Focal reducers

[Bugdozer]

I currently have an f/10 5" scope. A conversation in another thread mentioned that a focal reducer might help with looking at fainter fuzzy objects, and it was also suggested as a means of getting a wider field of view photographing at prime focus. 

This all sounds great. However... 

I can't help but feel this is like getting something for nothing? Surely I can't pump more photons out of my scope and get a bigger exit pupil without a corresponding negative to balance this in some way? I have no practical experience of reducers and how they are used. Can anyone advise me? 

[Budgie1]

You get a wider FOV and a slightly faster f ratio on the scope, but your trade off is that the objects appear smaller, further away and less detailed, because of that increased FOV.

To see what you would get, put you scope and camera or EP details into the FOV calculator in Astronomy Tools.

[Louis D]

That, and those cheap 0.5x reducers produce a highly curved field with lots of outer field aberrations.

The best use for one I've found visually is to increase the field of view of my binoviewers in scopes with limited back/in focus.  I have a 1.25" 2x Barlow element up front followed by 45mm of spacer rings followed by the 0.5x reducer.  There is some vignetting and outer field aberrations, but I can reach focus in my Dob with only 1.0x magnification instead of my normal 3x magnification of the Barlow element alone.  I've essentially created a relay lens.

Conceptually, the way it makes the exit pupil larger is by compressing the entire exiting light flux into a smaller image circle at a lower power.  This also means that widest true field eyepieces will be vignetted if used with a focal reducer.  For photography, you can't use as large of an imaging sensor when using a telecompressor (focal reducer) because of the reduced image circle.

[John]

If your scope is an SCT the F/6.3 reducers work quite well for visual observing as long as you stick with 1.25 inch format eyepieces.

I used to use one when I had a 5 inch F/10 SCT and with an 8 inch F/10 SCT. 

I don't image but I believe some folks do with the F/6.3 and F/3.3 reducers. The latter is not intended for visual use and has some camera limitations for imaging I believe.

 

 

[doublevodka]

If it's an SCT which I assume it is then people have some success with reducers such as this - https://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

Just don't be tempted by these https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html take it from experience, they are not worth the money, tried in multiple scopes and the distortion is just too much.

That said, I'm going to play devil's advocate here and say why not look at a different OTA if you already have a suitable mount, for example, you could pick up a 6-inch Newtonian used for not much more money than the 6.3 reducer. Generally going to be around f5, giving you a wider field of view and a faster system for grabbing the faint fuzzies, just an idea....

[John]

It is almost inevitable (it seems) that people who purchase long focal length scopes with a narrow-ish field of view soon investigate how to get a lower power, wider field of view from it and similarly, people who buy short focal length, wide field of view scopes soon develop the urge to get better high power views from them. 

@doublevodka probably hits the nail on the head above 🙂

Edited October 25, 2023 by John

[Bugdozer]

2 hours ago, John said:

It is almost inevitable (it seems) that people who purchase long focal length scopes with a narrow-ish field of view soon investigate how to get a lower power, wider field of view from it and similarly, people who buy short focal length, wide field of view scopes soon develop the urge to get better high power views from them. 

@doublevodka probably hits the nail on the head above 🙂

Well it's not really surprising, because we all want to be able to see everything. Very few of us are completely "just planets/binaries/moon" or "just galaxies/nebulae". I have gone for five years without any upgrades to my equipment and just two eyepieces. 

I did wonder how long it would be before the inevitable "buy a new telescope" comments turned up! What I want to do is maximise what I can do with the gear I have, rather than start from scratch to achieve an aim. I don't have the space for a second scope, especially a big Newtonian, and I would then also need another mount and tripod etc, putting it well beyond the cost of a focal reducer. I was asking for advice on focal reducers because I know little about them and wanted to understand them before considering whether or not to purchase one. 

[John]

3 hours ago, Bugdozer said:

Well it's not really surprising, because we all want to be able to see everything. Very few of us are completely "just planets/binaries/moon" or "just galaxies/nebulae".....

Absolutely. 

I don't think the suggestions about another optical tube assembly are flippant. Sometimes this is the most cost effective and least compromising route to achieve what is desired.

An F/6.3 focal reducer does take up a lot less room though.

