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SCT reducer for visual use


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Posted (edited)

Hello all, 

Due to the long delivery time of my widefield scope I'm considering making my C5 a bit more multi-purpose with the use of a reducer for visual use only (at the moment). I know a few people like @Ags have had successes doing something similar and some reports seem to suggest that the overall outcome is actually slightly better optically speaking. 

So far I know of two which work well the C5:

- the Celestron branded 0.63 reducer

- the TS-Optics 0.63 reducer

The TS one is about 20eur cheaper and available in Germany direct from them, as opposed to having to import the Celestron from Amazon US. 

Can anyone say whether one is better than the other, or if there are others I should be considering? Some reports seem to suggest that many of these reducers are actually the same optical components with specific branding. Can anyone confirm any of this? 

 

Thanks!

Edited by badhex
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Can't comment on the relative quality of the TS unit, but I am very happy with my Celestron-branded one. 

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Before investing in reducer - I would check what is the size of illuminated field on C5.

SCTs and MCTs have small fully illuminated field (or even partially illuminated field). Say your C5 has usable field of 25mm.

That sort of field can easily fit in 1.25" eyepiece - take 32mm plossl, it has 27mm field stop. It has larger field stop that illuminated field that scope can provide (in this example). Focal reducer simply won't help there.

Only thing that you are going to get is same FOV with shorter FL eyepiece. Given that the scope is F/10 - there is no need for short FL eyepiece - 32mm plossl will give you 3.2 exit pupil.

Here is a bit more in depth discussion on CN:

https://www.cloudynights.com/topic/360096-celestron-c5-and-63-focal-reducer/

some measurements even suggest that using focal reducer on scopes such as C5 and C6 lead to stopping down of aperture completely (C6 measured to be 133mm instead of 150mm) due to primary mirror baffle tube.

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Interesting - I will measure the exit pupil on my C6. I haven't noticed a loss in aperture (either with my eyeballs or by the impact on photographic exposure times), but it will be easy to verify. Wtihout aperture loss, I should have just under 5mm exit pupil with a 30mm eyepiece.

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Can't answer your specific questions but I use a Celestron reducer on my C6 with no issues. There is no vignetting with an APM UFF 24 or 32mm Plossl. 

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I use the Celestron f6.3 FF/FR with my 're-modded' ETX105 [and C6/SCT] and I have no known issues. As far as I am aware, a dedicated FF/FR for Maksutov 'scopes are hard to acquire or non-existant.

PIC025.JPG.cefae6dd0a831f1a44b582b1544d904c.JPGPIC022.JPG.ad2dde5173f39613817eb9f4f768708c.JPGPIC023.JPG.9015768a3cb121416d49ca9a58c896aa.JPGA5057402-94DE-4E35-A2DE-D8A6BDEFB67B.thumb.jpeg.2165097e2282e5347993d6249a14bd74.jpeg

note: The last image showing my ETX105 with an SCT extension tube between the OTA and 1.25" visual back and 1.25" star diagonal and no FF/FR.

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Simple way to asses vignetting would be to attach extension tube to back of SCT / MCT (this is just to give you reference point when placing eye - to simulate field stop of some diameter) - and then place your eye at the edges of this extension tube and look to see how much of secondary mirror you actually see.

For example like in this image:

image.png.faff3cf3c6b61d7b4136ced6415856f7.png

In the image above - it is clear that we can no longer see whole primary mirror reflection - part of it is obscured by primary baffle tube.

Humans have hard time distinguishing less than about 10% (7% actually) drop in light intensity. Rough measurement of how much vignetting there is would be to compare where intersection happens - at half of primary (baffle tube clips image at secondary shadow) - 50% vignetting - 1/4 to the edge ~25% vignetting.

If you can't see the whole image of primary when you put focal reducer in place - that means that scope is effectively stopped down.

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Thanks for the replies! A lot of stuff to digest but I'm a bit braindead right now from work so will have to have a read through in detail tomorrow. Very high level scanning though, seems that there's still a bit of a lack of consensus between real world usage and data. 

@Ags if you ever get as far as checking the exit pupil I'd be glad to know but don't go to any trouble on my account! 

@vlaiv thanks for the links - will have a look. The CN link seemed to have some hard facts, but again not and obvious consensus but I'll have to read in detail. I actually read some stuff recently on the site you linked with the photos and discussion on 2“ accessories, as I was looking to see if it was worth bothering.

 

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Oh, one other point - I hadn't intended to use 2" accessories, just in case that wasn't clear - I can't see how that would be possible with this reducer anyway, unless I'm missing something. 

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15 hours ago, badhex said:

Oh, one other point - I hadn't intended to use 2" accessories, just in case that wasn't clear - I can't see how that would be possible with this reducer anyway, unless I'm missing something. 

It's physically possible but reducer + 2" accessories equals significant vignetting.

