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Achieving focus with a reducer vs a flattener

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#1 aroughroad

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Posted 29 November 2021 - 04:20 PM

I have read the using a reducer requires you to rack your focuser in (toward the object you're viewing) relative to where you had it in focus when using only a flattener.  I was unable to achieve focus by doing this and have just set up during the day and achieved focus by doing the opposite (racking the focuser farther out!).  Is this sometimes the right thing to do or do I have something weird going on?

 

Setup with a flattener: WO FLT 132m with WO flattener 68iii.  Required back focus is 62.7mm and I achieve focus when focuser is racked out ~85mm.

Setup with a reducer: WO FLT 132m with WO 0.8x reducer.  Required back focus is 69.24mm and I achieve focus when focuser is racked out ~98mm.

 

Could someone help me understand the right way to think about this and whether I've actually solved my focus problem? :)



#2 aroughroad

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Posted 29 November 2021 - 04:34 PM

I have read the using a reducer requires you to rack your focuser in (toward the object you're viewing) relative to where you had it in focus when using only a flattener.  I was unable to achieve focus by doing this and have just set up during the day and achieved focus by doing the opposite (racking the focuser farther out!).  Is this sometimes the right thing to do or do I have something weird going on?

 

Setup with a flattener: WO FLT 132m with WO flattener 68iii.  Required back focus is 62.7mm and I achieve focus when focuser is racked out ~85mm.

Setup with a reducer: WO FLT 132m with WO 0.8x reducer.  Required back focus is 69.24mm and I achieve focus when focuser is racked out ~98mm.

 

Could someone help me understand the right way to think about this and whether I've actually solved my focus problem? smile.gif

I should add that both the flattener and reducer are adjustable so I am confident I have my back focus dialed in correctly with both.



#3 Midnight Dan

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Posted 29 November 2021 - 04:55 PM

To achieve focus during the day, even on a target a quarter mile away, you'll have to rack the focuser farther out compared to focusing on the stars.  It's unclear if you're comparing both during the day, or if you're comparing the flattener at night to the reducer during the day.  If it's the latter, that may explain the difference.

 

-Dan



#4 aroughroad

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Posted 29 November 2021 - 05:11 PM

To achieve focus during the day, even on a target a quarter mile away, you'll have to rack the focuser farther out compared to focusing on the stars.  It's unclear if you're comparing both during the day, or if you're comparing the flattener at night to the reducer during the day.  If it's the latter, that may explain the difference.

 

-Dan

I see.  I thought my apochromatic refractor is in focus at one and only one point along the focuser drawtube.  You are saying I may be in focus during the day at a distant object at one point, and in focus at night at the stars at a different point?  I am indeed comparing the flattener at night to the reducer during the day.

 

What does this mean for my efforts to achieve focus with the reducer later tonight when trying to image?  I was advised to achieve focus during the day so that I don't have to try to do it at night when it's darker and more difficult?



#5 chongo228

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Posted 29 November 2021 - 05:27 PM

Focusing on an earth object will be at a different point than focusing on stars. The person that told you that probably meant it to get you close to focus which makes finding stars much easier. Very out of focus stars don't even show up in your eyepiece or camera so you don't know which way to move the focuser. 

 

To really nail focus look into Bahtinov Mask. They are a easy way to manually focus on the stars. 



#6 aroughroad

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Posted 29 November 2021 - 05:35 PM

Focusing on an earth object will be at a different point than focusing on stars. The person that told you that probably meant it to get you close to focus which makes finding stars much easier. Very out of focus stars don't even show up in your eyepiece or camera so you don't know which way to move the focuser. 

 

To really nail focus look into Bahtinov Mask. They are a easy way to manually focus on the stars. 

Ok thanks for clarifying, but does it always, by definition, have to be the case that my focus point at night with a 0.8x reducer will require racking in the focuser farther than I do when I'm in focus at night with just the flattener?  In other words, if I'm in focus at night at step 85,000 on my focuser when I am using the flattener, can I declare with confidence that achieving focus at night with the 0.8x reducer will require me to place the focuser at some step position that is less than 85,000?



#7 aroughroad

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Posted 29 November 2021 - 05:44 PM

Ok thanks for clarifying, but does it always, by definition, have to be the case that my focus point at night with a 0.8x reducer will require racking in the focuser farther than I do when I'm in focus at night with just the flattener?  In other words, if I'm in focus at night at step 85,000 on my focuser when I am using the flattener, can I declare with confidence that achieving focus at night with the 0.8x reducer will require me to place the focuser at some step position that is less than 85,000?

Also, if I was able to achieve focus on a nearby object during the day, can I take that as confirmation that my back focus is in fact dialed in correctly?



