13 replies to this topic

[martinastro68]

I apologize in advance for the lengthy explanation of my question below. 

 

Two years ago I set the back focus on my C11 Edge HD, DSO image train at 146mm. With the Celestron 0.7 focal reducer in place, I measured from the rear of the threads on the focal reducer to the plane of the camera sensor. To get to 146.05mm I had to install two adaptor rings ( 11.5mm + 16.5mm) between my Celestron OAG and the ZWO EFW. See the attached photo of my image train. The PLL Esatto 2 LP focuser is in the middle of its 10mm travel. 

 

Then to get the image in focus in the camera, I had to move the C11 focus knob CCW so that the mirror is almost all the way forward towards the corrector plate. I noted at the time that the C11 mirror travel is 33 full turns of the focus knob from one end to the other. I was 1.5 turns from the mirror all the way forward. This worked fine. I was able to image fine, and aberration inspector in NINA told me my back focus error was very small. 

 

Now, I'd like to put the C11 mirror in the center of its travel and re-adjust the needed spacers in my image train. When I remove one of the mirror locking knobs on the back of the C11, I can see that 33 turns equates to about 20mm of mirror travel by watching the pin move in and out. This is an estimate. I do not know what the actual mirror travel range really is for a C11. Anyone know ?

 

From what I have read, moving the mirror towards the rear (CW on focus knob) moves the focal plane towards the scope. If this is the case I should be able to remove one of the spacers in my image train, right ? But then, the actual measured back focus would be 146.05mm minus the length of the spacer I removed.

 

This is confusing me. All the literature tells you to put the back focus of a C11 at 146.05mm. But it never mentions anything about the mirror position. One would think that the mirror position should be near the middle of its travel for this measurement. 

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[ABQJeff]

The Edge reducer for the C11 Edge is designed so it provides the same Edge correction with 146.05mm backfocus with the mirror in whatever position is necessary to achieve focus at that 146.05mm spot.  Celestron is aware the 0.7x reducer requires moving of the mirror forward.  If you remove a spacer, you will no longer have optimized coma and flat field correction (but you may not be able to notice).

[martinastro68]

Ok. That is info I was not aware of, that Celestron is aware that using the 0.7 reducer requires moving the mirror forward. 

 

What about the situation when not using the 0.7 reducer, running the C11 at native f 10. Would the mirror still be forward, or more towards the center of its travel ? 

 

I do understand that either way, f 10 or f 7, the back focus is the same. 

[Endymion]

Edited by Endymion, 13 February 2025 - 02:31 AM.

[MarMax]

It's interesting how there are some similarities between the Edge and a regular C11. My 2014 C11 has 37 focuser turns lock-to-lock with the midpoint at 18-19 turns from full forward or rearward. The Celestron C11 recommended backfocus sets the optimum position of the primary mirror at or very near the mid-point. I've confirmed this with several different image trains I use for visual and EAA. The scope is closest to the design 2800mm FL (f/10) at the mid-point.

 

The more forward the primary mirror the longer the focal length. So moving the primary mirror well forward of the mid-point can increase the focal length of the scope significantly. Using the Starizona LF corrector (which attaches like the Edge corrector and has the same backfocus) I need to move the primary mirror 10-11 turns forward of the midpoint to achieve focus. The LF corrector is designed to be 0.7x yet my scope comes in at f/7.4 which has the scope operating at f/10.6 (2960mm).

 

The Hyperstar v3 operates in focus with the primary within 1-2 turns of the mid-point. I can set up the 0.63x Corrector IV to operate about 4 turns forward of the mid-point and plate solves confirm f/6.3.

 

So why are the LF corrector and the Edge 0.7x corrector designed to operate with the primary so far forward of the mid-point?

[carolinaskies]

It's interesting how there are some similarities between the Edge and a regular C11. My 2014 C11 has 37 focuser turns lock-to-lock with the midpoint at 18-19 turns from full forward or rearward. The Celestron C11 recommended backfocus sets the optimum position of the primary mirror at or very near the mid-point. I've confirmed this with several different image trains I use for visual and EAA. The scope is closest to the design 2800mm FL (f/10) at the mid-point.

