Starfield 102 Vs Tak FC100D

[CraigT82]

Had a couple of hours free time this afternoon so decidedly to spend it wisely: Playing with telescopes.

I set up the my Starfield 102ED (FPL-53) side by side with my Takahashi FC100DF.

Chose to aim both at a wooden sign post about 50m away and snapped some shots through with my IPhone. 

Used the same diagonal prism and eyepiece in both scopes (Baader Zeiss T2 prism, and Svbony 3-8mm zoom)

First pic is a side by side comparison at 8mm on the zoom, and second pic is side by side at 3mm.

One of the main things I’ve taken from this is the focuser on the Starfield is so much nicer to use than the Tak, with its more blue microfocuser fitted. Finding focus was easy with both scopes but the Tak unit definitely had a less premium feel than the Starfield. 
 

To me the images show about the same CA in both scopes ( here isn’t much to see but some visible on the edges of the wood in second pic) but the Tak is noticeably sharper. This shows just how good FPL-53 glass is, and also shows just how good the figuring of the Takahashi glass is. 

Im hoping tonight to conduct a lucky imaging test on a star tonight to see what the difference in stellar images is. 
 

Starfield on left, Tak on right 

 

Edited August 24 by CraigT82

[Elp]

Have you tried the comparison looking at far off tree branches or electrical wiring or antennas? CA can show up on poorly corrected optics.

[CraigT82]

Yeah branches against the bright sky was my intended target but the wind was moving the around too much. Hence the solid non moving wooden post. The grain of the wood makes a nice test for sharpness though, if not CA. 

[vlaiv]

It is interesting to see more difference in sharpness at 8mm setting than on 3mm setting.

I wonder why.

[Elp]

I know EPs can make a difference in such a test, I always test them first during the day to spot marginal differences. Using a good one will remove the EP as a variable in such a test.

[CraigT82]

12 minutes ago, vlaiv said:

It is interesting to see more difference in sharpness at 8mm setting than on 3mm setting.

I wonder why.

Might be because I ‘zoomed’ in more with the 8mm image than the 3mm whilst editing them to be side by side for presentation.

The unmolested captures are below:

EDIT: just a thought but the sign hanging from the post was swinging a little in the breeze, so not a great test subject. The unmoving post is a better comparison 

Edited August 24 by CraigT82

[vlaiv]

4 minutes ago, CraigT82 said:

Might be because I ‘zoomed’ in more with the 8mm image than the 3mm whilst editing them to be side by side for presentation.

How do images compare to your subjective feel at the eyepiece?

[CraigT82]

4 minutes ago, vlaiv said:

How do images compare to your subjective feel at the eyepiece?

To be honest, I can’t see a different at the eyepiece at all. But I haven’t really had the chance to test them side by side in good seeing yet. Also my eyesight isn’t the sharpest. Hence why resorting to the camera sensor to try to separate them. Got to say that £899 gets you a hell of a good scope. 

Edited August 24 by CraigT82

[Mr Spock]

On that last shot of the rings the focus looks different.

I found, on actual targets (the moon), visually there was little difference.

[vlaiv]

1 minute ago, CraigT82 said:

To be honest, I can’t see a different at the eyepiece at all. But I haven’t really had the chance to test them side by side in good seeing yet. 

I guess that test methodology needs to be a bit improved then?

Maybe instead of phone camera - one can use for example planetary camera coupled with some sort of decent short focal length lens (I think cheap old 20ish mm M42 lens could do the job? - maybe even very short FL all sky lens with wide view - like ones that come with camera?).

Camera settings and focus position of that lens would be kept the same (at infinity) while actual focusing of the image would be done with telescope focuser.

Actually, when I think about it - focal length of lens should be selected so that there is enough sampling to capture all the detail (8mm setting on EP should be at least critically sampled).

[CraigT82]

14 minutes ago, vlaiv said:

I guess that test methodology needs to be a bit improved then?

Maybe instead of phone camera - one can use for example planetary camera coupled with some sort of decent short focal length lens (I think cheap old 20ish mm M42 lens could do the job? - maybe even very short FL all sky lens with wide view - like ones that come with camera?).

Camera settings and focus position of that lens would be kept the same (at infinity) while actual focusing of the image would be done with telescope focuser.

Actually, when I think about it - focal length of lens should be selected so that there is enough sampling to capture all the detail (8mm setting on EP should be at least critically sampled).

I’m hoping to get out tonight and use my ASI485mc straight through the scopes. Capture few thousand frames of a bright star, Vega maybe as it’s high. Will do scope 1 then scope 2 then scope 1 and scope 2 again, to guard against and seeing variances. Moon would be better but that won’t be a decent target for a while 

[CraigT82]

24 minutes ago, Mr Spock said:

On that last shot of the rings the focus looks different.

I found, on actual targets (the moon), visually there was little difference.

Possible. How would you tell the difference between focus blur, and blur from spherical aberration? 

[Mr Spock]

7 minutes ago, CraigT82 said:

Possible. How would you tell the difference between focus blur, and blur from spherical aberration? 

Top one the ring is in focus the wood isn't.  The ring is a lot sharper and the web on the hook is more clearly defined.
Bottom one the wood is in focus and the ring isn't.

