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Reducer Backspacing, Right?

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#1 SapperUp

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Posted 22 September 2021 - 11:17 AM

Hey guys, just want to see if you guys come to the consensus that this is in fact just reducer spacing? I have a few shims up to 1mm so I'll play around on the next clear night and find out.

 

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#2 OldManSky

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Posted 22 September 2021 - 01:28 PM

Looks that way, and -- if this is the 6" RC -- also collimation (since the comatic-looking stars look different in the 4 corners!).



#3 SapperUp

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Posted 22 September 2021 - 01:39 PM

Looks that way, and -- if this is the 6" RC -- also collimation (since the comatic-looking stars look different in the 4 corners!).

This is my SW80. Should've mentioned that, my bad!

 

I have a new sturdier focuser coming this week as I think there also may be some tilt/sag in the stock one.


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#4 imtl

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Posted 22 September 2021 - 01:43 PM

Do you have just a flattener? This doesn't look good for a refractor. How does it look without the reducer? Have you tried "scanning" the back space? Which reducer is this?


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#5 SapperUp

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Posted 22 September 2021 - 02:20 PM

Do you have just a flattener? This doesn't look good for a refractor. How does it look without the reducer? Have you tried "scanning" the back space? Which reducer is this?


I do have a flattener. I'll have to try it and without a reducer. What do you mean scan the backspace?

This is the Orion 0.8x for short refractor.

#6 imtl

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Posted 22 September 2021 - 02:22 PM

I do have a flattener. I'll have to try it and without a reducer. What do you mean scan the backspace?

This is the Orion 0.8x for short refractor.

What I mean is this reducer is a generic one and not dedicated for your scope. You need to change the backspace until the stars are optimized across the FOV. It might be that this scope and reducer combination do not work well together.



#7 SapperUp

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Posted 22 September 2021 - 02:27 PM

What I mean is this reducer is a generic one and not dedicated for your scope. You need to change the backspace until the stars are optimized across the FOV. It might be that this scope and reducer combination do not work well together.


Yeah I think I'll try to shim it and adjust the back spacing a bit.
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#8 TareqPhoto

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Posted 23 September 2021 - 06:42 AM

So using a dedicated or non dedicated reducer is only a job of back focusing to be precise?

#9 PiotrM

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Posted 23 September 2021 - 06:48 AM

So using a dedicated or non dedicated reducer is only a job of back focusing to be precise?

If you use a non-dedicated flattner/corrector then you don't know what usable imaging circle you will get and what "correct" spacing you need to get most of it - trial and error only. Non-dedicated corrector can under or over correct the field.



#10 TareqPhoto

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Posted 23 September 2021 - 07:53 AM

If you use a non-dedicated flattner/corrector then you don't know what usable imaging circle you will get and what "correct" spacing you need to get most of it - trial and error only. Non-dedicated corrector can under or over correct the field.

Forget about imaging circle and only focus on the correcting part, so why it will be either under correct or over correct if spacing is done right? I mean i didn't say the back focus will be correct from first time, but let's say you play with it until you reach that back focus, so is it still that it will never correct it because it is not the dedicated one? 

 

I read some topics where people struggles with dedicated reducers ands also others who used non dedicated successfully and great, so i keep thinking about is it because of the back focusing thing to be done properly even with trial and error? 

 

I saw a dedicated reducer for my scope from a site i don't want to buy from for a reason, i saw another reducers very very similar to that one if not the same, i even feel like one of them can do a better job than the dedicated one, because when i looked at the dedicated one it looks like just normal one but it can mount on the scope directly, from my understanding most of them needs 55mm from the edn of the reducer and not the length of reducer itself or what optics, hope i am wrong in that, just want to learn more about reducers if possible.



#11 PiotrM

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Posted 23 September 2021 - 08:03 AM

so why it will be either under correct or over correct if spacing is done right?

If you design a corrector for like an f/6 APO triplet to generate f/5 with full frame corrected imaging circle then you are designing it around a specific field curvature. If you put it int a say similar f/7 APO triplet that has a smaller curvature then it will overcorrect, if you put it in f/5 then it will under-correct - it will work in both cases but corrected field will be smaller - the smaller the bigger the mis-correction is.

 

Also same corrector/scope can be sold under different brands wink.gif


Edited by PiotrM, 23 September 2021 - 08:04 AM.


