Starfield 102 Vs Tak FC100D

[Elp]

My concern is the SF102 shows what I would deem excessive softness at the edges of a small 11 x 6mm sensor. Obviously for photographic work you'd use a flattener, but without it does seem soft, or somewhat out of focus almost. But, they're not usually built for daytime work so night testing is needed.

[CraigT82]

56 minutes ago, vlaiv said:

Yes, it is pretty hard to get the idea of how much difference that really makes.

I'm inclined to think that it is non trivial amount of difference as 485mc picks it up. It is OSC camera so in reality it has only the half of resolving power of mono version (if that is not remedied with Bayer drizzle approach and stacking) and with 2.9um pixels - it is really suitable for around F/14.5 systems - which is double of what these scopes have. We can say that you are under sampled by x4 when using this camera natively - and it still shows the difference between the two scopes.

Here is what I would do to try to make the sense of optical difference between the two scopes:

1. use high quality eyepiece - good ortho - in straight thru configuration (no need to add diagonal, or perhaps even with diagonal to simulate observation conditions)

2. use good smartphone to simulate target (these days any will do as most have good enough screens with good number of pixels).

3. Put image of Jupiter on smart phone screen (very detailed one - find Hubble Jupiter image online) and place smart phone at a known distance (use laser range measuring device to get precise reading - it will be needed).

4. Make sure that image of Jupiter on screen creates about say 45 arc seconds image at a given distance - whilst making sure that Jovian disk covers as much of screen / a lot of pixels on screen so that screen pixels can't be resolved

5. Use DSLR with manual focus lens of appropriate focal length - calculated so that DSLR oversamples by quite a bit (at least x2 given its pixel size as it is also OSC sensor). Focus lens at infinity and use the same settings to take the image between two scopes. Use focuser on each scope to get the best image

6. Scale each image so that pixel size is at critical sampling.

7. Look at such images but also compare images with auto brightness / contrast applied to each (our eye adapts if nothing else is in the view - it adapts to brightness and it tries to do "white balance" or color adaptation as well as contrast adaptation - it can't do miracles but it can do small adjustments - so that image looks "the best").

In this scenario what is the point of the ortho eyepiece? Why not just shoot straight through with the DSLR? 

[vlaiv]

25 minutes ago, CraigT82 said:

In this scenario what is the point of the ortho eyepiece? Why not just shoot straight through with the DSLR? 

You can't get optimum sampling with usual pixel size on such "fast" scopes - fast in this case being below F/10. If we really want to capture everything there is to capture - we should sample at optimum rate. That will give us precise view of optical performance of the telescope. Undersampling might "mask" issues - in the same way using less optically good eyepiece does or for example atmospheric influence (in bad seeing scopes might good the same image even if one of them is sharper).

This means that we really need to employ some sort of additional glass in order to asses the performance of each scope. There is method that does not require use of additional glass even with larger pixels - but it produces MTF graph as a result - and not image that we can compare visually.

Now, to get back to whole eyepiece thing - one could use barlow, but since we are interested in visual optical performance - it stands to reason to try to mimic conditions when doing visual - so use of eyepiece.

Eyepiece + camera lens will act as amplifier element. DSLR camera will be closer in QE response to human eye than dedicated astro camera (even with UV/IR filter it will still have much stronger Ha line for example). Choice of target is made so we have "a good reference point" - Almost all of us know what they can expect from viewing the Jupiter in different conditions - better seeing and worse seeing - but not many people know just how good their scope is in absolute best seeing - because such conditions are rare. Above test gets us as close as one can be to a perfect seeing.

It will also give us the idea of scale of difference in sharpness between the two scopes - as it will be on object that we are all familiar with.

[CraigT82]

1 hour ago, vlaiv said:

You can't get optimum sampling with usual pixel size on such "fast" scopes - fast in this case being below F/10. If we really want to capture everything there is to capture - we should sample at optimum rate. That will give us precise view of optical performance of the telescope. Undersampling might "mask" issues - in the same way using less optically good eyepiece does or for example atmospheric influence (in bad seeing scopes might good the same image even if one of them is sharper).

This means that we really need to employ some sort of additional glass in order to asses the performance of each scope. There is method that does not require use of additional glass even with larger pixels - but it produces MTF graph as a result - and not image that we can compare visually.

Now, to get back to whole eyepiece thing - one could use barlow, but since we are interested in visual optical performance - it stands to reason to try to mimic conditions when doing visual - so use of eyepiece.

Eyepiece + camera lens will act as amplifier element. DSLR camera will be closer in QE response to human eye than dedicated astro camera (even with UV/IR filter it will still have much stronger Ha line for example). Choice of target is made so we have "a good reference point" - Almost all of us know what they can expect from viewing the Jupiter in different conditions - better seeing and worse seeing - but not many people know just how good their scope is in absolute best seeing - because such conditions are rare. Above test gets us as close as one can be to a perfect seeing.

It will also give us the idea of scale of difference in sharpness between the two scopes - as it will be on object that we are all familiar with.

Ah gotcha. I don’t have any way of attaching a DSL to an eyepiece so would probably go with a barlow. But to be honest I’m not going to go to that extent to test the scopes, I’ve already made up my mind which one to keep. 
 

Cheers 

[vlaiv]

13 minutes ago, CraigT82 said:

Ah gotcha. I don’t have any way of attaching a DSL to an eyepiece so would probably go with a barlow. But to be honest I’m not going to go to that extent to test the scopes, I’ve already made up my mind which one to keep. 
 

Cheers 

Just for the record if anyone wishes to do the test (I'm guessing you won't be bothering) - you don't need to attach DSLR to an eyepiece - it can be as simple as using standard tripod with ball head and "aiming" DSLR lens into the eyepiece from small distance - let's say that front surface of the lens be at eye relief distance (although that is not strictly necessary - it can be a bit further away).