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C8 with f/6.3 reducer backfocus help with zwo 533

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#1 gimpei

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Posted 18 April 2021 - 11:15 PM

I've have Celestron SCT 8 with the Celestron f/6.3 focal reducer. I recently decided to give EAA a go. I'm trying to get it to work with a ZWO ASI533MC. I had a couple of questions around the procedure.

 

The camera is supposed to have a 55mm backfocus. Does the addition of the FR modify the backfocus distance needed? I've seen some posts here that suggest that it does and others that seem to imply the opposite.

 

The camera did not come with an adapter that could be threaded into the FR. Any suggestions on what adapter I should get that would go from FR to M42 (I would need to know the width too).



#2 Tapio

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Posted 18 April 2021 - 11:35 PM

What is relevant here is the reducer backfocus.
That should be 105mm. Means the distance from reducer back to camera sensor.
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#3 kel123

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Posted 18 April 2021 - 11:55 PM

The camera does not have a backfocus of 55mm. You are missing things up. Every reducer has its specified backfocus, which is the distance from the reducer to the sensor.
The backfocus of 55mm came about in the camera diagram because most reducers require a backfocus of 55mm. This why ZWO has that diagram of 55mm back focus measurement. In fact, the camera itself has a backfocus of 6.5 mm. But that's not the point here.
The effective backfocus of your setup is determined by the backfocus specified by the F/6.3 reducer which I think is 105.
So, I wouldn't know where you read a debate about whether a reducer modifies a backfocus. It is the reducer that makes backfocus requirement necessary.

#4 WoodlandsAstronomer

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Posted 19 April 2021 - 12:08 AM

Yep what he said. The important metric is how far back your sensor must be from the focal reducer to attain focus. Don’t forget their is no rear-drawing focuser in an SCT (moves internal). So you just need to get enough space to attain focus. For C8, I recommend a SCT female to T male (16mm), a T female to T male spacer (40mm), a Baader Varilock variable T female to T male spacer (20-29mm), and then don’t forget to account for filter wheels (20mm), offsets in camera (e.g., 6.5mm), etc, to achieve roughly 105mm back focus from the focal reducer and you should be good to go.

Edited by WoodlandsAstronomer, 19 April 2021 - 12:13 AM.


#5 aaube

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Posted 19 April 2021 - 02:29 AM

I've have Celestron SCT 8 with the Celestron f/6.3 focal reducer. I recently decided to give EAA a go. I'm trying to get it to work with a ZWO ASI533MC. I had a couple of questions around the procedure.

 

The camera is supposed to have a 55mm backfocus. Does the addition of the FR modify the backfocus distance needed? I've seen some posts here that suggest that it does and others that seem to imply the opposite.

 

The camera did not come with an adapter that could be threaded into the FR. Any suggestions on what adapter I should get that would go from FR to M42 (I would need to know the width too).

 

The FR won't modify the back focus needed.

 

Total backfocus for your configuration is 105mm.  That is from the camera sensor surface till the beginning of the threads of the reducer.  Since the 533 has a

relatively small sensor, you have some margin here, i would think +- 1 or 2 mm. 

 

The sensor surface is 6.5mm inside the camera body, as shown by the mark on the camera body.  The other mark, on the green tape, shows the start of the

threads for the reducer inside the screw ring of the Celestron T adaptor (because of the parallax, the mark does not appear in line with the ''0'' of the ruler but it is).

 

There is several ways to get the spacing right, here i have a Celestron T adaptor on the reducer (50mm) ;

https://www.celestro...rain-telescopes

then a Baader T2 Quick changer (15mm), and then, 33.5mm worth of other adaptors.

(6.5+33.5+15+50)

 

Hth,

 

Alain

Attached Thumbnails

  • C8 ASI533 Reducer Back Focus.jpg


#6 ERHAD

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Posted 19 April 2021 - 08:16 AM

Please be aware  that when it comes to the F/6.3 Celestron reducer no matter how many times the (in)famous 105mm of back focus is mentioned, it is still wrong. A BF of 105 mm will give a much higher reduction and will likely introduce considerable aberrations. The Bf requirement may vary slightly depending on individual scopes. I currently image with a reduced C9.25 and the same 533 camera as you do. I currently have a bit over 85mm of total back focus (from the back thread of the reducer to the sensor) and I am close to nominal back focus ( I am actually getting more reduction given that my BF slightly over 85 mm). So two things to consider here:

 

1. The back focus for the Celestron f/6.3 IS NOT 105mm as is usually claimed, it is closer to  85mm.

 

2. The exact BF requirements may vary slightly from setup to setup. Plate solving is your friend. I know that I am close to the desired  reduction because I have played with the back focus and plate solved consistently until I got to the desired focal ratio. Also,  I get much better stars in my off axis guider at 85mm back focus than I do at 105mm where the off axis performance is really affected and the starts are really deformed. Bear in mind that the C8 might behave a bit differently to my C9.25, but the 533 has a small-ish sensor and the tolerances are higher, so you can't go wrong by trying 85mm of BF. I have and am really happy with the results...

