27 replies to this topic

[NeutronStar79]

Good day,

I've just ordered a Celestron 9.25 which I'll be mounting on a Skywatcher Wave 150i.

I'm excited about the scope after reading generally very positive reviews.

I'm aware of its relatively narrow field of view, and I know that there are focal reducers available which reduce the FL to F/6.3.

I seem to understand the reducer is used mainly for AP. Does anyone have experience using it for visual? Would you recommend it? What's the trade off?

Thanks for any info you'll be able to share and clear skies!

[k.darwin]

I thought they were mainly for visual?

[Rustler46]

I use my Celestron 0.63X focal reducer for imaging ( in past), but now mostly for visual - for C-8 & C-11. It is supposed to flatten the field and have the main benefit of widening the field as well. While there are some negative consequences, the positives far outweigh these in my experience. There is a discussion of focal reducers on the following link:

• https://www.cloudyni...-focal-reducer/

The above discussion relates how a focal reducer works with a bino-viewer. But much can be learned about focal reducers in mono-viewing. In this case the light path length remains much as designed. So the negative effects are minimized.

 

As I recall the focal reducer was designed to increase the photographic speed for film-based photography.

 

Clear Skies,

Russ

Edited by Rustler46, 23 March 2025 - 12:34 AM.

[PJBilotta]

Absolutely. You'll find many threads here discussing this.

The Celestron and Antares (probably others, as well) are both reducers and correctors. In addition to reducing the focal length of the scope, they also compensate a bit for the SCT's inherent field curvature. I, and many other observers, find views significantly sharper and better corrected with an R/C than without.

I consistently find using the R/C at f/6.3 with a 24/68 to achieve maximum FOV in my 8" SCT significantly better than a 40/70 at native f/10.

My R/C never leaves my scope. It provides wider views and better visual correction than without.

[RichA]

Good day,

I've just ordered a Celestron 9.25 which I'll be mounting on a Skywatcher Wave 150i.

I'm excited about the scope after reading generally very positive reviews.

I'm aware of its relatively narrow field of view, and I know that there are focal reducers available which reduce the FL to F/6.3.

I seem to understand the reducer is used mainly for AP. Does anyone have experience using it for visual? Would you recommend it? What's the trade off?

Thanks for any info you'll be able to share and clear skies!

People who were weaned on wider field scopes will tell you it's better with the R/C, maybe even that it offers better views.  Not really.  They are wider, but CA which is more or less not visible without it is visible with it.  Also, 55mm 2 inch eyepieces down to around 40mm may not be that usable depending on how bright your skies are, you may see the secondary mirror as a dark shadow in the centre of the field.  

[Martinbruce]

I have an EVO 8 and use a R/C on everything but the planets for which I go native f/10. As my eyepieces and diagonal were 1.25 it was the way to go for me to achieve the widest FOV. Be aware of the impact a R/C has on magnification. Play with this tool to help. 
​https://astronomy.to.../field_of_view/

 

Good luck. 

[mayhem13]

I currently use the Starizona reducer but I still have the Celestron unit…..it does the job for visual. These are not created equal over time…..the older pieces were made in Japan to a much higher optical standard….if you can find a used one listed as Japan, buy that. They come up here on CL from time to time

 

…..or just splurge now on the Starizona. You’ll need a 2” visual back for the scope though. Starts getting expensive.

[Magnus Ahrling]

I have an old (1973) orange C8 and since many years I have had a Celestron (Japan made) focal reducer. Can`t live without it! (I am a 100% visual observer.) In my opinion it`s one of the most useful asseccory for an SCT. I use mine with a 1,25" diagonal. Hmm tradeoffs, can`t think of any, really..........

Good Luck & Clear Skies

 

Magnus

Edited by Magnus Ahrling, 23 March 2025 - 08:30 AM.

[NeutronStar79]

Thanks for the helpful answers everyone.

I have one follow up question: Would it be possible to use the focal reducer and a 2 inches star diagonal and eyepiece at the same time, or would that be pushing it, causing vignetting or other issues?

[Rick-T137]

Thanks for the helpful answers everyone.

