FSQ 106 crop vs TOA 130

[Rodd]

The length of time it takes to finish an image these days due to the unsettled weather pattern and short astro-darkness, coupled with the Moon stealing the last few clear days, left me with the opportunity to do a direct comparison that I have been thinking about for a long time.  The issue is would a significant crop of a widefield image taken with FSQ106 and .6x reducer compare favorably with a full image of the same crop taken with the TOA 130.  This addresses the question of whether the 2 scopes are redundant.  The small pixels of the ASI 1600 combined with the FSQ optics, allows cropping the widefield to create multiple images.  But are they as "good" as uncropped versions taken with a longer focal length, 1" larger scope.  Since I already had a Ha image of the Gamma Cygni region taken with the FSQ, and since I had already cropped the SHO version of that image to portray a close in of Banard 334,  It was a prefect opportunity to collect some Ha of Banard 334 with the TOA 130, which is currently on the mount.  The fact that the Moon is up and large left me with little else to shoot anyway after midnight.

To ensure the images were precisely the same in dimension, I registered the Wide filed FSQ FOV to the TOA restricted FOV--no need to crop (PI does it for you).  I then tried to process the images identically.  Same noise control, same sharpening, same histogram etc.  There could be some minor differences (I fixed the stars in the FSQ a best I could--I jused an extra iteration of curves.  But the deformed stars of the FSQ are clear--this image was taken before the collimation was fixed.  I do not know if the recollimation fixed the scope--it is still in the box.  I hope it did--the stars are obviously inferior--but, it is a rather substantial crop (zoom).  The stars look ok when viewing the full image.

Anyway--here are some thoughts, notes regarding this experiment.  

1) The FSQ with .6x reducer shoots at F3 and the TOA 130 is F7.7.  There is an extra inch of aperture with the TOA--but I was expecting to need much more data with the TOA for a comparable image.

2) The FSQ image has 53 300sec subs and the TOA image has 43 300sec subs.  Would using 10 more subs be noticeable--less noise?

3) Pixel scale of the FSQ was 2.48 arcsec/pix and for the TOA was 0.78 arcsec/pix

4 Other than the stars--which look like little diamonds in the FSQ image--with some registration artifacts, I find the images very close.  Are the stars this shape due to the miscollimation, or because of  a zoom factor that pushed the data beyond what it could handle.

5) The ultimate question--can the FSQ at F3 be used to capture widefield that can subsequently be cropped to yield multiple images such that the TOA 130 is redundant and therefore not necessary?  If the stars are like this because of the crop, then I'd say no.  But if the micollimation is the cause of the star shape, and it has been fixed, then I'd say...……….well, I'd create this post and ask SGL!  

As an aside--I had a really weird Flat issue.  lately, due to my ASI 1600 filter wheel not accurately positioning itself after rotation, I must shoot flats after each session--actually I must shoot flats prior to ANY filter change.  Its a pain, but I have learned to live with it.  BUT, I shot the Ha flats for the 18 subs I collected on the second night using the TOA and I noticed a very dark--virtually black--spot.  This spot was not on the subs.  And the spot seemed to change slightly between flats, though it remained in the same location.  I knew not what it was--and it disappeared when I collected Ha flats the next night.  Fortunately, those flats successfully calibrated the Ha subs collected the night before--I guess I got lucky.  I have 2 ideas--1) a small bug crawled onto the filter, or a droplet of moisture appeared on the filter.  The next day--it was gone.  Anybody have any ideas?

FSQ 106 with .6x reducer-large crop, 53 300 sec subs

 

 

TOA 130 native. full FOV image, 43 300 sec subs

 

 

 

 

[Whirlwind]

The TOA image is better I think when zoomed further in to look at the specific detail.  However at the full image scale it really is barely perceptible. 

You are spreading the flux out in the TOA by a much greater margin so really to let the TOA really shine the total exposure should be longer.  Both are great images though.

The square shaped stars are from the CMOS microlens.  You have a classic diffraction pattern around the stars which is making them look square.  The small "/pixel in the TOA is likely masking this effect because each stars light is spread out over many more pixels. 

[Stargazer33]

The TOA image is better IMO. The stars in the FSQ image look a little 'square'! Both are enviable images though.

[ollypenrice]

Regarding F ratio, you know my view...

The images do not have much to choose between them, it is true. However, Ha is not the wavelength in which to conduct this test unless it's the only wavelength you use. It gives the F3 scope a very easy ride compared with broadband and especially blue. Conduct this test in broadband and I think you'll find that the larger scope will pull away. Near monochromatic light needs little correction. However, as I've said before in comparing big scope-big pixels with small scope-small pixels, the latter does surprisingly well!

