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Barlow for EAA for short refractors

EAA equipment
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#1 khachik

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Posted 26 September 2020 - 01:21 PM

Hi everyone, I have been looking for some information about using barlows/focal extenders to achieve variable magnification for EAA with short refractors.

The idea is to use a barlow lens or focal extender to change the image scale in my EAA rig. This is going to be used with a variety of short refractors: F4-F6.2, 50mm - 80mm apertures. The max sensor size is 13.5mm.

The main question that I'm try to answer is to choose a barlow that can be used to achieve variable image scale, by changing the distance between the barlow and the camera. Theoretically all barlows provide a certain range for distance / scaling, but I don't know how practical is that in terms of image quality/aberrations. I'm looking for 1.25".

The barlows I have considered so far:

  • TV powermate: pros: predictable quality; cons: constant 2.5x magnification regardless of the distance.
  • GSO APO 2.5x: pros: reportedly good quality, reportedly variable magnification based on the distance; cons: unknown
  • GSO 2x ED barlow: pros: nothing special; cons: 2-lens element, compared to above, couldn't find any reference about the magnification range based on the distance.
  • Baader zoom barlow 2.25x: pros: reportedly good quality; cons: no user reports about distance/magnification.
  • Baader Q barlow: advertised 1.3x, 2.25x dual magnification, assuming any values in between are achievable; cons: doesn't seem to match the quality of any of the above.

On a separate note: I understand the consequences of increasing the F-ratio, I'm OK with it. Currently I'm experimenting with my F12 mak for EAA, quite happy with it if expectations are set up properly.

It would be great if someone could share their experiences. Thanks!



#2 Alien Observatory

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Posted 26 September 2020 - 08:44 PM

I use a Nikon 2X Teleconverter (Barlow) with a Bower Nikon T Ring and a ZWO Nikon adapter with the correct spacing for a ZWO cam (55 mm backspacing).  It will accommodate any sensor size up to a Full Frame 35 mm Sensor.  Not adjustable but works very well with no Vignetting and very good optical quality...  Pat Utah smile.gif

 

 

Attached Thumbnails

  • Nikon 2X Teleconverter for ZWO.jpg

Edited by Alien Observatory, 26 September 2020 - 08:44 PM.


#3 khachik

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Posted 28 September 2020 - 08:34 AM

Thanks Pat.

Not super relevant, but: I see 462 color cam in your signature, that is one of the cameras I'm planning to use with this barlow, for "large" scale imagines, will really appreciate if you could share your experience with the cam. I'm deciding between 290 (because it is back-illuminated), 224 (because lots of users seem to be very happy with its DSO performance) and 462 (advertised as very low read-noise camera). Over/under sampling is another consideration, that is why I'm looking for a barlow that would provide a range of magnification.



#4 Alien Observatory

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Posted 28 September 2020 - 09:45 AM

No Problem, The 462 is a 290 with improved performance at 850 nm.  It will Bloat out stars, moon and other bright objects because of this increased sensitivity.  Unless you ar going to use an IR cut filter or a 850 nm pass filter to explore the 850 nm area then the 290 is a better cam in my opinion.   I bought it to explore the 850 nm area.  The 224 and 290 are equivalent in size and performance.  The 385 is a larger 224 sensor and would be better do to the larger FOV.

 

I am not sure what you are expecting in a wide field FOV with a smaller refactor, but the 1/3 sensor size leaves a lot to be desired compared to a 183 cam.  Try using the FOV calculator (Imaging Mode) and see how your expectations might be met using different cams and telescopes.  Sample image is a 72mm F6 telescope with a 290/ 462 sensor (red) and a 183 sensor (yellow).  Pat Utah smile.gif

 

https://astronomy.to.../field_of_view/

Attached Thumbnails

  • Screen Shot 2020-09-28 at 8.33.14 AM.jpg

Edited by Alien Observatory, 28 September 2020 - 09:50 AM.



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