4 replies to this topic

[james7ca]

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):
FC-100DZ          1.70"
Stellamira 80ED  1.79"

 

Median Eccentricity (anything below 0.42 is usually considered to be round looking by most people, lower is better):
FC-100DZ          0.402
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):
FC-100DZ          1.56"
Stellamira 80ED  1.64"

 

Here is the theoretical Airy disk size in green light for each scope:
FC-100DZ          2.57"
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:
FC-100DZ          1.16"
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.

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Edited by james7ca, 01 March 2023 - 10:45 AM.

[james7ca]

Here are images of Tegmine that had a lighter histogram stretch. This shows what is probably the core of the Airy disk and retains more of the brightness differences between the three stars. Here it seems obvious that the Takahashi FC-100DZ did a better job at resolving the closest pair in this triplet. Or maybe more accurately, the session using the 100DZ returned a better looking result after the processing in Autostakkert! and PixInsight.

 

I also had to offset the exposure times by an even greater amount than I had for the images of the Eskimo Nebula and that may have degraded the results from the Stellamira (i.e. less "lucky" imaging). The 100DZ used 10ms exposures while the Stellamira had to use 17ms.

 

With all of this testing it would be much better to have results from multiple trials including a switch of the cameras and filters between the two scopes.

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Edited by james7ca, 01 March 2023 - 12:50 AM.

[james7ca]

Well, I processed another sequence of images from both scopes, this time on the open cluster M67. This was done on the same night as the previously shown captures, but somewhat later in the evening. Unfortunately, by the time that I took the subs on M67 I was having intermittent problems with high clouds. However, I blinked each sequence and used PI's SubframeSelector so that only the best 54% of the subs were used.

 

This time around the Takahashi FC-100DZ did notably better than the Stellamira 80ED. In fact, the FWHM results on the 100DZ were quite good. No sure what caused the difference, could have been changes in the seeing. But, I saw practically no difference between the two session involving the Stellamira (the one on the Eskimo Nebula and the later result with M67). And yes, both scopes were refocused before each sequence.

 

In any case, here are the FWHM and eccentricity results for the open cluster M67.

 

Median FWHM (Full Width Half Maximum or star size in arc seconds, lower is better):
FC-100DZ          1.58"
Stellamira 80ED  1.79"

Median Eccentricity (anything below 0.42 is usually considered to be round looking by most people, lower is better):
FC-100DZ          0.350
Stellamira 80ED  0.382

 

Total number of stars measured in the master integration by PI's FWHMEccentricity script (once again the 100DZ recorded more stars):

FC-100DZ          398

Stellamira 80ED  327

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):
FC-100DZ          1.50"
Stellamira 80ED  1.73"

 

It may be worth noting that the FWHM measurements above are actually very good. Going below 2 arc seconds in the broadband is something that requires both good seeing and a well-focused and properly functioning scope (IMO). In fact, the 1.58" result from the Takahashi FC-100DZ is getting reasonably close to the best that I've seen during the last year (although this past summer the Stellamira did even better which was quite a surprise to me considering the small size of that scope). However, I've done better in H-alpha when using a Tele Vue NP127is, a few times in the 1.2" to 1.3" range.

 

I still plan on posting image samples from both the Eskimo Nebula and M67 sessions, but quite frankly after a simple histogram stretch the differences between the two scopes are very minor.

Edited by james7ca, 01 March 2023 - 10:45 AM.

[eyespy]

Hi James,

 

Interesting report. As a matter of interest, have you compared both telescopes visually in focus and out of focus on either side of focus without the field flatterers, filters etc in the chain say at 40x per inch in each ? Maybe you only image ?

 

Doug…..

Edited by eyespy, 01 March 2023 - 10:50 AM.

[james7ca]

I think I've looked through the Stellamira 80ED once, seemed fine on the moon and bright stars (no evidence of any chromatic aberration). I've never looked through the Takahashi as it's always been used for imaging. Quite frankly, imaging is going to reveal a lot more about the quality of these scopes than would visual.

 

In any case, both scopes are really only for imaging, although once I get the Takahashi fully configured I may decide to use the Stellamira as a grab-and-go for visual.

 

I've posted several images here on CN from both scopes. If you want to check those out just search for Stellamira or 100DZ under my user name.

Edited by james7ca, 01 March 2023 - 10:52 PM.