I'm sorry you've had collimation woes with the new 106. I'm beginning to wonder if this isn't the norm rather than the exception since 4 out of 4 of the examples I've hosted have had collimation/tilt issues. Personally I feel that F3 is asking for trouble, especially in a refractor, but the scopes I've hosted were working at native and were still out. This should not be happening.

Olly

[tomato]

17 hours ago, Rodd said:

 

As an aside--I had a really weird Flat issue.  lately, due to my ASI 1600 filter wheel not accurately positioning itself after rotation, I must shoot flats after each session--actually I must shoot flats prior to ANY filter change.  Its a pain, but I have learned to live with it.  BUT, I shot the Ha flats for the 18 subs I collected on the second night using the TOA and I noticed a very dark--virtually black--spot.  This spot was not on the subs.  And the spot seemed to change slightly between flats, though it remained in the same location.  I knew not what it was--and it disappeared when I collected Ha flats the next night.  Fortunately, those flats successfully calibrated the Ha subs collected the night before--I guess I got lucky.  I have 2 ideas--1) a small bug crawled onto the filter, or a droplet of moisture appeared on the filter.  The next day--it was gone.  Anybody have any ideas?

 

Myself and  Tomatobro  have been experimenting with retrofitting Peltier coolers to ASI178 cameras. Early runs did produced some random black specks on the images, which were caused by tiny droplets of moisture falling onto the sensor window.  As you would expect, this was definitely worse when the refractor was pointing almost straight up.

Application of the ZWO heater strip at the front of the camera body, and cooler control, seems to have fixed the problem, but this has yet to be properly tested.

[Rodd]

4 hours ago, ollypenrice said:

Regarding F ratio, you know my view...

The images do not have much to choose between them, it is true. However, Ha is not the wavelength in which to conduct this test unless it's the only wavelength you use. It gives the F3 scope a very easy ride compared with broadband and especially blue. Conduct this test in broadband and I think you'll find that the larger scope will pull away. Near monochromatic light needs little correction. However, as I've said before in comparing big scope-big pixels with small scope-small pixels, the latter does surprisingly well!

I'm sorry you've had collimation woes with the new 106. I'm beginning to wonder if this isn't the norm rather than the exception since 4 out of 4 of the examples I've hosted have had collimation/tilt issues. Personally I feel that F3 is asking for trouble, especially in a refractor, but the scopes I've hosted were working at native and were still out. This should not be happening.

Olly

I did not realize this....then again I did not plan this really....I had the one image and the Moon was up so my options were limited.  I figured why not.     I am still surprised that the F7.7 required less data than the F3.  Then again, we could be seeing the effects of F3 on the narrowband filters (Astrodon said I should be fine, but if 10% is lost (shifted), it could be the difference in a close call. 

I hope the scope is fixed now.  The miscollimation was showing up at F5 as well.  TNR said it was out and that they fixed it.  But get this...if you buy a scope and it shows up out of collimation--the recollimation labor is not covered under warranty.  I managed to convince them that it should be and they did it, but I had to fight for it.  Their point is they can't control a scope once it leaves the premises.  My point was that if the scope arrives out of collimation, how can they blame the consumer?  Takahashi should either come up with a stabler configuration, control the transport, or whatever.  They wanted to charge me $400 (on top of the shipping which i did have to pay for).  It is ludicrous to pay $6,000 for a 4" scope and not have it work....and then have to pay to fix it!  Anyway...i is still in the box.  I change scopes so infrequently due to the limited clear sky I get.   Regarding the F3--I have come to love the wide FOV, and the ability to crop the image and get multiple images out of it.  Except for the star distortion, the crops nold up amazingly will to zoom.  I am anxious to see if they really did fix it, of if the star shapes are a product of the spot size and F3 

Rodd                                                                                                                    3

[Rodd]

46 minutes ago, tomato said:

Myself and  Tomatobro  have been experimenting with retrofitting Peltier coolers to ASI178 cameras. Early runs did produced some random black specks on the images, which were caused by tiny droplets of moisture falling onto the sensor window.  As you would expect, this was definitely worse when the refractor was pointing almost straight up.

Application of the ZWO heater strip at the front of the camera body, and cooler control, seems to have fixed the problem, but this has yet to be properly tested.

I got a heating strip for the ASI 1600--then I was informed that part of the heat exhaust from the cooling is directed at the window.  I have only had a dew issue once--and that was right after a heavy rainstorm.  I uncovered the scope (it sits outside under a thingamajig), and moisture was dripping from the tripod legs and parts of the mount.    I turned it on and cranked the cooling--bad idea.  That was the only time, so I have not used the heating strip.  I probably will install If I change scopes.  I need to clean the filters anyway.