I have one follow up question: Would it be possible to use the focal reducer and a 2 inches star diagonal and eyepiece at the same time, or would that be pushing it, causing vignetting or other issues?

I use my f/6.3 reducer all the time for visual with my Meade 2080. As to your follow up, I only use it with a 1.25" diagonal because I have experienced strong vignetting when I tried it with a 2" diagonal. Some CN'ers do use both together and quite like it so it may be something you might want to try...

 

[Dave Hederich]

I was going to suggest that Astronomics also offers an Astro-Tech branded 0.63 reducer. But a quick check of the site shows that it's no longer listed. Astronomics does carry the Antares and Celestron versions. The Astromania brand version is available at lower cost on Amazon. From what I've read on this forum they are all comparable in performance.

[mclewis1]

Yes to using 2" eyepieces with the SCT reducer BUT there are two major caveats. One already mentioned above by Rich is the problem with very long fl eyepieces (40+mm) and "shadowing". Most folks use 35mm or shorter fls with the reducer. 

 

The other issue is more mechanical, that is getting the correct spacing behind the reducer.  The Celestron SCT reducer should be mounted as close to the rear cell as practical and the eyepiece's field stop should be at the spacing prescribed by the reducer (about 105mm from the back shoulder of the reducer or 110mm from a point within the reducer's optics). This usually means that a 2" barrel mounted diagonal with some sort of SCT to 2" barrel adapter (2" visual back) will put the eyepiece too far away from the ideal 105/110mm spacing. The extra spacing increases the amount of reduction (so more than .63x and sometimes closer to .5x) but it also degrades the optical performance a bit (more noticeable at the edges of the fov).

 

One solution is to use a 2" diagonal with an SCT threaded nosepiece (and not a 2" barrel attachment) that goes directly onto the rear of the reducer. Again you have to watch the spacing, and ideally you want a setup with a very short mounting of the SCT threaded nosepiece.

[Bill Fischer]

I have compared the view with and without on a C8. The Reducer/corrector never comes off. I also use a 2” diagonal with an AT28/82, I like the view. I have tried a Celestron prism  1.5” diagonal, it seems a tiny bit sharper at 254x; but, I like the AT28/82 view so much I nerve take the 2” diagonal off. This is probably something you will need to try for yourself.

[ABQJeff]

As stated, yes you can use the 0.63 R/C for visual BUT you will be limited in what eyepieces (to smaller 2” focal lengths) and diagonals you can use to maintain ideal backfocus (105mm) and avoid vignetting.

Edited by ABQJeff, 23 March 2025 - 11:39 PM.

[Bill Fischer]

In this case you lose nothing when using an eyepiece that vignettes. The view outside the FOV of about 1.2 degrees just fades away in your peripheral vision. I like it, it seems more natural than a black circle. The illumination does not fall off quickly and the FOV outside 1.2 does show lots of stars. If you try it you might like it.

[PJBilotta]

In this case you lose nothing when using an eyepiece that vignettes. The view outside the FOV of about 1.2 degrees just fades away in your peripheral vision. I like it, it seems more natural than a black circle. The illumination does not fall off quickly and the FOV outside 1.2 does show lots of stars. If you try it you might like it.

I have experimented a lot with what combination yields the widest possible view with non-noticable vignetting, and for me is is the R/C with a shorter light path 2" diagonal, and a 28/68. This is pushing well beyond the theoretical 1.2 degrees, but still presents a clean image with a crisp field stop. There's definitely light fall-off present in the outer field, but it's not glaring, and plenty of stars are still visible in this outer region. I've tried going just a little wider with the 30 UFF and 30 XW, and they are very visibly vignetted. Not pleasing at all.

The critical element here is using a 2" diagonal with a shorter light path, such as the GSO model with the negative profile 1.25" adapter and the 2" APM prism. These both have optical paths about 10-15mm shorter than most other 2" diagonals, leaving you with enough remaining in-focus. It also reduces the distance from the R/C to the eyepiece so you don't over-reduce the scope to F/5.8-f/6. If you do over-reduce, you just introduce the vignetting earlier.

