of your chip. Your eye admits of a larger field of view and, on top of that, you can move your head and peer into the field stop of the EP, something which the chip cannot do. The reducer will certainly increase the FOV on your chip. No problem there.

It's perfectly possible that you can get a wider FOV with your present EP and reducer but I believe I'm correct in saying that, eventually, the reducer cannot increase the visual  FOV over the maximum allowed by a 2 inch EP unreduced. The baffle tube is the ultimate arbiter in my understanding. We need the SCT expert, Peter Drew, to settle the matter!

Olly

[Luna-tic]

5 hours ago, ollypenrice said:

'The greater advantage is that the reducer drops the effective f/ ratio, and can cut photographic exposure time by half or more.' Be warned, this is a deadly subject and a matter of much dispute. Eek!! Very obviously, since the reducer does not increase the aperture, it cannot increase the number of photons from 'an area of interest' which lands on the chip anyway. We've talked this one to death on the forum so I won't rehearse it again here but try Stan Moore's famous article which also appears in Rob Gendler's AP book, Lessons from the Masters. http://www.stanmooreastro.com/f_ratio_myth.htm

' I've viewed with and without the reducer in my C6, using both 1.25 and 2" back and EP's. The FOV is substantially larger with the reducer in place, especially when shooting prime. I am unable to get the entire disc of the Sun or full Moon in the camera's FOV without the  reducer in place.' In this sentence you're doing two things, 'viewing' and 'shooting,' which are not at all equivalent. The FOV when 'shooting' can be, and probably will be, limited by the size of your chip. Your eye admits of a larger field of view and, on top of that, you can move your head and peer into the field stop of the EP, something which the chip cannot do. The reducer will certainly increase the FOV on your chip. No problem there.

It's perfectly possible that you can get a wider FOV with your present EP and reducer but I believe I'm correct in saying that, eventually, the reducer cannot increase the visual  FOV over the maximum allowed by a 2 inch EP unreduced. The baffle tube is the ultimate arbiter in my understanding. We need the SCT expert, Peter Drew, to settle the matter!

Olly

Thanks for that, I keep forgetting the differences between my eyes and the camera's sensor. And I've wondered, since the baffle tubes are a fair amount smaller than a 2" back, how that translates to more light entering. I could only assume that the baffle tube was bigger than it needed to be for the tapering "beam" of light passing through it from the secondary mirror.

Maybe the whole intent of using a reducer is just for photography; if you want a wider field, use a lower power. The OP mentioned a reducer being good for getting wider fields from an EP, but what is the point of using a reducer to widen a field, and then using a higher power EP, which would narrow it again?

[Louis D]

1 hour ago, Luna-tic said:

Maybe the whole intent of using a reducer is just for photography; if you want a wider field, use a lower power. The OP mentioned a reducer being good for getting wider fields from an EP, but what is the point of using a reducer to widen a field, and then using a higher power EP, which would narrow it again?

Absolutely on the right track about magnifying a focal reduced image as making no sense.  The focal reducer should be removed for mid to high power viewing for the best possible image.  A similar thing happens with certain coma correctors.  Because they introduce some spherical aberration to the on-axis view, they should be removed when viewing low contrast objects (planets) at high power for the best possible on-axis view.  If you're going to let the subject drift across the FOV in an ultra wide FOV eyepiece, you should probably leave it in.

[TSRobot]

One other thing to watch with visual observations. With the 0.63 reducer (Antares NOT corrector) in my C8 (now sold) and an SW 32mm plossl EP you can just begin to see the central obstruction so contrast is gone. I find the Field of View calculator invaluable?

[Dave In Vermont]

This paper (photons & pixels) is useful, though written around the venerable MallinCam Video-Cams, as it's principles remain the same with regardless. So I though you might it useful and stash a copy away for the proverbial 'cloudy day.'