 

Hope it helps.

 

Erik


Edited by ERHAD, 19 April 2021 - 08:17 AM.

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#7 kel123

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Posted 19 April 2021 - 08:45 AM

Please be aware that when it comes to the F/6.3 Celestron reducer no matter how many times the (in)famous 105mm of back focus is mentioned, it is still wrong. A BF of 105 mm will give a much higher reduction and will likely introduce considerable aberrations. The Bf requirement may vary slightly depending on individual scopes. I currently image with a reduced C9.25 and the same 533 camera as you do. I currently have a bit over 85mm of total back focus (from the back thread of the reducer to the sensor) and I am close to nominal back focus ( I am actually getting more reduction given that my BF slightly over 85 mm). So two things to consider here:

1. The back focus for the Celestron f/6.3 IS NOT 105mm as is usually claimed, it is closer to 85mm.

2. The exact BF requirements may vary slightly from setup to setup. Plate solving is your friend. I know that I am close to the desired reduction because I have played with the back focus and plate solved consistently until I got to the desired focal ratio. Also, I get much better stars in my off axis guider at 85mm back focus than I do at 105mm where the off axis performance is really affected and the starts are really deformed. Bear in mind that the C8 might behave a bit differently to my C9.25, but the 533 has a small-ish sensor and the tolerances are higher, so you can't go wrong by trying 85mm of BF. I have and am really happy with the results...

Hope it helps.

Erik


Sometimes, the performance of these equipments vary as they are mass produced. You may speak for your setup and you are not wrong to say that there might be some variation but it is wrong to categorically say that the backfocus of 105mm is wrong.
A lot of users work with that backfocus of 105mm and the manufacturer of the reducer specified that backfocus. It is always better to work with the manufacturer's specification towards your own sweet spot.
You can actually make your point which is quite valid when it comes to your own requirements without claiming that others are wrong especially when they are quoting manufacturer specs.
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#8 Tapio

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Posted 19 April 2021 - 08:57 AM

Agreed, cannot say that 105mm is wrong but can say that 105mm (or the other common measure of 55mm) is a starting point.

 

Just plate solved and measured my f/6.3 configuration.

I have back focus of 114mm and 1277 mm focal length - so f/6.385 and I have pretty good stars (with my SXV-694 camera)


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#9 ERHAD

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Posted 19 April 2021 - 01:20 PM

I totally get your point. A couple things to note: plenty of vendors are quoting the back focus requirement for this reducer at 85mm; Also, there seem to be different opinions as to where the 105 mm should be measured from: some say from the threads at the end of the reducer, some say from the center of the optical element and some even claim from the threads at the back of the telescope. It also seems the reducer might operate differently with the different Celestron models (C8, C9.25, C11 etc..). May I kindly ask which tubes you’re using the reducer with and where you are measuring the BF from? I am genuinely curious, and I do apologize if my statement was a bit too categorical.

 

Erik



#10 Tapio

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Posted 19 April 2021 - 01:43 PM

I use it with C8 and measure it from the back shoulder of reducer.

I know that it should not be measured from the center of optical element.


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#11 aaube

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Posted 19 April 2021 - 04:32 PM

Please be aware  that when it comes to the F/6.3 Celestron reducer no matter how many times the (in)famous 105mm of back focus is mentioned, it is still wrong. A BF of 105 mm will give a much higher reduction and will likely introduce considerable aberrations. The Bf requirement may vary slightly depending on individual scopes. I currently image with a reduced C9.25 and the same 533 camera as you do. I currently have a bit over 85mm of total back focus (from the back thread of the reducer to the sensor) and I am close to nominal back focus ( I am actually getting more reduction given that my BF slightly over 85 mm). So two things to consider here:

 

1. The back focus for the Celestron f/6.3 IS NOT 105mm as is usually claimed, it is closer to  85mm.

 

2. The exact BF requirements may vary slightly from setup to setup. Plate solving is your friend. I know that I am close to the desired  reduction because I have played with the back focus and plate solved consistently until I got to the desired focal ratio. Also,  I get much better stars in my off axis guider at 85mm back focus than I do at 105mm where the off axis performance is really affected and the starts are really deformed. Bear in mind that the C8 might behave a bit differently to my C9.25, but the 533 has a small-ish sensor and the tolerances are higher, so you can't go wrong by trying 85mm of BF. I have and am really happy with the results...

