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Should I use my focal reducer every session?

Astro Tech Astronomics Astrophotography Refractor
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#1 fehr24

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Posted 28 May 2024 - 01:13 PM

Two questions.

 

I have an Astro Tech AT90CFT paired with a ZWO ASI2600MCP with the 0.8x reducer/flattener.

 

1) should I use the reducer every time I use this scope?

2) If I wanted to take advantage of the full 540mm it offers (albeit slower optics), how do I attach the camera/OAG to the back of the scope with proper backfocus?

 

Thank you all for any feedback since I am new to the refractor game!

 

Attached are photos of the imaging train with and without the reducer/flattener.

 

IMG_8776.jpg

 

IMG_8726.jpg



#2 hyiger

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Posted 28 May 2024 - 01:16 PM

You can certainly use without a reducer (I often do if I need more focal length) but you still need a flattener. Either the Explore Scientific or Hotech 1x flattener for f/5 to f/7 refractors will work with your scope. 


Edited by hyiger, 28 May 2024 - 01:18 PM.

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#3 GregsCNAccount

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Posted 28 May 2024 - 01:23 PM

The picture you show with the caliper at 55 mm does not take into account the backfocus taken up by the camera.  You need to know the distance from the front face of the camera to its imaging sensor.  Subtract that from the 55 mm backfocus called for the flattener to get the length of spacer you need between it and the camera.


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#4 GregsCNAccount

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Posted 28 May 2024 - 01:29 PM

One more thing with regard to the positioning of the OAG in the spacer stack.  Try to get the OAG as close to the camera as you can and still get the guide camera in focus.  This allows the pickoff prism or mirror to intercept the maximum amount of the light cone, getting the brightest guide stars possible with that setup.


Edited by GregsCNAccount, 28 May 2024 - 01:29 PM.

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#5 hyiger

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Posted 28 May 2024 - 01:35 PM

To add what Greg is saying, your camera back focus distance from the edge of the tilt plate to the sensor is 17.5mm So set your calipers to 37.5mm (55mm - 17.5mm) and set the spacers so the edge of the camera to the last optical element of your reducer is 37.5mm. And I agree, you OAG is too far away from the main camera but then your back focus is 17.5mm too long. 


Edited by hyiger, 28 May 2024 - 05:12 PM.

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#6 bbasiaga

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Posted 28 May 2024 - 05:47 PM

To be direct, if you use it without a flattener or flattener/reducer, your stars will all point inwards towards the middle of the frame.  It will be quite obvious and most folks find it pretty displeasing.  Its free to try though.  With no FF/FR or FF, there is no 'back focus'  there is just focus.   

 

When you set the back focus of the reducer, you are making sure the sensor is a specific distance away from the glass in the reducer/flattener.  This is the distance at which the optical corrections made in the reducer are fully achieved.   If its too close or too far away, the stars will point inward, or start to circle the center of the field, respectively.  

 

Once you have that right, you attach to the scope and have to use the focuser to place the camera's sensor at the focal plane created by the scope.  That occurs at a distance equal to the focal length of the scope, as measured from the front element.    You need the correct back-focus for the flattener, and the whole assembly to sit at the focal plane of the scope at the same time. 

 

With no FR/FF in place, you just need the camera to sit at the focal plane.  YOu'd do this by attaching the camera to the focuser directly and racking the focuser in and out until you get it in focus.  You may require some extra extension tubes, as many manufacturers of imaging scopes leave a lot of room there assuming some of the space will be taken up with a reducer, OAG, filter wheel etc.  

 

Brian


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