It's good to explore alternative options I think. What route you personally decide to take is entirely up to you of course 🙂

 

Edited October 25, 2023 by John

[vlaiv]

18 hours ago, Bugdozer said:

I can't help but feel this is like getting something for nothing?

Well, this is indeed true.

I've recently experimented with 4" refractor and focal reducer to see if I'd be able to get full view of M31 in the eyepiece.

There is couple of ways this can be achieved with said scope, and I've chosen one that was available for me. I already had (rather expensive) focal reducer that I knew would work well with this scope, and I also had eyepiece I wanted to try this all out with.

If that was not the case - I'd probably go the following route - get one of these:

https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-super-plossl-eyepiece.html

2" 55mm Plossl.

In any case - when trying to get wide field of view from such instrument - there is really only two things that you need to be careful about:

1. Exit pupil must not exceed your pupil when fully dilated / dark adapted. This is usually quoted at 7mm but people tend to loose ability to dilate their pupils with age and it's best to actually measure it.

2. Fully corrected and illuminated field of the telescope can't be enlarged and is subject to restrictions. For example - in 2" system, max field of view is around 47mm, while in 1.25" one that is about 27mm.

Try to "squeeze" more field into that - and you will have vignetting and poor stars at the edge of the field.

In my case above - 4" F/10 can provide good field up to 2" size - so 47mm and 2" 55mm Plossl is one of the few eyepieces that has field stop that large. Also, that eyepiece would give 5.5mm exit pupil so it is a good match (and only drawback is 50 degrees AFOV of plossl - for those that love their wide field EPs).

On the other hand, same scope with x0.67 reducer will "squeeze" 47mm down to 47 * 0.67 = ~31.5mm. Eyepiece that I already have is Explore Scientific 28mm 68 degrees. It has field stop of 31.8mm - so again very good match (maybe tiniest bit of vignetting but it was not noticeable when observing). It also produces 2.8mm exit pupil so that checks out as well.

They both achieve the same (or rather very similar) thing - one with and one without focal reducer. This is because the scope is capable of showing that much. If I tried to use 55mm plossl with that reducer - it would not work (at least not very nice) - as vignetting would be very pronounced. So I would not see more of the sky (then the scope is capable of showing) - that is physically impossible. Focal reducer just makes it easier in some cases.

Similarly - when you try to match sensor size for photography with illuminated field - focal reducer can be used to widen the field of view if scope is capable of otherwise rendering such image on larger field of view. Same effect can be achieved by using larger sensor - so it is matter of economics and convenience - what is affordable and what works well for you - larger sensor or smaller sensor + reducer.

There are a few more minor things that need to be taken into consideration. Reducers often work at prescribed distance - so you must take care of that. I for one had to remove nose piece and eyepiece adapter from my diagonal and 3d print direct adapters for reducer and eyepiece - because of distance required by reducer.

Reducers also move focus point inward and you might not reach focus if your focuser does not have enough inward travel.

 

[Bugdozer]

1 hour ago, vlaiv said:

1. Exit pupil must not exceed your pupil when fully dilated / dark adapted. This is usually quoted at 7mm but people tend to loose ability to dilate their pupils with age and it's best to actually measure it.

2. Fully corrected and illuminated field of the telescope can't be enlarged and is subject to restrictions. For example - in 2" system, max field of view is around 47mm, while in 1.25" one that is about 27mm.

Try to "squeeze" more field into that - and you will have vignetting and poor stars at the edge of the field.

 

OK, firstly I cannot use a 2" eyepiece, mine is a 1.25" sort. 

Secondly, I thought larger exit pupil was a good thing? In another thread I was recommended a 40mm plossl over a 32mm because even though in my scope they will give the same field of view the 40mm has a larger exit pupil. 

Thirdly, your point 2 about FOV in mm is confusing me. Eyepiece FOV always seems to be expressed in degrees. If I look through a given eyepiece I have no idea if I am looking at a FOV of 10mm, 20mm, or any other number. But I can estimate it in degrees. 

[Louis D]

19 minutes ago, Bugdozer said:

OK, firstly I cannot use a 2" eyepiece, mine is a 1.25" sort. 

Secondly, I thought larger exit pupil was a good thing? In another thread I was recommended a 40mm plossl over a 32mm because even though in my scope they will give the same field of view the 40mm has a larger exit pupil. 

