SW Quattro 10" vs 12" for imaging

[Northernlight]

Hi olly, uou tal

14 hours ago, ollypenrice said:

That makes perfect sense. I'll make my usual point that 'faster optics' and 'larger aperture' really mean the same thing but I take your point entirely.

Olly

Olly, do you mean the F-Ratio Myth - think i recall you saying something about for a given aperture the light hitting the sensor will be the same no matter what the F-Ratio ?  I could be wrong as it was a few years back. Never understood it then and still dont now 🤣 

Back when you told me, i thought to myself - so why do they even bother mentioning the F-Ratio if it doesn't matter, why not just state the aperture and why do scope manufacturers make the same aperture scopes with different F-Ratio ? Is it purely for different FOV ? 

Rich

 

[ollypenrice]

47 minutes ago, Northernlight said:

Hi olly, uou tal

Olly, do you mean the F-Ratio Myth - think i recall you saying something about for a given aperture the light hitting the sensor will be the same no matter what the F-Ratio ?  I could be wrong as it was a few years back. Never understood it then and still dont now 🤣 

Back when you told me, i thought to myself - so why do they even bother mentioning the F-Ratio if it doesn't matter, why not just state the aperture and why do scope manufacturers make the same aperture scopes with different F-Ratio ? Is it purely for different FOV ? 

Rich

 

It's not so much that F ratio doesn't matter but that it introduces confusion. If we compare two scopes of the same focal length, one F5 and one F10, then we can say for sure that the F5 is four times faster. Why can we say this? Because the area of the objective of the F5 must be four times bigger than the F10, so admitting four times as much light. But what happens if we take the F10 scope and put in a O.5 times reducer to make it F5. Does that make it four times faster? No, because the area of the objective has not been increased. There is no new light. All that happens is that you use the same amount of light to make a smaller, brighter picture of the object in the same time. You could also do this by binning 2x2 without the reducer.

When daytime photographers use F ratio to mean aperture (a regrettable habit!) they are able to do so because their focal length is fixed and they are using an iris to increase or decrease the area of aperture they are using. They are not varying their focal length, they really are varying their aperture. However, when we use focal reducers we are not changing our F ratio by adding more light, we are changing it by reducing our focal length. This is not at all equivalent.

When we look at a scope spec we're surely going to begin with focal length since that decides our field of view. Do we want a close up view of small targets or a widefield view of large ones? Once we've decided on an approximate FL, and only then, should we become interested in the F ratio. In fact, though, when the FL is fixed the F ratio and the aperture are describing the same thing, namely aperture, which his how daytime photographers think.

Olly

Edited August 23, 2020 by ollypenrice

[tomato]

Interesting discussion, as a UK imager I am firmly in the territory of knocking out quick but unexceptional images.

However, I have recently been persuaded by @gorann's RASA 8 and CMOS camera image of M31 that quick and good is possible, so much so that I have ordered said scope.  I am hoping  I can get close to replicating this kind of  result from the UK. 

 

[Northernlight]

The Rasa looks a great scope, and It certainly makes an interesting alternative to a small refractor or the likes of the Takahashi Espsilons. You get the same FOV but with much bigger optics to pull in the light. For me I already have a 102mm Frac, which is wide enough for me @571mm FL with the reduce. Whilst it's only a doublet, it's fine as i only ever image in narrowband, but the rasa would make an interesting replacement. At the moment i'm only interested in longer focal lengths 1m-1.2m as i want to go after the galaxies and smaller DSO.

The trouble with RASA scopes are that they forces you change your camera to a round body camera like the ZWO or QHY and in most instances a OSC, as it would be an absolute pain, running with single filter and having to change individual filters.

But im with you on the short sub lengths in the UK, as i just got my QSI camera back from repair and started imaging last night and with the smaller frac, evan running at F5.6 i tend to find that i need to do 15-20min subs to get a really nice result and in the UK, that can be problematic and last night being a good example, i only managed 5 subs before the clouds rolled in. So if i can get these subs down to 300-600s then it would be a huge help.

Rich.

 

 

[gorann]

5 hours ago, Northernlight said:

The Rasa looks a great scope, and It certainly makes an interesting alternative to a small refractor or the likes of the Takahashi Espsilons. You get the same FOV but with much bigger optics to pull in the light. For me I already have a 102mm Frac, which is wide enough for me @571mm FL with the reduce. Whilst it's only a doublet, it's fine as i only ever image in narrowband, but the rasa would make an interesting replacement. At the moment i'm only interested in longer focal lengths 1m-1.2m as i want to go after the galaxies and smaller DSO.

The trouble with RASA scopes are that they forces you change your camera to a round body camera like the ZWO or QHY and in most instances a OSC, as it would be an absolute pain, running with single filter and having to change individual filters.