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Thanks again all for the info. I've read through most of it and I have to say I'm still a little unclear as to a definitive answer, but unless I'm misunderstanding, it seems that there's one thing most people agree on - quote from http://www.waloszek.de/astro_ce_c5_2z_e.php

Quote

You will, however, not achieve a larger field of view this way! Using the f/6.3 reducer/corrector with the "maximum" focal lengths for 1.25" is more or less equivalent to using 2" maximum focal lengths without the reducer/corrector.


So my takeaways are:

  • On paper it is problematic, but some real-world reports say they have a good experience
  • If using an EP with a field stop smaller than/equal to the baffle tube diameter, I should not experience vignetting. Need to find/take measurements for my 24mm 68˚ Maxvision, my longest FL 1.25" EP
  • 2" accessories not useful (was not planning to use anyway)
  • A reducer may cause effective stopping down of the scope if the full objective cannot be viewed when the reducer is attached per @vlaiv's test 

Does that sound right?

Thanks
Joe

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@badhex

That test that I mentioned is really just "eyeballing it". There is much better test done with actual eyepiece in place. You need just torch / flashlight that gives concentrated/narrow beam of light (it does not have to be very narrow).

You put eyepiece in scope and focus at infinity. Then point the scope at white wall and shine torch at eyepiece (along optical axis of eyepiece) from some distance away - like one meter or so. Just make sure everything is perpendicular / in line (scope perpendicular to wall and beam in line with eyepiece)

Look at the wall and measure resulting circle diameter - as per diagram:

image.png.b712f0b24062445fbab8667c316bc9a5.png

Also be careful - if you place torch too close to eyepiece - you'll get larger image on the wall as you start introducing light at angles. Point is that you hold torch far enough so that any angles are rather small (and they get further reduced by magnification of scope/eyepiece combo).

Of course, you'll probably need to dim / turn off the lights in the room to be able to see the circle on the wall - it will be very faint.

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I have to agree that 2" EPs and focal reducers don't really go together on an SCT. Even on my C8, which has a considerably larger image circle than either the C5 or the C6, the use of the 0.63x reducer cuts the image circle to about 28 mm, so APS-C or 1.25" EP size. Anything with a wider image circle shows clear vignetting. Since I switched to a 2"visual back for the C8, I have used my reducer only for imaging.

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37 minutes ago, vlaiv said:

@badhex

That test that I mentioned is really just "eyeballing it". There is much better test done with actual eyepiece in place. You need just torch / flashlight that gives concentrated/narrow beam of light (it does not have to be very narrow).

You put eyepiece in scope and focus at infinity. Then point the scope at white wall and shine torch at eyepiece (along optical axis of eyepiece) from some distance away - like one meter or so. Just make sure everything is perpendicular / in line (scope perpendicular to wall and beam in line with eyepiece)

Look at the wall and measure resulting circle diameter - as per diagram:

image.png.b712f0b24062445fbab8667c316bc9a5.png

Also be careful - if you place torch too close to eyepiece - you'll get larger image on the wall as you start introducing light at angles. Point is that you hold torch far enough so that any angles are rather small (and they get further reduced by magnification of scope/eyepiece combo).

Of course, you'll probably need to dim / turn off the lights in the room to be able to see the circle on the wall - it will be very faint.

Thanks @vlaiv - I'll try this over the next few days without the reducer (as I don't have one!) for a baseline. 

 

11 minutes ago, michael.h.f.wilkinson said:

I have to agree that 2" EPs and focal reducers don't really go together on an SCT. Even on my C8, which has a considerably larger image circle than either the C5 or the C6, the use of the 0.63x reducer cuts the image circle to about 28 mm, so APS-C or 1.25" EP size. Anything with a wider image circle shows clear vignetting. Since I switched to a 2"visual back for the C8, I have used my reducer only for imaging.

Thanks @michael.h.f.wilkinson - I'm looking to see if I can squeeze a bit more versatility out of the scope rather than full maximum FOV, but I know it will always be a bit of a compromise without 2" being possible. 

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Just now, badhex said:

Thanks @vlaiv - I'll try this over the next few days without the reducer (as I don't have one!) for a baseline. 

 

Thanks @michael.h.f.wilkinson - I'm looking to see if I can squeeze a bit more versatility out of the scope rather than full maximum FOV, but I know it will always be a bit of a compromise without 2" being possible. 

I think the main use of a reducer in an SCT is actually for imaging, unless you are restricted to 1.25" EPs (as I was previously) and wanted a larger FOV. If something like a 24 mm 68 deg EP or a 32 mm Plössl already capture the entire image circle of the C5, there is not much use for a reducer, except for DSO imaging.

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Thanks for all the replies. I've been thinking it over for the last few days.

I do think it would be an interesting experiment and I always like to see things first-hand and in practice, especially since quite a number of people have had good experiences. I also have a load more questions that probably illustrate my lack of detailed knowledge about optical physics. 

Having said all of that, I've now also cancelled my existing delayed order for the widefield scope and ordered a Zenithstar 73 which will be here in the next week or two, so it's probably not worth me rushing out and buying a brand new Celestron reducer, but rather grabbing one second hand if the chance arises. 

Thanks again all for the interesting and informative replies! 

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