Hemisphere:  Northern  ·  Contains:  NGC1819
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Comparison of 130mm Refractor vs. 11 inch SCT for Small Targets, 


            Gary Imm
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Comparison of 130mm Refractor vs. 11 inch SCT for Small Targets

Imaging telescopes or lenses: Takahashi TOA-130NFB  ·  Celestron EdgeHD 11

Imaging cameras: ZWO ASI 183MM Pro Cool  ·  ZWO ASI 1600MM Cooled Pro

Mounts: Astro-Physics Mach1GTO

Guiding telescopes or lenses: Orion 60mm Guidescope

Guiding cameras: ZWO ASI174MM  ·  Orion StarShoot AutoGuider Pro Mono

Software: Main Sequence Software Sequence Generator Pro  ·  Stark Labs PHD2 2.6.3

Filters: Astrodon Lum 31mm Gen2 I-Series

Accessory: ZWO EFW  ·  Celestron OAG  ·  MoonLite Focuser for EdgeHD 11  ·  Takahashi Flattener TOA-67  ·  Feathertouch Focuser Boss II Electronic Focusing Control

Dates:Jan. 20, 2020

Frames:Astrodon Lum 31mm Gen2 I-Series: 30x120" (gain: 111.00) -20C bin 1x1

Integration: 1.0 hours

Avg. Moon age: 25.21 days

Avg. Moon phase: 19.65%

Astrometry.net job: 3199449

RA center: 5h 12' 3"

DEC center: +5° 11' 55"

Pixel scale: 0.282 arcsec/pixel

Orientation: 1.024 degrees

Field radius: 0.178 degrees

Resolution: 3736x2583

Locations: Backyard (Mag 20.8 - New Moon), Onalaska, Texas, United States

Data source: Backyard


I have enjoyed using a refractor to image small targets for a number of years. After a while, I debated the merits of buying a large diameter SCT for this purpose. I looked for comparison images to help in my decision, but could not find much. So I am presenting this here to help any of you who may be in the same situation.

This is a side-by-side comparison of one hour of integrated Lum (2 minute subs) of NGC 1819 (1.3 arc-minutes wide). The images were taken 2 weeks apart under skies of similar seeing. I did not do any processing of the images for noise reduction, etc.

The details of my setups:

My Refractor:
Scope: Takahashi TOA-130NFB with 60 mm Guidescope
Camera: ZWO ASI 183MM Pro Cool
Pixel Scale: 0.50 arc-sec/pixel
Focal Length: 1000 mm

Scope: Celestron EdgeHD 11 with OAG
Camera: ZWO ASI 1600MM Cooled Pro
Pixel Scale: 0.28 arc-sec/pixel
Focal Length: 2800 mm

Mount (same for both): Astro-Physics Mach1GTO

I did not use the same ZWO camera for both. The small pixel size of the 183 works for the refractor setup, but a larger pixel size makes more sense for the SCT setup. The refractor image FOV is about twice that of the SCT, so the refractor image has been cropped to have the same FOV as the SCT image.

I collimated the SCT the best I could using the Hotech Advanced Laser SCT Collimator followed by a star test, but the scope is still new to me so I might be able to improve the optical quality as I gain more experience with it. The refractor, of course, is not readily adjustable.

The result is about what I had expected. The bright areas of the image are not that much different. The 4x larger area of the SCT optics really helps in less bright areas, especially the noise of the background sky. Faint details such as the star streams and faint galaxies are picked up better by the SCT. The SCT did better than I expected in a comparison of star quality. It is not shown here, but I also have been pleasantly surprised with the color performance of the SCT.

There is a good possibility that the SCT, and perhaps also the refractor, was seeing limited because the imaging pixel scales of both of these systems are on the low end. Better skies could make the above comparison more favorable to the SCT. I plan to obtain a reducer for the SCT which will reduce its focal length to 2000mm and be more compatible with my seeing. I may update this comparison, ading the SCT reducer case, once I receive it.

FYI: The weights of these 2 scopes are almost identical. The refractor costs about twice as much, although there are much cheaper brands out there. The refractor provides a larger field of view, providing more flexibility for larger targets, but the SCT offers the Hyperstar option which can do even larger targets.

The bottom line is that each of these scopes works great. The decision on which option is "best" for you will depend upon your individual needs and the objects you prefer to image.



Gary Imm
License: None (All rights reserved)

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Comparison of 130mm Refractor vs. 11 inch SCT for Small Targets, 


            Gary Imm