 

The more forward the primary mirror the longer the focal length. So moving the primary mirror well forward of the mid-point can increase the focal length of the scope significantly. Using the Starizona LF corrector (which attaches like the Edge corrector and has the same backfocus) I need to move the primary mirror 10-11 turns forward of the midpoint to achieve focus. The LF corrector is designed to be 0.7x yet my scope comes in at f/7.4 which has the scope operating at f/10.6 (2960mm).

 

The Hyperstar v3 operates in focus with the primary within 1-2 turns of the mid-point. I can set up the 0.63x Corrector IV to operate about 4 turns forward of the mid-point and plate solves confirm f/6.3.

 

So why are the LF corrector and the Edge 0.7x corrector designed to operate with the primary so far forward of the mid-point?

The reason is pretty basic optics.  Reducer design is for a specific optical curvature. 

Optical design wise, the longer the FL the less curvature of the focal image being reduced, this limits the possible aberrations produced by the reducers own optical curvature.  Further, the actual field at the forward position has a larger fully illuminated image circle(because the primary is closer to the secondary), meaning more light to optimize and represent the final illuminated field which exits the telescope. 



  

[MarMax]

The reason is pretty basic optics.  Reducer design is for a specific optical curvature. 

Optical design wise, the longer the FL the less curvature of the focal image being reduced, this limits the possible aberrations produced by the reducers own optical curvature.  Further, the actual field at the forward position has a larger fully illuminated image circle(because the primary is closer to the secondary), meaning more light to optimize and represent the final illuminated field which exits the telescope. 

That makes sense yet it still does not provide for the "optimum" system. If the Edge and LF correctors achieved focus at the mid-point of primary mirror travel the overall optical performance of the system would be better. The physical limitations of the scope design preclude moving the correctors closer and costs likely preclude making the correctors "different" such that they achieve the design reduction with the primary mirror at the mid-point.

[MarMax]

trim

 

From what I have read, moving the mirror towards the rear (CW on focus knob) moves the focal plane towards the scope. If this is the case I should be able to remove one of the spacers in my image train, right ? But then, the actual measured back focus would be 146.05mm minus the length of the spacer I removed.

 

This is confusing me. All the literature tells you to put the back focus of a C11 at 146.05mm. But it never mentions anything about the mirror position. One would think that the mirror position should be near the middle of its travel for this measurement.

I forgot to respond to this part of your question. Moving the primary rearward decreases the focal length of the scope. It's counter intuitive but the closer the primary is to the secondary the longer the focal length.

 

You will not be able to reach focus regardless of the spacers with the primary mirror at the mid-point of travel. What needs to happen is the Edge 0.7x corrector needs to be moved closer to to the secondary mirror which is physically impossible. The only way you will achieve focus is to use the original image train with 146mm backfocus.

 

Have you plate solved images and what is your net correction? As mentioned in the initial response, my C11 with the Starizona LF corrector plate solves at f/7.4 and this is because the primary mirror is far forward of the mid-point.

 

I fully agree with your last statement so it's a mystery to me as well.

[speedster]

Performance doesn't care where the focus arm is on the threaded rod.  The system is designed for best performance at f/10 regardless of where that puts the arm on the rod.  If you like, you can find f/10 without the reducer, leave the focus alone,  then add the reducer and adjust spacers until you are very close to in focus.  Check the star shapes at the edges and they will indicate which way distance needs to be adjusted from there, if any. 

[thierry martin]

 

I try to find the true power of the reducer  for  the C11 Edge. Celestron is saying is 0.7 x  for 2788 mm of the C11 Edge.  But the fact in order to use it we have to turn  8.375 turns foward  acording Endymion   

 

So far we know the thread of the focus shaft is M10 x 0.75mm. If we want to know the focus  the primary moves of  8.375 turns = 6.28125mm  With Oslo Edu I can managed to simulate the C11 . I can't say is what Celestron  has done with the C11 Edge, but without having a C11 I try to use the design Celestron is given for the C11 with the pdf I have dowloaded. 

I give my findings free to use  with OsloEdu. My C11 is quite coma free,   nearly limited diffraction up to the Edge,  and  flat field for the focus,  with some chromatic aberrations. This is only a simulation. This is far better than the classical C11.