#8 Midnight Dan

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Posted 29 November 2021 - 06:06 PM

Ok thanks for clarifying, but does it always, by definition, have to be the case that my focus point at night with a 0.8x reducer will require racking in the focuser farther than I do when I'm in focus at night with just the flattener?  In other words, if I'm in focus at night at step 85,000 on my focuser when I am using the flattener, can I declare with confidence that achieving focus at night with the 0.8x reducer will require me to place the focuser at some step position that is less than 85,000?

Not exactly.  The focuser position is a different thing than  your actual focus position.  A little explanation is in order.

 

 First, a refractor has a fixed focal plane position behind the objective lens when viewing objects at infinity.  As you bring objects closer, that focal plane position will move farther back, away from the scope.  Hence the day/night difference.

 

When imaging, without any reducer or flattener, the sensor must be at that same focal plane position to be in focus.  If you add a reducer in the image chain, then the focal plane position moves closer to the scope.  This is always true.

 

But your focuser position is affected by other things that may be in the image path.  For example, lets say that without a reducer, your focal plane is 100mm behind the fully-racked-in focuser.  If you have a 50mm long extension tube between the focuser drawtube and the sensor, then your focuser would have to racked out by 50mm for the sensor to be at the focal plane.  If you have a 40mm long extension tube, then the focuser would have to be racked out by 60mm.

 

In this example, the focal plane position has not changed, but the focuser position has.   Point is, the focuser position is NOT an indication of where the focal plane is, or whether it has changed.

 

So, back to your situation.  You have two different devices in the image chain, a flattener, and a reducer.  I'm not familiar with those two, but I would assume they are not the exact same length.  So they will change the focuser position in the same way as my previous example, regardless of whether they change the focal point or not.

 

Bottom line, measure the distance from the rear of the scope (not focuser) to the sensor when you're in focus.  That distance should get shorter when you use a reducer.

 

-Dan



#9 aroughroad

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Posted 29 November 2021 - 06:56 PM

Not exactly.  The focuser position is a different thing than  your actual focus position.  A little explanation is in order.

 

 First, a refractor has a fixed focal plane position behind the objective lens when viewing objects at infinity.  As you bring objects closer, that focal plane position will move farther back, away from the scope.  Hence the day/night difference.

 

When imaging, without any reducer or flattener, the sensor must be at that same focal plane position to be in focus.  If you add a reducer in the image chain, then the focal plane position moves closer to the scope.  This is always true.

 

But your focuser position is affected by other things that may be in the image path.  For example, lets say that without a reducer, your focal plane is 100mm behind the fully-racked-in focuser.  If you have a 50mm long extension tube between the focuser drawtube and the sensor, then your focuser would have to racked out by 50mm for the sensor to be at the focal plane.  If you have a 40mm long extension tube, then the focuser would have to be racked out by 60mm.

 

In this example, the focal plane position has not changed, but the focuser position has.   Point is, the focuser position is NOT an indication of where the focal plane is, or whether it has changed.

 

So, back to your situation.  You have two different devices in the image chain, a flattener, and a reducer.  I'm not familiar with those two, but I would assume they are not the exact same length.  So they will change the focuser position in the same way as my previous example, regardless of whether they change the focal point or not.

 

Bottom line, measure the distance from the rear of the scope (not focuser) to the sensor when you're in focus.  That distance should get shorter when you use a reducer.

 

-Dan

 

Ok, I think I follow.  I just held up the flattener and the reducer next to each other when they are both adjusted for their respective back focus requirements, and they were identical -- meaning the distance between their scope side edge and the point at which they connect to the next item in the train (the OAG) is the same.  Given this, and the requirement that there should be less distance between the rear of the scope and the sensor when using a reducer, I can conclude that I MUST have to rack the focuser IN somewhat at night, and that what I did during the day was of no real use.  Correct?



#10 Midnight Dan

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Posted 29 November 2021 - 07:02 PM

Ok, I think I follow.  I just held up the flattener and the reducer next to each other when they are both adjusted for their respective back focus requirements, and they were identical -- meaning the distance between their scope side edge and the point at which they connect to the next item in the train (the OAG) is the same.  Given this, and the requirement that there should be less distance between the rear of the scope and the sensor when using a reducer, I can conclude that I MUST have to rack the focuser IN somewhat at night, and that what I did during the day was of no real use.  Correct?

Sounds right to me. 

 

-Dan



#11 aroughroad

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Posted 30 November 2021 - 03:07 PM

Sounds right to me. 

 

-Dan

It worked.  Had to rack it in.  Think I was just struggling the previous night because of some cloud cover I couldn't quite see with my eyes.  Thanks a lot for your help.




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