Focal Reduction For Dummies.pdf

Starry Skies,

Dave

 

[25585]

11 hours ago, Luna-tic said:

Thanks for that, I keep forgetting the differences between my eyes and the camera's sensor. And I've wondered, since the baffle tubes are a fair amount smaller than a 2" back, how that translates to more light entering. I could only assume that the baffle tube was bigger than it needed to be for the tapering "beam" of light passing through it from the secondary mirror.

Maybe the whole intent of using a reducer is just for photography; if you want a wider field, use a lower power. The OP mentioned a reducer being good for getting wider fields from an EP, but what is the point of using a reducer to widen a field, and then using a higher power EP, which would narrow it again?

As the OP my query is based on getting the widest TFOV from a given ep. So using a long FL ep, could a reducer expand its FOV. 2 ins seems not, but then there are some 31.7mm wide field long FL eps too. 

Are Celestron Edge & any equivalent models given different optics to standard SCTs? Or are you paying for a package with optics that can be bought independently turning a long FL SCT into an Edge's shorter FL?

I presume Edges can be changed to the native Fl if desired.  

[Louis D]

57 minutes ago, 25585 said:

Are Celestron Edge & any equivalent models given different optics to standard SCTs? Or are you paying for a package with optics that can be bought independently turning a long FL SCT into an Edge's shorter FL?

It's my understanding that there are optical correcting elements at the base of the rear baffle to flatten and correct the edge of the field.  As a result, you can't shove a traditional tube fan up inside of it to equilibrate it.  It also means a traditional SCT field flattener/corrector has the wrong prescription to work well with it; so it needs a new, dedicated field flattener.  I don't know if there are any field flatteners/correctors which will bring a tradtional SCT up to the level of edge correction of a stock EdgeHD.  If there were, why would anyone buy an EdgeHD?

[Luna-tic]

3 hours ago, 25585 said:

As the OP my query is based on getting the widest TFOV from a given ep. So using a long FL ep, could a reducer expand its FOV. 2 ins seems not, but then there are some 31.7mm wide field long FL eps too. 

Are Celestron Edge & any equivalent models given different optics to standard SCTs? Or are you paying for a package with optics that can be bought independently turning a long FL SCT into an Edge's shorter FL?

I presume Edges can be changed to the native Fl if desired.  

 

2 hours ago, Louis D said:

It's my understanding that there are optical correcting elements at the base of the rear baffle to flatten and correct the edge of the field.  As a result, you can't shove a traditional tube fan up inside of it to equilibrate it.  It also means a traditional SCT field flattener/corrector has the wrong prescription to work well with it; so it needs a new, dedicated field flattener.  I don't know if there are any field flatteners/correctors which will bring a tradtional SCT up to the level of edge correction of a stock EdgeHD. If there were, why would anyone buy an EdgeHD?

It depends on the specific Edge where the field flattener is located. In the diagram below, you see it's in the baffle tube near the position of the primary mirror on the 8" and 11", but at the back of the tube on the 9.25" and 14". This might also explain why the reducers for the Edge models are specific to the model diameter.

To opine a possible answer to the bold print; mirror locks, maybe?

[25585]

Interesting difference between 8 & 9.25. I always wondered why 2 models with a bit over an inch difference in aperture (price apart). Perhaps to woo Meade 10 inch users/buyers. 

[Stu]

One brief reminder that we are in the eyepieces section here, so visual comments are what is relevant to the OP.

I have one of these, an Alan Gee Telecompressor which reduces an SCT to f5.9 and flattens the field. It works very nicely for visual in my C9.25.

https://www.teleskop-express.de/shop/product_info.php/info/p732_Baader-Alan-Gee-f-5-9-Telecompressor-Mark-II-for-Schmidt-Cassegrains.html

[Louis D]

6 hours ago, Stu said:

One brief reminder that we are in the eyepieces section here, so visual comments are what is relevant to the OP.