 

Then from this simulation I have moved the primary mirror by 6.28125mm. The internal corrector  has not moved  (Same distance secondary internal corrector).   I give the result too,

This gives me the final focal length and the ugly spots diagram. Of course this is very bad, because when you fix the aberrations in a conical ligth, you are very limited to play with the focal length. The respaced  C11 should be much better,  but also  it is more cumbersome to carry. Anyway. Celestron is selling its C11 for F10. I you want to use at F7 you have to buy its reducer.

From Oslo we get the   Focal length is 3583.9mm  

 

The final focus is located at 457.23mm from the primary which as been moved  . To compare with  focus position at the F10 position (265.1706 mm) ,this is 192.059 mm futher away of the focus position at F10 which is positioning at  146.05mm from the back of the SCT 

 

The focus of the SCT at F/12.8  is at 192.059+146.05 = 338.1094 mm from the back of the SCT  I don't  know the lengtn of the reducer . lets say it's 70mm 

 

. I try to compare the two lengths from the reference for measuring the backfocus of the reducerder

 

   this reducer is installed at the back of the SCT. when it is at F12.8   the remaining length  after the reducer,:  338.1094  - 70 =  268.1094mm 

  The reducer has a backfocus of  146.05mm  after the last lens surface. The two lengths can be compare.

. What is the % of the movement?  146.05/268.1094 = 0.544

   The reducer has reduced the focus of the 3583.9mm by a factor 0.544  ( not the 110' ) .  the final focal legth is  3583.9 x 0.544 = 1952.2mm

 

Celestron  has chosen to name its reducer 0.7x C11 reducer  which gives  2788 x 0.7 = 1,951.6mm.   Maybe I'm lucky .

 

 The Celestron Edge owners about reducer  must move fowards the primary in order to increase the focal length and eventually to reduced it ,

Celestron will never gives us how many turns to get the back focus spot on at 146.05mm, because the confusion. Here, we realize the fact that we can't  take into account the 0.7 factor given by Celestron to calculate how may turns of  the focuser to move  the primary.

 

This is why some Celestrons  owners came to the surprise not having enougth focuser turns to get the reducer working  when they install the motor....

 

The need  to  push foward the mirror gives more backfocus for the SCT , in order to install  the reducer  which  takes some space at the back of the SCT, and of course the reducer must have a good backfocus . 

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Edited by thierry martin, 16 February 2025 - 06:42 PM.

[thierry martin]

The reason is pretty basic optics.  Reducer design is for a specific optical curvature. 

Optical design wise, the longer the FL the less curvature of the focal image being reduced, this limits the possible aberrations produced by the reducers own optical curvature.  Further, the actual field at the forward position has a larger fully illuminated image circle(because the primary is closer to the secondary), meaning more light to optimize and represent the final illuminated field which exits the telescope. 



  

The meaning is not correct.Lost of confusion, I'm afraid. 

 

Reducing the distance between the two mirrors increases the F/D. When we have to calculate the ID of the baffle we use this method

Calculation of the Diameter D of the central deflector

 

D is a function of primary diameter, depends on: obstruction  O ( 0.34 for exemple ) from secondary mirror , from m "the Barlow effect of  secondary", from T the backfocus of the SCT  as a function of F1

. D = (1 + T) / (m + 1/O).    T will incrase very fast  because the seconday mirror multiply the motion of the primary by a factor (m² +1) .  Because the baffle  is calculated for F/D 10,  with the idea not having  direct ligth reaching the baffle, we realise here,  if F/D increase ,  then the baffle at F/D will not be big enougth.  Less ligth which is it sad,  when you push foward the primary,  in order to use a reducer for wider field of view!  

 

But it is not finished. The length of the baffle must be taking account too

 

Calculation of the length L of the baffle depending on F1

L = (1+T) /(1+1 /(m x O)) -T 

 

 if T is  increasing with F/D,   the length L is getting smaller

 

when the SCT has a bigger  F/D,  it  requires shorter baffle but with a bigger ID  !

 

To make Closer the reducer of the exit of the SCT, brings bigger field of full illumination. Do not install extra rings before the reducer. This will increase the baffle length. 

