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.7 focal reducers in series?

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#1 Jeffmar

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Posted 02 June 2020 - 03:50 PM

I had kind of a wild thought. I was wondering if anyone has tried putting two .7 EdgeHD focal reducers in series to get .5 reducer. I have three of the pricey things for my EdgeHD scopes. Just wondering if any good could come from trying it.


Edited by Jeffmar, 02 June 2020 - 06:53 PM.


#2 photoracer18

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Posted 02 June 2020 - 04:51 PM

Nothing good but it won't hurt to play with them anyway. Problem is each reducer expects the light cone to be just so when it gets to it. However in the case of the second one it won't be what the second one's optics is expecting so no telling. I have heard of someone trying this before. As long as the reducer is not a field flattener also you might get something out of it in the center of the field. I have fooled around with stacked Barlows and also camera teleconverters and in one case a reducer combined with a camera teleconverter behind an SCT (to make it into an autofocusing catadioptric camera lens).


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#3 Jeffmar

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Posted 02 June 2020 - 06:55 PM

Don’t focal reducers shorten the light cone? 



#4 Der_Pit

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Posted 03 June 2020 - 08:11 AM

Two identical ones for sure not.  They are usually made for a specific F-ratio of the incoming beam, and deliver a faster beam on output.  Therefore, the next one has a 'wrong' input.

Also, they are usually made to also 'fix' things like field curvature.  If the first one corrects that, the second one would try to fix it, but instead start introducing it in the opposite direction.

 

Finally, I'd have not the foggiest idea about proper back spacing.  For one reducer this is a given value.  You can place your camera in proper distance to the last of the two reducers.  But what distance to put between the two?

 

In short - if you have them, play with it.  But don't expect (too) much.


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#5 Jeffmar

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Posted 03 June 2020 - 05:49 PM

Two identical ones for sure not.  They are usually made for a specific F-ratio of the incoming beam, and deliver a faster beam on output.  Therefore, the next one has a 'wrong' input.

Also, they are usually made to also 'fix' things like field curvature.  If the first one corrects that, the second one would try to fix it, but instead start introducing it in the opposite direction.

 

Finally, I'd have not the foggiest idea about proper back spacing.  For one reducer this is a given value.  You can place your camera in proper distance to the last of the two reducers.  But what distance to put between the two?

 

In short - if you have them, play with it.  But don't expect (too) much.

The focal reducers made for Celestron EdgeHD scopes probably do not flatten the field because these telescopes are supposed to already have flat fields. You are likely to be correct about expectations for my experiment. If it doesn’t work, it won’t cost me anything, and It certainly won’t be a big disappointment. All of my focal reducer are made for different size telescopes, so that may make it even worse, but I have no clue either way. I was hoping someone belonging to this forum had tried it and could shed some light.

 

It is hard to find focal reducers that work with large sensors. So many astronomy cameras have sensors closer to the size that cell phones use. My full frame cameras barely work with Celestron’s .7 reducers that boast full frame compatiblity.


Edited by Jeffmar, 03 June 2020 - 05:54 PM.


#6 Stelios

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Posted 03 June 2020 - 06:20 PM

I would file this under "attempt after you have watched all available shows on Netflix."


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#7 Noah4x4

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Posted 04 June 2020 - 03:29 AM

People in the EAA Forum are doing this with combinations of x6.3 and x 0.5 Focal Reducers. They report vignetting, which can be cropped, but most seem pleased. However, it might be different with an Edge HD x0.7. I mention as you might find information there. I have two x6.3 FRs, but have not yet tried at night as conditions have been unfavourable, but I have attempted on a daylight Moon and got good focus. If successful, it might assist with rear end clearance on Alt-az mounts, which is a problem with most cooled cameras. 



#8 Der_Pit

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Posted 04 June 2020 - 11:31 AM

The focal reducers made for Celestron EdgeHD scopes probably do not flatten the field because these telescopes are supposed to already have flat fields. You are likely to be correct about expectations for my experiment. If it doesn’t work, it won’t cost me anything, and It certainly won’t be a big disappointment. All of my focal reducer are made for different size telescopes, so that may make it even worse, but I have no clue either way. I was hoping someone belonging to this forum had tried it and could shed some light.

 

It is hard to find focal reducers that work with large sensors. So many astronomy cameras have sensors closer to the size that cell phones use. My full frame cameras barely work with Celestron’s .7 reducers that boast full frame compatiblity.

The curvature was only an example, could also be coma or something.  I just had the impression that reducers that 'absolutely only reduce' are rather rare.  Might be wrong there.

 

As for vignetting - what do you expect?  It's IMO less a problem of the reducer, but of the image circle of the original telescope.  The reducer shrinks the image circle, too.  There's no way around that.  So if the OTA has 50mm image circle (which is already quite a lot), a .63 reducer will shrink that to 31.5mm which would just about fit an APS-C sensor...



#9 Jeffmar

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Posted 07 June 2020 - 09:56 PM

The curvature was only an example, could also be coma or something.  I just had the impression that reducers that 'absolutely only reduce' are rather rare.  Might be wrong there.

 

As for vignetting - what do you expect?  It's IMO less a problem of the reducer, but of the image circle of the original telescope.  The reducer shrinks the image circle, too.  There's no way around that.  So if the OTA has 50mm image circle (which is already quite a lot), a .63 reducer will shrink that to 31.5mm which would just about fit an APS-C sensor...

I have always expected vignetting with focal reducers. It just depends on how much I can compensate for vignetting given the target.

 

“a .63 reducer will shrink that (50mm image circle) to 31.5mm which would just about fit an APS-C sensor”.

That makes perfect sense. It looks like putting two .7 reducers together will shrink a 50mm image circle to roughly 25mm. That is 4/3 sensor size. It will be interesting to see what other issues will arise. 



#10 Chuckwagon

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Posted 08 June 2020 - 12:36 AM

I have always expected vignetting with focal reducers. It just depends on how much I can compensate for vignetting given the target.

 

“a .63 reducer will shrink that (50mm image circle) to 31.5mm which would just about fit an APS-C sensor”.

That makes perfect sense. It looks like putting two .7 reducers together will shrink a 50mm image circle to roughly 25mm. That is 4/3 sensor size. It will be interesting to see what other issues will arise. 

As an FYI, no matter how large the illuminated spot of the telescope is rated to be, the reducer will produce an illuminated spot that is no larger than the smallest clear aperture in front of it (including itself) times the reduction factor.  And that won't be an un-vignetted spot either.  I don't know what the clear aperture of the .7 is, but for the normal .63 reducers, that have 41mm of clear aperture, that means 41 x .63 = 25.83mm.  The second reducer would then shrink that further.  If you have anything that restricts the aperture, like a 1.25" filter, in front of the reducer, then that becomes your smallest clear aperture and reduces the spot even more.  So you can calculate the largest spot you can have by figuring out your clear aperture in front of the reducers (or the reducer itself) and then multiply by the reduction factor.


Edited by Chuckwagon, 08 June 2020 - 12:38 AM.



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