[CraigT82]

6 minutes ago, Mr Spock said:

Top one the ring is in focus the wood isn't.  The ring is a lot sharper and the web on the hook is more clearly defined.
Bottom one the wood is in focus and the ring isn't.

I see what you mean, but I was focusing on the hanging sign for that image rather than the ring/post. Which was a mistake anyway as the sign was feee to move. The 8mm image of the solid post is a better indicator. 

[CraigT82]

Well…. Got everything set up last night to test both on and star and my Astro laptop wouldn’t turn on 🤷🏻‍♂️. 

[dweller25]

I suspect there is almost no difference until you start to use high powers on the planets, then one may be a little sharper and have a little more contrast than the other.

But the extra difference in the optically better scope may not equate to the price difference.

Edited August 25 by dweller25

[Mr Spock]

13 minutes ago, dweller25 said:

I suspect there is almost no difference until you start to use high powers on the planets, then one may be a little sharper and have a little more contrast than the other.

I found little difference on the moon at around the x220 mark. I wouldn't use either above that. I've had the Tak quite a while now and x224 with the 3.3mm TOE is the best it gets. I've tried the 2.5mm TOE on the moon and it isn't that sharp. Jupiter I stick to the 4mm for x185 - I do the same with the 12" - Jupiter looks better at more modest powers.

I will use the word 'perhaps' here. With the Tak lunar shadows were perhaps a shade darker, and lunar highlights perhaps a shade lighter. The difference is so small you would only notice side by side. Same on stars, both very sharp with the airy disk perhaps a little more etched in the Tak. The Starfield cost £899 and the Tak, with FT, over £3k. 

A sensible person would pick the Starfield every time. But if you want to be reassured that your optics can't get any better off the shelf or want to make a lifetime investment, then the Tak may well be the scope for you despite the huge cost.

I'm happy with mine. I've not heard of anyone not satisfied!

[RobertI]

It’s an interesting comparison and I’d be interested in further testing. But I think the real limiting factor here is that you are using your eyes to focus, and as you say, your eyesight may not be perfect, so can never be sure that you are comparing perfect focus of both scopes. Perhaps a star test with Bhatinov mask might be more accurate, but even then there is some visual judgement required unless you are using software.

Edited August 25 by RobertI

[CraigT82]

2 minutes ago, RobertI said:

It’s an interesting comparison and I’d be interested in further testing. But I think the real limiting factor here is that you are using your eyes to focus, and as you say, your eyesight may not be perfect, so can never be sure that you are comparing perfect focus of both scopes. Perhaps a star test with Bhatinov mask might be more accurate, but even then there is some visual judgement unless you are using software.

Yes further testing definitely required, and planned. Will use an Astro cam straight through on both scopes, firstly in daytime on a far off chimney pot I can see from my back garden. I will try to use the edge detection focusing aid in sharpcap to make sure I get best focus in each scope. Also planning to get some stellar images too. However I think I need a new laptop as my old thinkpad seems to have given up the ghost . 
 

 

[Louis D]

On 24/08/2024 at 12:54, vlaiv said:

It is interesting to see more difference in sharpness at 8mm setting than on 3mm setting.

I wonder why.

In my inexpert testing and in visual use, the Svbony 3-8mm zoom is considerably less sharp at 3.5mm (not really 3mm) than at 8mm.  I compared the Svbony side-by-side with my Pentax XW 3.5mm, and the unsharpness seen in the Svbony's view went away when using the XW.  It is definitely the eyepiece and not the scope or seeing conditions.

Point is, when you are using a less than super-sharp eyepiece, the differences in sharpness between two OTAs is going to be that much harder to pick out.

I'd be curious how these Chinese scopes would stack up against Astro Physics or TEC scopes when it comes to absolute sharpness at the limits.

Another test to check for would be scatter.  Flourite is supposed to have much less scatter than ED glass:

[vlaiv]

5 minutes ago, Louis D said:

In my inexpert testing and in visual use, the Svbony 3-8mm zoom is considerably less sharp at 3.5mm (not really 3mm) than at 8mm

Good point.

I guess such comparisons should really be done with high quality ortho EP.

[Elp]

Hence my earlier suggestion about EPs.

[vlaiv]

4 minutes ago, Elp said:

Hence my earlier suggestion about EPs.

Good one ...

We could actually take EP out of the equation - but we would need very small pixel size - not commonly found on astro cameras - something like ~1.4um in mono for F/7 scopes. I know that there are such sensors, but I'm not sure that I've seen astro camera with one.

On F/5 scopes we would need to go down to 1um ...

Well - almost ASI715 has 1.45um pixel size but it's OSC ....

[Elp]

678mm has 2um so not that close. Could you not double the f ratio and determine that way though introducing more glass will skew results, or stopping down?

[vlaiv]

1 minute ago, Elp said:

678mm has 2um so not that close. Could you not double the f ratio and determine that way though introducing more glass will skew results, or stopping down?

Adding barlow is similar to adding eyepiece, although good barlow is both cheaper and simpler than good eyepiece. It adds glass to the light path in any case and can "skew" results.

Stopping down will work - but that will be comparing only "part of the lens" rather than whole lens. Stopped down lens always works better (well, in most cases anyway) because there is less deviation in smaller surface. That approach also skews results and probably much more than barlow or eyepiece (making things sharper than they are).