About one month ago I did some star testing on a Takahashi FC-100DZ (see __HERE__ on CN for the earlier report) and recently I started on an imaging comparison between the 100DZ with Takahashi's 1.04X Multi Flattener and a Stellamira 80ED (an f/10 doublet using SFPL-53 and Lanthanum for its objective).
The Takahashi seems to have "won" this round, but it was fairly close and probably within the variance you might see from periodic changes in the seeing or just a slight mis-focus on the Stellamira. The seeing conditions were reasonably good (for a change) but I've had better on a few nights over the last two months since I've been using the 100DZ.
One problem with this comparison is that I had to change the exposure times and gains in order to try and match the subs between the two setups. I used different cameras, an ASI183MM Pro on the 100DZ and an ASI178MM Cool on the Stellamira, but both are mono cameras with 2.4um pixels and they probably come from the same basic "family" of Sony sensors. I used a side-by-side mounting on an A-P Mach1GTO so that I could have both scopes working at nearly the same time.
To account for the differences in the f-ratios (around f/8.1 on the 100DZ and f/10 on the Stellamira) I started out by increasing the exposure times by 50% for the Stellamira (as 10^2 / 8.1^2 ≈ 1.5). This means the 100DZ had somewhat of an advantage since the shorter exposures lessened the impact of any seeing and tracking errors. I also changed the gain to try and match the appearance of the nebulosity that was recorded on some test shots that I did on the Orion Nebula. I used a region of interest (ROI) from within SharpCap Pro to bring down the ASI183MM's larger sensor to match that of the ASI178MM. Then, I reframed each sequence to match each scope's center of coverage.
All testing was done using green filters (a Baader CCD RGB for the 100DZ and a ZWO for the Stellamira).
Given the differences in the f-ratios, apertures, and cameras I couldn't get an exact match, but I tried to make the subs look as close as possible. That said, given all of the above the test conditions should have definitely favored the 100DZ.
I still have some sequences to finish but I've completed sets on both the triple star Tegmine (Zeta Cancri) and the Eskimo Nebula.
For the Eskimo Nebula I took 192 subs with each setup using an exposure time of 2s for the 100DZ and 3s for the Stellamira. Then I ran PixInsight's SubframeSelector tool and selected the best 58% of the subs for each scope (rejecting mostly on FWHM, but also star count, median -- I had some intermittent haze -- and eccentricity). This gave me 111 subs for each scope that I then combined in PixInsight to produce the final masters. Then I took the resultant linear masters and measured them with PixInsight's FWHMEccentricity script. The results were as follows:
Median FWHM (Full Width Half Maximum or star size in arc seconds, lower is better):
Stellamira 80ED 1.79"
Median Eccentricity (anything below 0.42 is usually considered to be round looking by most people, lower is better):
Stellamira 80ED 0.404
The best median FWHM from any SINGLE sub as determined by PI's SubframeSelector (these subs were used as the registration reference for each scope):
Stellamira 80ED 1.64"
Here is the theoretical Airy disk size in green light for each scope:
Stellamira 80ED 3.21"
Also, the 100DZ recorded more stars, most likely because of differences in the cameras, effective exposures, and the scope's apertures. However, the FWHMEccentricity script measured over 200 stars in each master. You should note (again) that I used a region of interest on the larger ASI183MM sensor to match the field coverage provided by the ASI178MM. There was also a slight difference in the image scales, as the 100DZ has a plate-solved focal length of 813mm with the Multi Flattener, while the Stellamira 80ED has a focal length of approximately 800mm.
On the triple star Tegmine I again tried to match the effective exposure as closely as possible and I used AutoStakkert! to process the "best" 1000 subs from sequences of 10,000 frames. I did a 3X drizzle as the closest pair in this system has a separation of just 1.1 arc seconds with respective magnitudes of 5.30 and 6.25 for the primary and closest secondary. This is actually below Dawes' Limit for both scopes, well below for the 80mm aperture Stellamira. The image scale on these images of Tegmine is approximately 0.2 arc seconds per pixel (major pixel peeping and well beyond critical sampling for both scopes).
Here is Dawes' Limit for each scope:
Stellamira 80ED 1.45"
The images of Tegmine are shown below. I think both scopes did pretty well, but there does appear to be better separation with the 100DZ (as should be expected, given the differences in apertures).
My own analysis is that the Takahashi 100DZ did slightly better in both tests which should not come as any surprise.
More specifically, the 100DZ seems to be producing smaller stars than the Stellamira 80ED (but only by about 5%, whereas theory would suggest up to 25%) and the Takahashi is definitely making better looking stars (mostly in the freedom of haze or flare around the brighter stars and in the halo's symmetry). In the case of the star shape and appearance, I've noticed this difference consistently when looking at results from both scopes, the 100DZ just produces better looking stars even on axis.
I'll add PixInsight AberrationInspector results on the Eskimo Nebula from both scopes in my next post.
Lastly, I'm not sure whether these results are really definitive. What I need is a night of really excellent seeing conditions to determine where the limits of performance are on the 100DZ. This past summer I had some very good seeing when I was using the Stellamira and based upon those results I'm still hoping to get some better performance out of the Takahashi. Also, the resolution differences between the scopes should be greater than what was suggested by these tests, at least when looking at the theoretical limits (of course, seeing is critical as is focus and tracking). But, only time will tell and it could be months before I have the conditions that I need to adequately test the Takahashi.
Edited by james7ca, 01 March 2023 - 10:45 AM.