[luxo II]

You're all over-thinking this.

 

Reach for a bigger 2" eyepiece - in my case a 48mm Brandon. There are many others similar.

 

Forget the reducer.

Edited by luxo II, 25 March 2025 - 01:47 AM.

[C0rs4ir_]

You're all over-thinking this.

 

Reach for a bigger 2" eyepiece - in my case a 48mm Brandon. There are many others similar.

 

Forget the reducer.

No chance if you want to use Naglers or Ethos with a classic sct. There is definitive a gain from the R/C.

[Thomas F]

I'm using a orange C8 from 1980.

For Fotos with a 0,63 reducer by Meade to get a fast er system

For visual observations I now use a 2" mirror and 2" 38mm Ocular which gives the largest possible field of view.

CS Thomas

Edited by Thomas F, 25 March 2025 - 06:28 AM.

[luxo II]

No chance if you want to use Naglers or Ethos with a classic sct. There is definitive a gain from the R/C.

The field is determined by the diameter of the field stop in the eyepiece... which is limited by the barrel.

Unless Televue is using barrels larger than 2" your field stops are no bigger than mine.

Edited by luxo II, 26 March 2025 - 02:23 AM.

[C0rs4ir_]

The field is determenied by the diameter of the field stop in the eyepiece. Unless Televue is making 3” barrels the field stop of yours are no bigger than that in the Brandon.

I mean the correction of coma, the maximum possible field size should be similar.

[akdwivedi]

the reducers are good accessories to have.. I have a C6 and C8 and I use the reducer with both of them. I wish they had a larger backfocus like the C9.25 Edge reducer so that we could use a binoviewer with the reducer without any glass path corrector. but the backfocus on standed C6/C8 reducer is 105mm only which makes it difficult to use a binoviewer with reducer.

[Eddgie]

Unless you have the reducer somehow connected directly to the mirror box of the diagonal, the C9.25 will probably use some aperture.  The C9.25 has a different geometry than the C8, C11, and C14, and ray tracing suggests that when used with thee reducer, the C9.25 will tolerate only about 100mm of back focus  before aperture loss occurs. Of course you might get a wider field of view but you can get that using 2" diagonal and a wide field eyepiece.

 

Also, the combination may increase spherical aberration, but you would have to test the system to know that.  In this case, it would be best to remove it if you were going to view planets. 

 

If the goal is a wider field of view though, you can get a bit wider field, but the outside of the field suffers extreme illumination falloff which may extinguish fainter stars. 

 

My experience using both is that for an SCT, it is better to use a 2" diagonal and a good 2" wide field eyepiece than use a reducer/corrector. You preserve full aperture, and the field illumination is much better at the edge. The reducer is best used with a 1.25" prism diagonal, but you can get a wider field though, and for some, the aperture loss is not as important as getting the wider field.  To me though, it is better to use a telescope designed for a wider field, but that is just me. 

 

All of this is shown in ray trace done by Ray Hutchinson. 

 

 

Ray traces are ray traces though, and while I have asked many people over the years to actually measure their effective aperture when using a focal reducer with a 2" diagonal, no one has ever posted the actual figures. 

Edited by Eddgie, 26 March 2025 - 07:51 AM.

[Bill Fischer]

Eddgie to help me clarify my definitions, please look at these photos of my C8. I read 115mm of “back focus” on the 1.5” diagonal and 160mm on the 2” diagonal with the reducer. Am I using the term back focus correctly for this discussion? I am measuring from the back of the C8 to the top of the edge of the eyepiece holder.

In this 2” set up, is the reducer causing a loss of aperture? The reducer is 25mm. The 2” diagonal measures 135mm.

[gnowellsct]

Unless you have the reducer somehow connected directly to the mirror box of the diagonal, the C9.25 will probably use some aperture.  The C9.25 has a different geometry than the C8, C11, and C14, and ray tracing suggests that when used with thee reducer, the C9.25 will tolerate only about 100mm of back focus  before aperture loss occurs. Of course you might get a wider field of view but you can get that using 2" diagonal and a wide field eyepiece.