 

Hope it helps.

 

Erik

 

Well, mine gives me a F/L of 1248-1250mm after countless platesolves @ 105mm back focus, no aberrations.  I must be doing something very wrong then.


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#12 ERHAD

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Posted 19 April 2021 - 04:45 PM

Well, mine gives me a F/L of 1248-1250mm after countless platesolves @ 105mm back focus, no aberrations.  I must be doing something very wrong then.

Well, I think it has been established that there seems to be a HUGE variation in BF requirements for this FR. Again, I didn’t want to sound all categorical. My comment comes from having seen the BF advertised as 85mm by several vendors and having been recommended 85mm myself by my own telescope stuff provider. On top of that, I have spoken to quite a few other people who also believe that the correct spacing is much shorter than 105mm. To the point that I am starting to wonder if there might be different FR models with different specs. Or maybe the 105mm is for the C8 model in particular? Definitely not working as advertised in my C9.25 or any of my club’s tubes...

 

Erik.


Edited by ERHAD, 19 April 2021 - 04:46 PM.


#13 roelb

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Posted 19 April 2021 - 04:58 PM

The 85 mm backfocus is for the Meade f/3.3.

The Celestron f/6.3 needs 105 mm backfocus.

Here you can find the exact required backfocus for SCT with moving mirror: http://www.wilmslowa...rmulae.htm#FR_b


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#14 ERHAD

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Posted 19 April 2021 - 05:32 PM

The 85 mm backfocus is for the Meade f/3.3.

The Celestron f/6.3 needs 105 mm backfocus.

Here you can find the exact required backfocus for SCT with moving mirror: http://www.wilmslowa...rmulae.htm#FR_b

No, the BF for the f/3.3 (which I also own) is 60mm, not 85. Incidentally, you can check it in the very same link you provided.



#15 roelb

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Posted 19 April 2021 - 06:21 PM

No, the BF for the f/3.3 (which I also own) is 60mm, not 85. Incidentally, you can check it in the very same link you provided.

Sorry, you are correct. I confused the BF with the FR focal length.


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#16 ERHAD

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Posted 19 April 2021 - 07:22 PM

Sorry, you are correct. I confused the BF with the FR focal length.

No big deal. The topic of focal reducers and back focus keeps coming up year after year and I just find it shocking that it can still be such an object of contention and confusion. I’ve just spent the best part of an hour reading everything I could find in this forum on the topic and there seems to be nothing but confusion and disagreement. The one thing I think I can state is that the 105mm figure indeed seems to not be true for the C9.25. An issue that is so clearly black and white with refractors... Go figure.

 

Erik.



#17 kel123

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Posted 19 April 2021 - 08:27 PM

This is not such a big deal. If you own an SCT, a camera or a mount, spacers and extra extension tubes are peanuts. If you can figure other issues out in astrophotography, getting the correct backfocus should be a piece of cake.

#18 gimpei

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Posted 19 April 2021 - 09:37 PM

Thanks everyone for the help. This clears a lot up. I guess I'll try the range between 105 and 85. 



#19 GGK

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Posted 21 April 2021 - 07:41 AM

I am visual only, but read a lot about back focus to best optimize my view with and without a focal reducer. 

 

Here's some other information I found a while back that might be helpful for what you need:

 

This CN thread gives some interesting details, measurements and charts related to using the f6.3 reducer:

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

 

Also, Celestron identified these values as the optimum back focus distances for their non-edge SCTs in their Knowledge Base in 2012:

  What are the optimum back focus distances for Celestron SCTs?

  November 4, 2012

  The back focus distances are as follows:

  SCT Back focus distance from primary mirror baffle tube lock ring (in)
  C5           5 in
  C6           5 in
  C8           5 in
  C925       5.475 in
  C11        5.475 in from 3 in-2 in Reducer Plate
                 5.975 in from 3 in Primary Mirror Baffle Tube Lock Ring
  C14        5.475 in from 3 in-2 in Reducer Plate
                5.975 in from 3 in Primary Mirror Baffle Tube Lock Ring

This is the link to that page:  https://www.celestro...-celestron-scts

 

Regarding the 105mm optimum back focus for the f6.3 reducer, I read it in the Agena Astro's guide to focal reducers at this link:  https://agenaastro.c...cers-guide.html

   The naming convention of SCT focal reducers is a little confusing. An f/6.3 reducer is designed to reduce the focal ratio of an f/10 SCT to f/6.3. So it provides a 0.63x design reduction factor when used with an f/10 SCT at the specified working distance. The working distance (backfocus) of the Celestron f/6.3 reducer is specified to be 105mm from the base of the male SCT thread on the camera side.