Thirdly, your point 2 about FOV in mm is confusing me. Eyepiece FOV always seems to be expressed in degrees. If I look through a given eyepiece I have no idea if I am looking at a FOV of 10mm, 20mm, or any other number. But I can estimate it in degrees. 

1. You could mount a 2" visual back and 2" diagonal to a C5 (assuming that's your scope).  You'll just get vignetting like I do with my 127mm Mak and 2" visual back.  The fully illuminated image circle is limited by the approximately 1 inch diameter rear baffle and rear port.
2. Exit pupils larger than your fully dilated eye pupil will waste aperture collected photons on your iris instead of sending them to your retina.  Sometimes, this is acceptable when trying to achieve maximum TFOV at any cost.
3. You're mixing up eyepiece AFOV (apparent field of view, measured in degrees and is completely independent of the scope) with TFOV (true field of view which is also measured in degrees on the sky, but is dependent on the scope it's mounted to).  However, TFOV is dictated by the field stop diameter of the eyepiece.  Between two eyepieces, the one with the larger field stop will yield the larger TFOV in any scope used in common between the two eyepieces.

[vlaiv]

6 minutes ago, Bugdozer said:

OK, firstly I cannot use a 2" eyepiece, mine is a 1.25" sort. 

That is ok, just remember one thing we will need for further explanations - maximum field stop diameter with 1.25" eyepieces is ~27mm.

17 minutes ago, Bugdozer said:

Secondly, I thought larger exit pupil was a good thing? In another thread I was recommended a 40mm plossl over a 32mm because even though in my scope they will give the same field of view the 40mm has a larger exit pupil. 

It can be both good and bad thing - it depends.

Larger exit pupil makes object both brighter and smaller in size. Up to a point. If exit pupil is larger than the pupil of your observing eye - then you are wasting light.

Look at following image:

It describes what happens when you have exit pupil larger than your pupil (in this case, observer is not dark adapted and their pupil is only 2-3mm wide, while exit pupil from eyepiece is 7mm wide).

Disregard "result" comment at the bottom of the image as it is actually wrong in this case . In any case - some of the light will hit iris of your eye and will fail to enter your eye. That light is lost.

For this reason you want to keep exit pupil of the eyepiece below or equal to how much your pupil dilates in the dark.

On second note regarding exit pupil size - I've mentioned that object observed gets brighter when you use larger exit pupil. Same thing happens with the background sky. If we were floating in outer space and we did not have this background light from the sky - largest exit pupil would be ideal, but since we have atmosphere and that atmosphere scatters some light and is not completely dark, then it is matter of contrast.

As we increase exit pupil size - we brighten both target and background sky. At some level of target brightness vs sky brightness - contrast will be the best (this also depends on the size and shape of target). For this reason, largest exit pupil is not always the best - we need to try different exit pupils depending on our sky conditions (level of light pollution) to find one that will show target the best against background sky.

26 minutes ago, Bugdozer said:

Thirdly, your point 2 about FOV in mm is confusing me. Eyepiece FOV always seems to be expressed in degrees. If I look through a given eyepiece I have no idea if I am looking at a FOV of 10mm, 20mm, or any other number. But I can estimate it in degrees. 

Ok, yes - I will explain this a bit better.

Telescope and eyepiece are really the same thing but in reverse.

Telescope takes angle and projects it onto a plane - focal plane, and eyepiece does the opposite - it takes point of focal plane and makes light ray out of it at certain angle.

This angle is related to distance of the point from optical axis (center of focal plane) by simple equation that depends on focal length.

This diagram explains it nicely - there is entrance angle alpha which gets projected onto focal plane (little vertical arrow between two lenses) and then eyepiece turns that into exit angle beta.

alpha = tangent of length of small arrow over focal length of telescope

beta = tangent of length small arrow over focal length of eyepiece

This is why magnification of a telescope and eyepiece combination is given by ratio of their focal lengths - it is actually the same as ratio of beta / alpha (magnification is change in angles).

Ok, so now imagine that this little vertical arrow in the middle can only be of a certain length. This is what field stop / field diameter is.

It is diameter of black ring in the image you see:

This is usually limited by physical dimensions of the telescope and eyepiece - or to be more precise focuser tubes and telescope tube.

You've mentioned that you have 32 and 40mm plossls and that they show the same amount of sky just at different magnifications / AFOVs and exit pupils.