 

Why?

in fact the full length of the baffle has to be taking  for the calculation of the field of full  ligth illumination. The rule is F/D= 10 is 10% ot the full length of the baffle  to be removed of the ID  entrance of the baffle. if F/D increases then the loss too

 

For exemple The baffle is 300m long and  has 50 mm ID.   At F/D 10  we have the full ligth illumination = 50 - (300 x 0.1) = 50-30 = 20mm.

if F/D= 12 we get 50 -(300x 0.12) = 50-36 = 14mm

 

How to get better result with a focal reducer?

 

In fact it is what Celesstron has done with the Edge at F10 by placing the corrector inside the baffle  bringing more ligth of full ligth iillumination. 

The closer the stop to the entrance of the baffle  (usually the first lens ), bigger is the full ligth illumination field at the exit.  Celestron could have designed a removal corrector inside the baffle instead of a permanent corrector. And with less lenses , it could have manufactured a reducer to swap with the original corrector. liberating the back of the SCT using the existing thread at the back , maybe having the same 146.05mm original  position providing bigger full ligth illumination than the one given by the 0.7 reducer.....

Edited by thierry martin, 16 February 2025 - 04:05 PM.

[martinastro68]

Thanks for all the detailed and technical responses. I know the back focus measurement is critical to minimize coma, and also to reduce elongated stars (aberration) near the corners of the FOV. What I did not understand is why I had to move the primary mirror almost all the way forward towards the secondary to achieve focus. I was concerned I would run out of mirror travel. But it did work and I was able to take images fine. Aberration inspector in NINA told me my back focus was very close.

 

I currently have my Planetary image train on the C11. My DSO image train in the top picture is on my dining room table. I'm reconfiguring it a bit. I'm replacing the ASI 1600mm with a new ASI 2600mm Pro, a new 36mm filter wheel loaded with Chroma LRGB and 3nm SHO filters. I moved the OAG as close to the filter wheel as possible. I will put the two spacers back in, but now they will be between the Esatto focuser and the OAG. The spacers were 48mm diameter ones, so I have ordered some 54mm diameter spacers to increase my clear aperture down the inside. I will keep the back focus as close to 146.05mm as possible. I expect the mirror will end up being most of the way forward as before.

 

MarMax in one of the responses asked if I have plate solved, and what is my Net correction. I do use plate solving in NINA. But I'm not familiar where to find the Net Correction value. 

[MarMax]

thierry martin> I've always found the best performance with the C11 and the Starizona Corrector IV to be when the Corrector IV is placed well inside the baffle tube, probably around 30mm forward of the 3.25" flange. This is also where the actual correction is 0.63x and the primary mirror is only 3-4 turns forward of the mid-point. 

 

The recommended backfocus distances also place the primary mirror at or very near the mid-point.

 

https://www.celestro...-celestron-scts

 

I've always found the C11 to be at its best at or near the mid-point of focus. The Edge scopes may be different but I doubt it.

[thierry martin]

thierry martin> I've always found the best performance with the C11 and the Starizona Corrector IV to be when the Corrector IV is placed well inside the baffle tube, probably around 30mm forward of the 3.25" flange. This is also where the actual correction is 0.63x and the primary mirror is only 3-4 turns forward of the mid-point. 

 

The recommended backfocus distances also place the primary mirror at or very near the mid-point.

 

https://www.celestro...-celestron-scts

 

I've always found the C11 to be at its best at or near the mid-point of focus. The Edge scopes may be different but I doubt it.

Your findings is directly linked to Celestron decision  to built the compact SCTs. A good visual telescope at F10.to be used always closed to this primary position  when possible; otherwise, big optical aberrations will be visible.

The best full ligth illumination is given with short baffle or by  insttaling a corrector inside the baffle. Alan Gee Mark II was the first reducer made for the C8 installed 38mm inside of the baffle. The best reducer for full ligth illumination.

 

To get a better  the classical C11 will be to do the focus by the secondary mirror ( a modified Optec  FastOptic kit ) to drop the primary at its bottom position , to shorten the baffle, or even better to modifiy the baffle to get a bigger entrance ID to improve full ligth illuminination when the reducer is used. To install the reducer inside the baffle to minimize  focal length  required for using it , because the short backfocus of C11 can't accomodate the reducer with good backfocus length.

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Edited by thierry martin, 18 February 2025 - 11:24 AM.