#12 TareqPhoto

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Posted 23 September 2021 - 09:48 AM

If you design a corrector for like an f/6 APO triplet to generate f/5 with full frame corrected imaging circle then you are designing it around a specific field curvature. If you put it int a say similar f/7 APO triplet that has a smaller curvature then it will overcorrect, if you put it in f/5 then it will under-correct - it will work in both cases but corrected field will be smaller - the smaller the bigger the mis-correction is.

 

Also same corrector/scope can be sold under different brands wink.gif

That was my point, if we have 3 or 4 reducers and mentioned they are all for F5-F8 for example and they are up to APS-C, so can't those be used with any scope in that criteria for say small sensors and up to APS-C, and lets say they have same length maybe and even same thread, but i agree that sometimes they are sold under different brands almost same as some telescopes.



#13 PiotrM

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Posted 23 September 2021 - 10:27 AM

That was my point, if we have 3 or 4 reducers and mentioned they are all for F5-F8 for example and they are up to APS-C, so can't those be used with any scope in that criteria for say small sensors and up to APS-C, and lets say they have same length maybe and even same thread, but i agree that sometimes they are sold under different brands almost same as some telescopes.

There are some semi-universal flatteners for refractors. They should work but there are no guarantees there won't be any caveats with them. There can be some difference between doublet/triplet refractor or optics made from some other glass type giving different curvature. That's why it's best to have something dedicated for given OTA and if that isn't available then something that may be compatible - and best if someone already tested it.



#14 TareqPhoto

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Posted 23 September 2021 - 12:18 PM

There are some semi-universal flatteners for refractors. They should work but there are no guarantees there won't be any caveats with them. There can be some difference between doublet/triplet refractor or optics made from some other glass type giving different curvature. That's why it's best to have something dedicated for given OTA and if that isn't available then something that may be compatible - and best if someone already tested it.

For example Sharpstar 61EDPH II isn't good with their own or dedicated reducer, it turns out to be as an old reducer but it is for 61 anyway, i don't know why it was ok for doublet and not for triplet unless that doublet is already not good so doesn't matter.

 

When i ordered my 90mm triplet there isn't any flattener or reducer dedicated for it, now they have, but looking at it it sounds very similar to another reducers or flattener i saw somewhere, even almost same specifications or description, not much there as details anyway so i assume both are identical, and i want to go with that as i found it a bit cheaper and good shipping, but then it is again about the back focus, if both reducers have almost same or similar optics quality i don't know why the dedicated one is better always because they calculated for that, you pointed out about it, my 90mm triplet is exactly same specifications of WO 91mm, why can't use that WO reducer then?!!!

 

The topic is about back focusing right, how to know if it is right, only by round stars all over the corners?



#15 TareqPhoto

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Posted 23 September 2021 - 12:19 PM

One more thing, and sorry for that, i have a quintuplet scope, which has build in flattener, and it sounds i have enough space to use for back focus and not a big deal or not an issue, when i use their dedicated reducer they stated to use 55mm, why using that back focus for the reducer if the native scope itself has a flattener built in?



#16 TareqPhoto

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Posted 23 September 2021 - 12:20 PM

This is my SW80. Should've mentioned that, my bad!

 

I have a new sturdier focuser coming this week as I think there also may be some tilt/sag in the stock one.

Why didn't you buy the dedicated SW 0.85x reducer?



#17 PiotrM

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Posted 23 September 2021 - 12:39 PM

One more thing, and sorry for that, i have a quintuplet scope, which has build in flattener, and it sounds i have enough space to use for back focus and not a big deal or not an issue, when i use their dedicated reducer they stated to use 55mm, why using that back focus for the reducer if the native scope itself has a flattener built in?

As it's likely designed to have best image at that distance (based on light cone coming in and out of that optics). You can test what happens when you put the camera closer or further away from it. The quadruplet may be corrected but the reducer may add it own field curvature so likely that's why it has a specific optimal distance to get as little of it as possible.


Edited by PiotrM, 23 September 2021 - 12:42 PM.


#18 TareqPhoto

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Posted 23 September 2021 - 01:00 PM

As it's likely designed to have best image at that distance (based on light cone coming in and out of that optics). You can test what happens when you put the camera closer or further away from it. The quadruplet may be corrected but the reducer may add it own field curvature so likely that's why it has a specific optimal distance to get as little of it as possible.

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