Rodd

Rodd                                                                                                                                                                                                                

[ollypenrice]

3 hours ago, Rodd said:

I did not realize this....then again I did not plan this really....I had the one image and the Moon was up so my options were limited.  I figured why not.     I am still surprised that the F7.7 required less data than the F3.  Then again, we could be seeing the effects of F3 on the narrowband filters (Astrodon said I should be fine, but if 10% is lost (shifted), it could be the difference in a close call. 

Rodd                                                                                                                    3

The 106 collects only 66% of the photons collected by the 130. I'll say this really, really quietly so nobody notices: F ratio myth!

lly

 

Edited June 24, 2019 by ollypenrice

[Rodd]

19 minutes ago, ollypenrice said:

The 106 collects only 66% of the photons collected by the 130. I'll say this really, really quietly so nobody notices: F ratio myth!

lly

 

Ahhh....after many attempts I have finally managed to post data that speaks directly to the myth...and I didn't even realize it! I didn't even consider the myth...but of course, I suppose is direct evidence that the myth is real.  If Focal ratio controlled--the F3 image would be better (not intending to open a can here).  But, despite the difference between F3 and F7.7, a single inch of aperture was enough to cancel any imagined benefit of shooting at f3.  

Rodd

[andrew s]

My only contribution would be to point out that while total photons goes a d^2. S/N ratio goes as d (assuming photon limiting and not read noise etc.) so the gain due to aperture is less than might wish for.

Regards Andrew 

[Rodd]

8 minutes ago, andrew s said:

My only contribution would be to point out that while total photons goes a d^2. S/N ratio goes as d (assuming photon limiting and not read noise etc.) so the gain due to aperture is less than might wish for.

Regards Andrew 

why is the gain due to aperture less than what one might hope for (which I take to mean what theory says)? 

Rodd

[ollypenrice]

7 minutes ago, andrew s said:

My only contribution would be to point out that while total photons goes a d^2. S/N ratio goes as d (assuming photon limiting and not read noise etc.) so the gain due to aperture is less than might wish for.

Regards Andrew 

Doh! Partially thwarted!!!

lly

[andrew s]

4 minutes ago, Rodd said:

why is the gain due to aperture less than what one might hope for (which I take to mean what theory says)? 

Rodd

Simply the gain in photons goes d^2 which you thing gives you a parabolic bang for your buck. However due to photon statistics you only get a linear improvement in S/N as it improves as d. Both a theoretically correct

Regards Andrew 

[Rodd]

5 minutes ago, andrew s said:

Simply the gain in photons goes d^2 which you thing gives you a parabolic bang for your buck. However due to photon statistics you only get a linear improvement in S/N as it improves as d. Both a theoretically correct

Regards Andrew 

If something is theoretically correct, but not practically correct--what good is theory?  If there are 2 theories and only one yields the "correct" value....both can't be correct.   One, while theoretically correct, is in fact not correct in reality.

[andrew s]

10 minutes ago, Rodd said:

If something is theoretically correct, but not practically correct--what good is theory?  If there are 2 theories and only one yields the "correct" value....both can't be correct.   One, while theoretically correct, is in fact not correct in reality.

Both are both practically and theoretically correct. One refers to the mean number of photon collected and the other the variance in the number collected. Two statistics measuring two different things about a signal. Roughly...

Say you were doing photometey rather than imaging then the first gives you (after some calculations)  the magnitude of the star and the second how confident you are in that value. 

For an image the first tell you how intense it will be and the second how much noise it will have.

Regards Andrew 

 

[tomato]

Where’s Vlaiv when you need him?☺️

[Rodd]

46 minutes ago, andrew s said:

Both are both practically and theoretically correct. One refers to the mean number of photon collected and the other the variance in the number collected. Two statistics measuring two different things about a signal. Roughly...

Say you were doing photometey rather than imaging then the first gives you (after some calculations)  the magnitude of the star and the second how confident you are in that value. 

For an image the first tell you how intense it will be and the second how much noise it will have.

Regards Andrew 

 

Ahh--so you are saying--for the second (the first does not apply)  that the F7.7 will have more noise and more signal, and the F3 will have less noise and less signal......or is that reversed?  We're you being clever, or is there an important consideration here?

Rodd

[andrew s]

3 hours ago, Rodd said:

Ahh--so you are saying--for the second (the first does not apply)  that the F7.7 will have more noise and more signal, and the F3 will have less noise and less signal......or is that reversed?  We're you being clever, or is there an important consideration here?

Rodd

No. The larger aperture is better for collecting photons and will have a better S/N ratio. 

It is just a matter of how much. Collecting photons improves faster than S/N as aperture increases.

Regards Andrew 

[Stardust1]

Rodd, thanks for sharing. Would love to see if possible both full size images of the FSQ and TOA. I'm curious of the FOV of both scopes.