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asi290mm spacing with .5x reducer

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9 replies to this topic

#1 calypsob

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Posted 16 April 2018 - 08:59 AM

Ive been helping a fellow imaging friend get up and running with a .5x reducer and the asi 290mm.  I am wondering what people are using in terms of spacer distance between the camera and reducer.  Its a generic .5x reducer. When you screw the camera directly onto the reducer the stars are pretty bad.  I am guessing this could be fine tuned to achieve the round stars I have seen others achieve with this type of pairing.  



#2 DonBoy

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Posted 16 April 2018 - 11:15 AM

A 1.25" .5x GSO reducer will provide a reduction of 0.5x when the center of its lens assembly is placed 51mm from the focal plane of the sensor.  A Antares 1.25" .5x requires 47.5mm.  2" versions require more spacing.


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#3 calypsob

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Posted 16 April 2018 - 11:52 AM

A 1.25" .5x GSO reducer will provide a reduction of 0.5x when the center of its lens assembly is placed 51mm from the focal plane of the sensor.  A Antares 1.25" .5x requires 47.5mm.  2" versions require more spacing.

Ok perfect, would I subtract the 17.5mm backfocus distance of the asi290mm from that? 

 

This one is an SVBONY reducer so Im not sure which spacer would be needed.  

 

https://www.ebay.com...=item3d5b700beb


Edited by calypsob, 16 April 2018 - 11:56 AM.


#4 DonBoy

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Posted 16 April 2018 - 12:42 PM

The spacing I quoted is for those two specific products, what the ebay one is I can't say.  

 

Spacing is from center of lens to sensor.  Spacer need would be calculated as follows: Spacer = spacing - 17.5mm - distance to center of reducer lens assembly.


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#5 calypsob

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Posted 16 April 2018 - 01:12 PM

The spacing I quoted is for those two specific products, what the ebay one is I can't say.  

 

Spacing is from center of lens to sensor.  Spacer need would be calculated as follows: Spacer = spacing - 17.5mm - distance to center of reducer lens assembly.

Thanks, sounds like a helical t2 will be needed to dial it in correctly.  Im guessing it will be similar to the other two in terms of distance.  


Edited by calypsob, 16 April 2018 - 01:12 PM.


#6 mclewis1

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Posted 16 April 2018 - 10:21 PM

Ok perfect, would I subtract the 17.5mm backfocus distance of the asi290mm from that? 

The ASI290mm spacing to the sensor is 12.5mm. The ZWO 1.25" adapter that comes with the camera is around 30mm long (I've never seen an exact measurement so this is extrapolated from similar adapters).

 

This setup (reducer on the 1.25" adapter/nose piece) will be pretty close the required spacing for the focal reducer. It may be a little bit under the exact number but that should be fine for optical quality.



#7 calypsob

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Posted 21 April 2018 - 07:33 PM

Well after alot of trial and error it turns out that the SVBONY .5x reducer prefers no spacing at all.  Any amount of spacing introduced coma.  With the reducer screwed directly onto the asi290 paired with a uv/ir cut filter, stars were perfect and reducer worked great, no internal reflections at all.  $12 well spent! 



#8 mclewis1

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Posted 22 April 2018 - 03:30 PM

Wes, When you say directly onto the ASI, do you mean onto the 1.25" adapter?

 

If so that would be 12.5mm + 26mm (not 30 that I used in my earlier post above) + about 5mm (for the internal position of the lenses within the reducer's body), so around 44mm?

 

Have you done a plate solve of an image from the new setup to see exactly what fov you are getting?



#9 calypsob

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Posted 22 April 2018 - 07:13 PM

Wes, When you say directly onto the ASI, do you mean onto the 1.25" adapter?

 

If so that would be 12.5mm + 26mm (not 30 that I used in my earlier post above) + about 5mm (for the internal position of the lenses within the reducer's body), so around 44mm?

 

Have you done a plate solve of an image from the new setup to see exactly what fov you are getting?

Mark let me break down the specs to be sure I am explaining all of this correctly.

 

The camera here is the Asi 290mm Autoguider version https://www.highpoin...era-asi290miniĀ 

By screwing the reducer directly onto the camera it seems that .5x was achieved This is actually right in line with what agena astro has listed as specs for their reducer

https://agenaastro.c...al-reducer.html

 

The reducer in this case was an sv bony 1.25" .5x redcuer. No spacers used to achieve .5x

 

I will share a link to one of Berts images taken with the .5x later.  Right now he is still processing the data. 



#10 bdyer22

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Posted 22 April 2018 - 09:55 PM

I got confused by that table the first time too.  0mm --> .5x (nominal reduction) on the chart is relative to the working distance of the focal reducer which is 79mm for the Antares 2" reducer linked to.

 

Even if attached directly to the camera you'll have a spacing greater than 0mm because the sensor is inset into the camera body by 12.5mm.

 

Once you have your image upload it to http://nova.astrometry.net/upload to get your working focal reduction with the reducer spacing.

 

Enjoy the camera and don't forget to share! :)




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