 

Also, the combination may increase spherical aberration, but you would have to test the system to know that.  In this case, it would be best to remove it if you were going to view planets. 

 

If the goal is a wider field of view though, you can get a bit wider field, but the outside of the field suffers extreme illumination falloff which may extinguish fainter stars. 

 

My experience using both is that for an SCT, it is better to use a 2" diagonal and a good 2" wide field eyepiece than use a reducer/corrector. You preserve full aperture, and the field illumination is much better at the edge. The reducer is best used with a 1.25" prism diagonal, but you can get a wider field though, and for some, the aperture loss is not as important as getting the wider field.  To me though, it is better to use a telescope designed for a wider field, but that is just me. 

 

All of this is shown in ray trace done by Ray Hutchinson. 

 

Screenshot 2023-07-24 130739.png

 

Ray traces are ray traces though, and while I have asked many people over the years to actually measure their effective aperture when using a focal reducer with a 2" diagonal, no one has ever posted the actual figures. 

Ironic that Ray does ray traces?

 

My experience is as you say.  There seems to be some confusion on this point, and the sensitivity to vignetting is dependent to some extent on what kind of sky you have.  In a bright sky (as in light polluted) you may not notice the vignetting much and for whatever reason prefer to use the focal reducer.  The darkening doesn't show.  Apart from photography, the main benefit is field flattening.  

 

The unvignetted widest fov of a c8 is about 1.2 degrees but using the standard calculation (AFOV/magnification) you can get a tad under 1.4 degrees.  That's with a 40 mm 70 degree eyepiece.  The fuzziness of the argument is that there are a lot of SCTs, Maks, and Newts that have illumination falloff and/or vignetting at their lowest magnifications, they vary in degree.  Some levels of vignetting are not really bothersome.

 

Some are very bothersome.  I passed my reducer around at our Bortle 3 (so it has been measured) observing site to a couple of SCT owners and it came back with very negative reviews.  In particular, if you take a 35mm wide field like a Pan Optic or a 41 Pan or  40mm XW and drop them behind a reducer on an SCT, the effect proves this: you don't get something for nothing.  From the back end, at f/10 or f/11, the SCT is not a widefield instrument and wide fields are best left to instruments that are good at it, like refractors and fast Newts (particularly those 200mm and greater, which have less illumination drop off).  

 

Here is what some people think:

.63 * 2020 = 1273  (focal length is shrunk by the reducer)

1273/40= 32  (the shrunken focal length divided by the eyepiece focal length=magnification)

70/32 = 2.2 degrees (the eyepiece afov divided by the magnification, yielding field of view)

 

But unless you're imaging with fastar, you're not going to get 2.2 degrees out of a c8.   Not 2.2 degrees that you're going to like anyhow.

 

Then there are the folks who say well that's fine just use it with an eyepiece in the 32mm and lower range.  Yes you can do that, and people refer to it as a reverse barlow.  It has nowhere near the convenience of a barlow in terms of putting it on and taking it off.  You have to remove the diagonal unscrew the reducer replace the diagonal and put in an eyepiece.  And then if you want the wider view again you have to put the reducer back on.

 

The views are really much nicer through something like a Pan 41 or XW 40.   And these are very easy to swap in and out.  It is true that the eyepieces are pricier.  Twenty years ago when I was watching every nickel I did pursue this budget option, but I was very glad to get some widefield eyepieces and move past this phase of my observing.  

 

There are a number of SCT users however who use the focal reducer almost 100% of the time.  I would think such users are less than 10%.  But it's not like Celestron is publishing any data.  I haven't run into visual observers who use reducer correctors on their SCTs at the star parties I've been to.   The reducers lower magnification, which can make the sky background grayer, and they add an extra layer of glass to the optical train, one more thing to keep clean.  I suppose there is a benefit in keeping the rear end of the SCT sealed.   

 

Anyhow reducers are fairly inexpensive (as these things go) so it's easy to find out for one's self.  But put up a wanted ad on CN or Astromart a lot of people have them who don't want them.  So you may save a few bucks.

 

Greg N