I finally completed modifications to my Orion ST-80 (80 mm f/5) telescope and gave it a test run on the night of October 30th into the morning hours of October 31st. The modifications included replacing the original 1.25-inch focuser with a GSO 2-inch focuser, an Orion field flattener (to reduce field curvature distortion of star images), and a Baader Fringe Killer filter (to reduce violet-blue halos around the stars caused by chromatic aberration). The animation below shows asteroids (55) Pandora and (60) Echo, the targets for this first light test of the upgraded ST-80 paired with my ZWO ASI 482MC camera.
I chose these targets because I wanted a field of view with two asteroids for visual interest, and I wanted to see how deep this telescope-camera combination scope could go. Conveniently, the field of view containing these two asteroids also contained two faint galaxies, IC 1492 and IC 1496.
The Minor Planet Center (MPC) predicted that (55) Pandora would be at Visual magnitude 11.6, and (60) Echo at 12.3. Both within easy reach of this scope-camera combo. The galaxies, however, were considerably fainter with IC 1492 at Vmag 15.2 and IC 1496 at Vmag 15.5. The galaxies would be the test of how deep this scope-camera combo could see. Additionally, I wanted to see how deep this scope-camera combo could go just using ten-second exposures.
The animation above shows the result. It consists of eight images taken over a two-hour period. Each image is made up of a stack of ninety 10-second sub images providing a total integration time per image of 900 seconds/15 minutes.
My hope in this project was to upgrade this inexpensive little scope to make it useable as a lightweight travel, EAA, outreach, and astrophotography telescope.
As I’ve previously posted, the Orion ST-80 is a simple achromatic refractor that trades off optical quality for very low price. One of these scopes sells now for $110 new.
Unfortunately, while very reasonably priced, as would be expected from an achromatic telescope, the scope suffers from severe chromatic aberration and field curvature. These two optical defects, common to achromats, cause noticeable violet-blue fringing around star images and distorted star images toward the outer edges of the field of view. Previous use of this scope showed that the field curvature was so bad that only about the central 30% of the field of view presented undistorted star images and the chromatic aberration produced large violet-blue halos around the stars across the entire field of view.
My plan to correct these issues was to insert an Orion Field Flattener and a Baader Fringe Killer filter in the optical path between the camera and the telescope. I had previously tested the Baader Fringe Killer and found that it did a good job eliminating the ugly violet-blue fringe halos around the stars. The field flattener, however, is new. But, because it is a two-inch accessory, I had to replace the ST-80’s original 1.25-inch focuser with a focuser having a two-inch draw tube.
Trying to complete this project on a limited budget, I chose the GSO 2-inch Crayford-style focuser from Agena Astro. With its two-inch threaded draw tube, the Orion Field Flattener screwed right on to the end of the focuser tube, and my ZWO ASI 482MC camera – with the 1.25-inch Fringe Killer filter screwed inside the camera just above the imaging chip – attached to the back of the field flattener with just the right spacing. As a bonus, this dual speed focuser provides for much finer focusing adjustments than the original focuser.
Looking at the images that produced the animation above, I am quite satisfied with the performance of this upgraded scope. The Baader Fringe Killer filter eliminated most of the violet-blue fringing around the stars, much of which disappeared anyway when I drastically lowered the color saturation to eliminate noise.
The Orion field flattener did a really good job of correcting field-curvature distorted star images across approximately 80% of the field of view. If you look close, you can see that the outer 10% on the right and left sides of the image contain slightly elongated stars. I could have cropped them out, or used one of the astronomy plugins for GIMP that have star rounding tools. But, in this case, other than some very slight cropping of the edges to remove stacking artifacts, I’ve opted to keep the full field-of-view image and not apply star-rounding correction.
Image Details
October 31, 2023 04:04:12-06:04:00UT
Two-hour time-lapse animation.
Minor Planet Center Predicted Vmags: (55) Pandora 11.6; (60) Echo 12.3
8 images, each a stack of 90 @ 10 seconds. Gain 350
FOV: 93.8 x 51.5 arcmin/1.56° x 0.86° (cropped size for animation)
North up. East left.
Seeing: Poor Transparency: Good Sky Condition: Bortle 7 with nearly full Moon 70° to the East.
Telescope: Orion ST-80 (80 mm f/5) + Orion Field Flattener + Baader Fringe Killer Filter
Camera: ZWO ASI482MC
Guide Scope: SVBONY SV165 (30 mm f/4)
Mount: Celestron CGEM
Captured with SharpCap Pro
Processed with Deep Sky Stacker, GIMP
Guiding: PHD2