 

What is the optical path length of the focal reducer?  I measured mine at 22mm (might be a mm or 2 off).

 

5 inches optimum back focus for the 8 inch SCT is 127mm.  Subtracting the 22mm for the f6.3 focal reducer equals 105mm.  Maybe a coincidence -  I really don't know.

 

Good luck.  Please post again if your get any performance data.  It's always interesting and helpful to see the numbers from tests and confirmations.

 

Gary


Edited by GGK, 21 April 2021 - 07:45 AM.

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#20 Borodog

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Posted 21 April 2021 - 08:00 AM

GGK, I think you are exactly right. The 5” number is measure from the end of the SCT threads on the rear cell. 105 mm is measured from the base of the SCT threads on the camera side of the reducer. The difference is the 22mm of glass in the reducer. 
 

If someone is using a back focus substantially different from that on a C8 and getting good results, (a) good for them, but (b) that seems to be the exception rather than the rule; they may have something else going on in the scope that is changing the optimal back focus, or perhaps an out of spec reducer.


Edited by Borodog, 21 April 2021 - 01:37 PM.


#21 ERHAD

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Posted 21 April 2021 - 08:36 AM

GGK, I think you are exactly right. The 5” number is measure from the end of the SCT threads on the rear cell. 105 mm is measure from the base of the SCT threads on the camera side of the reducer. The difference is the 22mm of glass in the reducer. 
 

If someone is using a back focus substantially different from that on a C8 and getting good results, (a) good for them, but (b) that seems to be the exception rather than the rule; they may have something else going on in the scope that is changing the optimal back focus, or perhaps an out of spec reducer.

There must be plenty of people with issues in their scopes or out of spec reducers then, because I am absolutely not the only one who reaches the specified reduction with a much shorter back focus. Now, I am not saying that 105mm is not the official BF stated by manufacturer, but surely there is something else at play here because it is  (as attested by the number of threads and accounts on this issue) unfortunately not that straightforward.


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#22 WadeH237

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Posted 21 April 2021 - 08:47 AM

There must be plenty of people with issues in their scopes or out of spec reducers then, because I am absolutely not the only one who reaches the specified reduction with a much shorter back focus. Now, I am not saying that 105mm is not the official BF stated by manufacturer, but surely there is something else at play here because it is  (as attested by the number of threads and accounts on this issue) unfortunately not that straightforward.

I have to wonder if the discrepancy is the result of placement of the reducer/corrector, and not the spacing between the r/c and the sensor.

 

If the r/c is attached directly to the back of the scope, the resulting focal length will be different than if there is anything between it and the scope.  So even if there is exactly 105mm between the r/c and the sensor, the distance between the r/c and the scope can significantly change the resulting focal length.



#23 ERHAD

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Posted 21 April 2021 - 09:18 AM

I have to wonder if the discrepancy is the result of placement of the reducer/corrector, and not the spacing between the r/c and the sensor.

 

If the r/c is attached directly to the back of the scope, the resulting focal length will be different than if there is anything between it and the scope.  So even if there is exactly 105mm between the r/c and the sensor, the distance between the r/c and the scope can significantly change the resulting focal length.

I can only speak for myself, but at least in my setup there is nothing between the scope and the reducer, so to be clear, the reducer is threaded to the SC threads at the rear end of the scope.


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#24 Borodog

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Posted 21 April 2021 - 01:47 PM

ERHAD,

 

Trust me, I believe you are 100% correct about your scope and plenty of others. It’s just that, having read thread after thread after thread on f/6.3 SCT focal reducers (Celestrons and Meades and Antares, oh my) recently while researching in preparation to get one for my C8, I have come to only 2 conclusions:

 

1) The topic is a giant morass of confusing, conflicting information, and

2) The correct back focus for a C8 for the Celestron unit is usually, but not always, 105mm, measured from the base of the SCT threads on the camera side of the reducer.

 

For what it’s worth, I finally acquired a new Celestron unit, and it measures out very close to 0.63 at 105mm. I have not, however, made an extensive study of optimum coma correction yet.


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#25 Borodog

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Posted 21 April 2021 - 02:00 PM

In fact, ERHAD, it sounds like your scope/reducer combo is even more out of spec, as you have a C925 not a C8. I believe, according to Celestron, the correct back focus for the 925 is actually 117mm from the base of the camera side reducer SCT threads. That is a full 32mm longer than what you are using!


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