This is because both of those eyepieces are limited by 27mm of their field stop. 40mm plossl will show same part of sky limited by same black ring - only smaller (less magnified).

Focal reducer helps with following:

- if telescope "field stop" (or illuminated field to be precise) is larger than eyepiece field stop - it helps to "squeeze" that field produced by telescope into field shown by eyepiece

Hope above makes sense.

 

[Louis D]

3 minutes ago, vlaiv said:

Focal reducer helps with following:

- if telescope "field stop" (or illuminated field to be precise) is larger than eyepiece field stop - it helps to "squeeze" that field produced by telescope into field shown by eyepiece

Further, assuming the OP has a C5, using a 0.63x focal reducer will show more field with a 32mm or 40mm Plossl, but with outer field vignetting as with a widest TFOV 2" eyepiece used with 2" accessories on the C5. TANSTAAFL

[vlaiv]

7 minutes ago, Louis D said:

Further, assuming the OP has a C5, using a 0.63x focal reducer will show more field with a 32mm or 40mm Plossl, but with outer field vignetting as with a widest TFOV 2" eyepiece used with 2" accessories on the C5. TANSTAAFL

I'm not sure that Maks and SCTs have as much vignetting due to size of rear port as people think.

Beam is very slow and focal length very long, so I don't think that much of light gets blocked in percentage - and drop of about 10% is just on the edge of being perceived as difference (just noticeable difference for light is 7% if I'm not mistaken).

[bomberbaz]

Just to help here it was I who suggested a reducer for increasing the exit pupil for use with narrowband filters in a different thread.

The op was looking at viewing the eagle nebula and was struggling to see it with no filter and a 25mm ep at 2.5mm exit pupil.

It was suggested by one commenter that a uhc filter would help but I chipped in with reducer for the larger exit pupil or a 40mm plossl as an additional measure to improve results.

My personal experience points to a 5mm exit pupil being optimum for line filters with 4-5mm being ok for the more forgiving uhc when used on diffuse nebula. I have read numerous articles in various forums and have yet to find anything to convince me otherwise.

So if you can live with the vignetting to have a strong response from a dim object at the centre of the eyepiece then it is still something to consider, a 40mm plossl is the other contender. 

 

[Louis D]

51 minutes ago, bomberbaz said:

Just to help here it was I who suggested a reducer for increasing the exit pupil for use with narrowband filters in a different thread.

The op was looking at viewing the eagle nebula and was struggling to see it with no filter and a 25mm ep at 2.5mm exit pupil.

It was suggested by one commenter that a uhc filter would help but I chipped in with reducer for the larger exit pupil or a 40mm plossl as an additional measure to improve results.

My personal experience points to a 5mm exit pupil being optimum for line filters with 4-5mm being ok for the more forgiving uhc when used on diffuse nebula. I have read numerous articles in various forums and have yet to find anything to convince me otherwise.

So if you can live with the vignetting to have a strong response from a dim object at the centre of the eyepiece then it is still something to consider, a 40mm plossl is the other contender. 

 

In that context, that should be fine.  Just as when I use a 40mm 2" eyepiece to center an object in an f/5 scope producing an 8mm exit pupil.  I don't really care about the wasted photons in that use case, I just want the object within the field of view to avoid searching about for it.

[John]

At what exit pupil does the secondary shadow start to show with an SCT ?

 

 

Edited October 25, 2023 by John

[Louis D]

1 hour ago, vlaiv said:

I'm not sure that Maks and SCTs have as much vignetting due to size of rear port as people think.

Beam is very slow and focal length very long, so I don't think that much of light gets blocked in percentage - and drop of about 10% is just on the edge of being perceived as difference (just noticeable difference for light is 7% if I'm not mistaken).

Here's my post on the subject:

As I say in the post, I've photographically measured about a 35% drop off in illumination toward the outer field.  Visually, I can't see it at all.

[Louis D]

1 minute ago, John said:

At what exit pupil does the secondary shadow start to show with an SCT ?

I haven't tried it with an SCT, but in my f/6 and faster Newts, I start noticing secondary shadow on the moon and sun at around 5mm to 6mm exit pupil.  It's quite strong by 8mm.  When fully dark adapted and looking at DSOs, I can't recall ever seeing secondary shadow at any exit pupil.

