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Focal Reducer Spacing

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#1 rgsalinger

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Posted 17 June 2018 - 08:50 PM

I own a Televue TV127is. I've had it for about 8 years now and I really like it.  Recently I decided that this summer I wanted to get a wider FOV. So,  I got the .8 focal reducer for the scope. I read the instructions which said that the back focus is 55 mm. I have an ASI1600 camera, ZWO 7 position filter wheel, and ZWO OAG. Total back focus is 42.5 mm. I put 13 mm of spacers on between the wheel and the camera to get me to 55. Terrible results. I called Televue and they said 58.5. So, I switched to 13 mm of of spacers and still got terrible stars.They look like comets in the corners. I extended it out another 3 mm but no that made no improvement.

 

So, I'm asking if anyone has the same scope and focal reducer and has used it with a CCD camera/wheel/OAG and gotten good results.  If so, what was your spacing. Televue also said that if I'm using 3 mm filters (which I am) that I needed to add one additional millimeter of spacing to make it perfect. From what I can see, I'm not even close. I had a friend with me doing this so either both of us cannot use calipers or we never got the right spacing.

 

Here's a corner ----

 

LUMngc7000_120sE.00011.jpg

 

Rgrds-Ross



#2 Peter in Reno

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Posted 17 June 2018 - 09:13 PM

This could also depend on the illumination circle of the focal reducer. Focal reducers naturally reduces illumination circle and sometimes by quite a bit. If the illumination circle is smaller than your camera's sensor diagonal, then it's possible that your CMOS sensor is too big and you may have to crop it. How are the stars at the center of the image? Where on the image do the stars start to distort?

 

Peter



#3 rgsalinger

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Posted 17 June 2018 - 10:35 PM

Not a big sensor, unfortunately. The distortion starts quite near to the center. 

Rgrds-Ross



#4 Peter in Reno

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Posted 17 June 2018 - 10:50 PM

Is this the focal reducer you are using?

 

http://www.televue.c...r NP Scopes.pdf

 

It does not specifically say where on the focal reducer you must start in order to determine the spacing for your setup.

 

Or it's possible you may have a defective focal reducer.

 

Peter



#5 rgsalinger

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Posted 17 June 2018 - 10:58 PM

That's the reducer and that's the problem - it's completely impossible to work out how to measure the correct back focus IF you are not using a DSLR. If you are using a Canon DSLR and the appropriate TV parts, it's all taken care of for you and works perfectly.

 

I am hoping to find someone who actually has done this and get a measurement to confirm that latest "information" that I got from TV. They said that it should be 62mm measured from the start of the threads plus 1 mm for the filters or 63mm. I can try this, of course, but it would be really nice to know that someone had used those settings and it worked.



#6 Goofi

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Posted 17 June 2018 - 11:31 PM

Ross, I purchased that reducer for use with my NP101 but couldn't get it to work.

Part of the problem was I couldn't use it and the Moonlite focuser - and I opted for the Moonlite.

 

But, part of the problem was trying to figure out spacing when I used the stock focuser. I never managed and tried about a dozen different configurations.  All I can say is, Good luck!



#7 Poochpa

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Posted 18 June 2018 - 09:40 AM

I’ve had the TV 1073 reducer for many years and it works fine with my NP101, with both DSLRs and my ASI1600.  I use approx. 55mm spacing. Does not need to be precise. My guess is that you have a defective reducer.

Mike



#8 rgsalinger

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Posted 18 June 2018 - 11:10 AM

Poochpa - using it with the ASI1600, from where do you measure the 55mm spacing? I'm thinking that it has to be from the very back of the reducer. Do you have a filter wheel?

 

Rgrds-Ross



#9 Poochpa

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Posted 18 June 2018 - 01:55 PM

Ross,

You measure the 55mm spacing from the reducer's rear flange, not the end of the threads. I do not use a filter wheel, as my camera is the 1600 osc version, but I do have a Starizona Filter Slider between the reducer and camera. My specific image train from the reducer to the camera is:

TV AFT-1105 filter adapter (6mm)

Blue Fireball male-to-male M48 adapter (1mm)

Blue Fireball M48 adapter (13mm)

Starizona Filter Slider (17.5mm)

ZWO M42 adapter (11mm)

ASI 1600 (6.5mm)

Total Spacing: 55mm

 

According to Televue, you have +/- 2mm latitude in the spacing.

 

Mike



#10 rgsalinger

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Posted 18 June 2018 - 04:59 PM

Thanks. Now I know the measurement starting point. I can set mine up with the same spacing and see what happens.

Rgrds-Ross




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