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Contains:  M 27, Dumbbell nebula, NGC 6853, The star 14Vul
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M27 Dumbbell Nebula 8SE with HyperStar v. Prime, 


            Gilbert Ikezaki
M27 Dumbbell Nebula 8SE with HyperStar v. Prime

M27 Dumbbell Nebula 8SE with HyperStar v. Prime

Technical card

Resolution: 1354x761

Dates:June 20, 2015

One Shot Color: 2x240" (gain: 20.00) bin 1x1
One Shot Color: 8x30" (gain: 20.00) bin 1x1
One Shot Color: 8x480" (gain: 20.00) bin 1x1

Integration: 1.3 hours

Darks: ~8

Avg. Moon age: 3.82 days

Avg. Moon phase: 15.60% job: 1519775

RA center: 299.918 degrees

DEC center: 22.738 degrees

Pixel scale: 2.235 arcsec/pixel

Orientation: -90.339 degrees

Field radius: 0.482 degrees

Locations: SDAA Tierra Del Sol, Boulevard, California, United States

Data source: Traveller


This is one of a pair of photos of M27 that I took to compare the effects of pixel scale. (See M27 Dumbbell Nebula EdgeHD 800 Prime v. HyperStar for the other photo.)

While working through the formulas for telescope specifications I was surprised to calculate that for the same sized diameter aperture, the telescope specifications of focal length, focal ratio, and pixel scale trade off and control the image specifications of magnification, apparent resolution, and exposure time. The aperture sets the telescope's maximum resolution and its total energy flux. These are fixed limits that impose what can and cannot be done with the telescope. What surprised me most was using a short focal length and fast focal ratio to achieve the same pixel scale should produce equivalent results as a long focal length and a slow focal ratio. While these two example configurations are not exactly comparable, but they do illustrate the theory. However, in the real world, theory is not the same as operational practice. (See Aperture, f-ratios, myths, etc. for Craig Stark's more detailed explanation.)

This photo was taken with a HyperStar on a 8SE using a guide scope, the companion photo was taken with a EdgeHD 800 at prime focus is using a OAG. Because of limited back focus an OAG cannot be used with a HyperStar on a 8" telescope. I get a guiding error of 12" total ptt with the guide scope, and 6" total ptt with OAG both on the same AVX mount. Together with the much larger pixel scale using the HyperStar, the less sharp optics of the HyperStar compared to the internal corrector in the EdgeHD, and the larger central obstruction of the Schmidt camera configuration, the HyperStar photo is much less sharp.

In practice the HyperStar configuration is 3x easier to use because I can set it up in 1/3rd the time with fewer mistakes. This is mainly because OAG is so difficult to use. On the other hand the OAG does provide 2x the performance due to better correction of PE. Focusing at f/2.1 with the small depth of field is no more or less difficult than focusing at f/10. With the same sized sensor, the HyperStar provides a much wider field of view.

Overall, I prefer using the HyperStar because I end up with more successful nights with it. With a better mount and a camera with smaller pixels, I think I could get very close to the prime lens sharpness using a HyperStar, and still retain the ease of use. The prime lens will always make sharper pictures because the large central obstruction of the camera and wires of a Schmidt camera cannot be avoided using the HyperStar.



Gilbert Ikezaki
License: Attribution-NonCommercial-ShareAlike Creative Commons

Sky plot

Sky plot


M27 Dumbbell Nebula 8SE with HyperStar v. Prime, 


            Gilbert Ikezaki