Getting to a large exit pupil at f/10 in an SCT is much more difficult than at f/5 in a Newt.  Perhaps with the SCT + F/R operating at f/6.3 it would be more of an issue.

[globular]

20 minutes ago, John said:

At what exit pupil does the secondary shadow start to show with an SCT ?

Roughly speaking, when the exit pupil multiplied by secondary obstruction % is about half or greater than your dilated pupil then you'll see it.

So in Louis examples;
lets guess his secondary obstruction was 30%;
and he started to see it around 5mm exit pupil;
when secondary exit pupil was 30% * 5 = 1.5;
so his dilated pupil when looking at the moon or sun was probably around 1.5 * 2 = 3mm.

When dark adapted his dilated pupil is probably 6 or 7 (if he's lucky);
so the exit pupil of the secondary would need to be 7 * .5 = 3.5;
and the full exit pupil would, in that case, be 3.5 / 30% = 11.5mm;
you'd be mad to create an 11.5mm exit pupil when your own pupil is only 7mm - seeing the shadow won't be the only problem you'd face.

(And Louis isn't mad - hence why he's never seen it when dark adapted).

[John]

4 minutes ago, globular said:

....you'd be mad to create an 11.5mm exit pupil when your own pupil is only 7mm - seeing the shadow won't be the only problem you'd face.

 

Yep. The largest I currently use is 5.25mm. I'm over 60 so I assume that my max dilated pupil has started to get smaller now.

[globular]

Just now, John said:

Yep. The largest I currently use is 5.25mm. I'm over 60 so I assume that my max dilated pupil has started to get smaller now.

I'm around that too - 5.5mm last time I measured it.
So my f10.5 scope isn't too limiting as I get 5.4mm with my 56mm plossl - and, when the sky is nice and dark, DSOs pop with the right filters.
Personally I see no reason to go the reducer route for visual astronomy - just pick the right eyepiece.

[bomberbaz]

36 minutes ago, John said:

Yep. The largest I currently use is 5.25mm. I'm over 60 so I assume that my max dilated pupil has started to get smaller now.

 

24 minutes ago, globular said:

I'm around that too - 5.5mm last time I measured it.
So my f10.5 scope isn't too limiting as I get 5.4mm with my 56mm plossl - and, when the sky is nice and dark, DSOs pop with the right filters.
Personally I see no reason to go the reducer route for visual astronomy - just pick the right eyepiece.

Seems us older chaps adopt similar,  my biggest exit pupil are at 5.3 (age [cough] 60) as a future proof should see me good for circa 10 years.

But this wasted light discussion brings us back to my rationale for adopting a larger exit pupil in the opposite way, maximise your light grasp when needed (when using light limiting filters) on dimmer nebula dso. 

 

[John]

This illustration might help. I created it using the Field of View (FOV) calculator which can be accessed under the Resources > Astronomy Tools tab at the top of the page. The target I used is Messier 45, the Pleiades cluster.

The yellow circle is the true field of view delivered by a 32mm plossl 1.25 inch format eyepiece with the F/6.3 reducer used in a 125mm F/10 (thus reduced to F/6.3) telescope. The red circle is the true field of view delivered by a 2 inch format 40mm eyepiece with a 70 degree apparent field of view (as large as you can get in the 2 inch format) in the same telescope but with no focal reduced used. EP = Exit Pupil diameter in mm:

 

 

 

 

Edited October 26, 2023 by John

[globular]

If you’d drawn the 32mm with reducer compared to a 50mm without the reducer then they would have given near identical fov, mag and exit pupil.

“But”, I hear the OP scream, “my scope has a 1.25 inch visual back so I can’t use a 2 inch 50mm eyepiece!”

Well, you could buy a 2” to 1.25” adapter (£20 or so) and accept the vignetting of the outer portion of the fov.

Or buy the reducer (£130 or so) - but it too will vignette to exactly the same extent. Because both are limited by the same 27mm effective field stop caused by the telescope back. (Or more likely there will be vignetting and distortions, because reducers tend to squeeze and distort while they compress).

I’m sure vlaiv is right that the vignetting won’t be that bad.  But my point is that it will be about the same with or without a reducer.

I think the best views you’ll get with that scope is with an eyepiece with a 27mm field stop and a focal length that gives you an exit pupil that best matches your sky conditions and your dilated pupil size.