I have one of these, an Alan Gee Telecompressor which reduces an SCT to f5.9 and flattens the field. It works very nicely for visual in my C9.25.

https://www.teleskop-express.de/shop/product_info.php/info/p732_Baader-Alan-Gee-f-5-9-Telecompressor-Mark-II-for-Schmidt-Cassegrains.html

It would make for an interesting shoot-out between your AGT corrected SCT and an EdgeHD both native and with a dedicated telecompressor.  If you could take a vintage SCT and bring it up to EdgeHD levels of correction for the price of an AGT, that would be worth trying.  I was never impressed with the sharpness of SCT images visually until I looked through an EdgeHD.  This comparison image sort of shows this difference.  Fine details are more visible and stars are less bloated.  Stars are also pinpoint right out to the edge of wide field views.

[Stu]

2 hours ago, Louis D said:

It would make for an interesting shoot-out between your AGT corrected SCT and an EdgeHD both native and with a dedicated telecompressor.  If you could take a vintage SCT and bring it up to EdgeHD levels of correction for the price of an AGT, that would be worth trying.  I was never impressed with the sharpness of SCT images visually until I looked through an EdgeHD.  This comparison image sort of shows this difference.  Fine details are more visible and stars are less bloated.  Stars are also pinpoint right out to the edge of wide field views.

Yes, would be interesting to see. I too really enjoyed the 8" Edge I had and disagree with the view that they are only of benefit for imaging. To me, the optics gave lovely sharp views with widefield (i.e. 82 and 100 degree afov) eyepieces.

[YKSE]

23 hours ago, Stu said:

One brief reminder that we are in the eyepieces section here, so visual comments are what is relevant to the OP.

I have one of these, an Alan Gee Telecompressor which reduces an SCT to f5.9 and flattens the field. It works very nicely for visual in my C9.25.

https://www.teleskop-express.de/shop/product_info.php/info/p732_Baader-Alan-Gee-f-5-9-Telecompressor-Mark-II-for-Schmidt-Cassegrains.html

That's an interesting reducer Stu

Have you tried it with C9.25 or Edge C8 with an EP with max TFOV for 1.25", such as 24 Pan? If so, did you see any vignetting in either scopes?

[Stu]

1 hour ago, YKSE said:

That's an interesting reducer Stu

Have you tried it with C9.25 or Edge C8 with an EP with max TFOV for 1.25", such as 24 Pan? If so, did you see any vignetting in either scopes?

It is Yong. I have used it in the C9.25, although conditions have never been particularly good. I will give it another go soon and check for vignetting. I'm pretty sure the aperture is large enough not to vignette a 24mm Panoptic but I will check.

In theory a 24mm Panoptic with the Alan Gee reducer gives the same field of view as at f10 with 40mm TMB Paragon, for example.

[YKSE]

Yes, stu, it'll be very interesting to know the visual result.

My line of thinking has been in simplified form: 46mm FS, with the f6.3 reducer, it corresponds the 29mm FS (46*0.63=29), so the f5.9 reducer should result in 27mm non-vignetted view (46*0.59=27). But, as we know, our eyes are not that sensitive to gradual vignetting, and 2" diagonals filter thread is actually 48mm, so the Alan Gee reducer might produce visually acceptible wider TFOV than using a 2" EP with 46mm FS.

[Stu]

3 minutes ago, YKSE said:

Yes, stu, it'll be very interesting to know the visual result.

My line of thinking has been in simplified form: 46mm FS, with the f6.3 reducer, it corresponds the 29mm FS (46*0.63=29), so the f5.9 reducer should result in 27mm non-vignetted view (46*0.59=27). But, as we know, our eyes are not that sensitive to gradual vignetting, and 2" diagonals filter thread is actually 48mm, so the Alan Gee reducer might produce visually acceptible wider TFOV than using a 2" EP with 46mm FS.

Yes, sounds like it should be correct. The spec does say a useable illuminated field diameter of 32mm but not sure if that just relates to imaging circle. Will check vignetting and come back to you.