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Next Stargazing ✦ March 9th
Next Stargazing ✦ March 9th

2" Advanced Convertible Photo-Visual Barlow


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Our Advanced Convertible Barlow has a unique design that provides several configurations to increase the focal length of your telescope. It was designed to match the characteristics of small and medium-sized CCD chips. It is also ideal for high-power visual observation and lunar-planetary-solar photography. This redesigned optic features a flatter field, allowing it to be used for deep-sky photography when increased magnification of smaller objects is desired.

It can be used conventionally by inserting an eyepiece or a camera’s nosepiece into its 2" locking adapter.  Alternatively, the optic section can be separated from the 2" locking adapter and screwed directly into a camera’s faceplate, or threaded nosepiece, or extension tubes for a flexure-free fitting. It has both 42 mm and 48 mm threads for versatility. See system chart for configurations.


  • Flat field for imaging and visual
  • Very effective high-tech coatings eliminate ghost images
  • Screws into front nosepiece of cameras and diagonals
  • Ideal for medium size CCD sensors
  • Greater than 98% light transmission
  • Large field optic eliminates vignetting even with f/6 optics

This redesigned optic features a flatter field, allowing it to be used for deep-sky photography when increased magnification of smaller objects is desired. Naturally, additional exposure time will be required. Changing the spacing distance will change the magnification of the Barlow (90 mm back spacing is ideal, yielding 2.0x magnification). The “sweet spot" of the flatter field is between 1.8x and 2.5x.

Note the following positions and resulting magnifications (distance from rear of Barlow optical section flange to focal plane, measured from 2" barrel):

  • Min recommended mag: 1.8x,                      70 mm back focus distance 
  • Normal magnification:     2.0x,                      90 mm back focus distance (best flat field)
  • Max recommended mag:  2.5x,                   134 mm back focus distance

The 2-element design preserves the fine color correction of the main objective. The optical elements are precision ground and polished to insure that no aberrations are introduced into the system. The Barlow focal length is -102 mm which is longer than conventional types. This minimizes the divergence of the edge rays and therefore reduces the apparent vignetting common in many eyepieces. The Barlow features a brass locking ring that clamps eyepieces and accessories securely, yet will not mar their fine surfaces.

The Barlow’s flat field is optimized for APS-C and smaller sensors (such as the 8300 and 3200 sensors). Using this Barlow with an f/8 refractor provides round stars over 90% of the field and over 80% of the field with an f/6.3 refractor.

The field of view is compared in the images to the left. The far left image is taken using the AP 175EDF at its native focal length of 1400 mm. The immediate left image is taken with the Advanced Barlow at approximately 2.0x magnification. The image at the right shows a final processed image. Please note the excellent star shapes across the image.
Click image for larger size.

These two images compare the resolution with and without the Advanced Convertible Barlow using a QSI 683 (5.4 micron pixels) camera with the AP 175EDF at 1400 mm focal length. The left image is shown at 400% screen size and the right image with the Barlow is at 200% screen size, keeping the two images equal in perspective. Click image for larger size.