But im with you on the short sub lengths in the UK, as i just got my QSI camera back from repair and started imaging last night and with the smaller frac, evan running at F5.6 i tend to find that i need to do 15-20min subs to get a really nice result and in the UK, that can be problematic and last night being a good example, i only managed 5 subs before the clouds rolled in. So if i can get these subs down to 300-600s then it would be a huge help.

Rich.

 

 

With the RASA8, which I bought second hand (apparently hardly used at all), I got the dedicated Baader filter drawer, so changing filters for NB imaging should not be much of a hazzle but I have yet to try it (I am waiting for the moon to get up before I change to my ASI1600mono and the filter drawer. I will report how it works. The ASI2600MC I now have on the RASA8 is probably sensitive enough to do NB even if is an OSC, but the chip distance is 17.5 mm so there is not enough space for a filter drawer with the RASA. Then of course you need the Baader special fast focal ratio filters at f/2. I have the Ha one but yhe Oiii is on back order until October (since Baader is making a new batch).

[ollypenrice]

8 hours ago, gorann said:

but yhe Oiii is on back order until October (since Baader is making a new batch).

Goran, be warned that if this back order is going to be like the one for the newer, narrower OIII filter in 1.25 mounted, you may be dead and buried before it actually appears! About every six months I receive news of further delay from TS...

15 hours ago, tomato said:

Interesting discussion, as a UK imager I am firmly in the territory of knocking out quick but unexceptional images.

However, I have recently been persuaded by @gorann's RASA 8 and CMOS camera image of M31 that quick and good is possible, so much so that I have ordered said scope.  I am hoping  I can get close to replicating this kind of  result from the UK. 

 

 

Like you I think Goran's setup looks very convincing. For me it's the small, tight stars which distinguish it from the images from most other ultra fast systems. As you say, here we have 'fast and good.' The core also suggests that there is no problem from well depth, which is restored by the deep stack.

Regarding the stars, while processing Yves Van den Broek's mega mosaic in Cygnus, I was struck by the tiny star sizes. His scope was an FSQ106. I have one myself (the earlier model) and I don't believe the small stars arise either from the optics or the focus. I can believe both might be a tad better than I achieve but the difference is well beyond that. It must be the camera, in my view. I'm not aware of this having been discussed on the forums but I haven't been looking.

This may contribute to the success of Goran's scope-camera combination.

Olly

[tomato]

10 hours ago, ollypenrice said:

Goran, be warned that if this back order is going to be like the one for the newer, narrower OIII filter in 1.25 mounted, you may be dead and buried before it actually appears! About every six months I receive news of further delay from TS...

Like you I think Goran's setup looks very convincing. For me it's the small, tight stars which distinguish it from the images from most other ultra fast systems. As you say, here we have 'fast and good.' The core also suggests that there is no problem from well depth, which is restored by the deep stack.

Regarding the stars, while processing Yves Van den Broek's mega mosaic in Cygnus, I was struck by the tiny star sizes. His scope was an FSQ106. I have one myself (the earlier model) and I don't believe the small stars arise either from the optics or the focus. I can believe both might be a tad better than I achieve but the difference is well beyond that. It must be the camera, in my view. I'm not aware of this having been discussed on the forums but I haven't been looking.

This may contribute to the success of Goran's scope-camera combination.

Olly

We might get an insight into this Olly, as I’ll be using an Atik Horizon II OSC, same size pixels as the 2600, but the Panasonic sensor.
 

The RASA is on the Mesu and ready to go, the Esprits had to come off as I don’t have nearly enough counterweights, but true to form, Storm Francis is about to blow in...

[Northernlight]

so putting aside size, weight and mount requirements - if i were to choose the 12" F4 over the 10" F4,  i'm assuming that this will help slightly to reduce my Sub lengths further, given that it grasps 44% more light than the 10" model.  But the big question, is how much shorter can i make my subs  44%, 20% 10% etc ? just rougly trying to decide if a 12" is worth the extra cost and effort, i.e. will it pay dividens in terms of reducing aquisition time.

I assuming that the EQ8 should be enough to handle the 12" and combining with my QSI 683 with built in OAG should help guiding as it will rule out any flex issues. I would also intend to reinforce the tube rings with a losmand bar on the top and bottom.

[SamAndrew]

9 hours ago, Northernlight said:

so putting aside size, weight and mount requirements - if i were to choose the 12" F4 over the 10" F4,  i'm assuming that this will help slightly to reduce my Sub lengths further, given that it grasps 44% more light than the 10" model.  But the big question, is how much shorter can i make my subs  44%, 20% 10% etc ? just rougly trying to decide if a 12" is worth the extra cost and effort, i.e. will it pay dividens in terms of reducing aquisition time.

This isn't how it works; sub lengths will be the same length if the F ratio is the same, the 12" has a longer focal length, so a smaller area of the sky is projected onto the camera which offsets the increased light gathering.

I had a 10" but now use the 8" as I wanted a slightly larger field of view, and the 8" with an ASI 1600mm gives 1" per pixel. 

[Northernlight]

Sam, really not sure about that   - I can't believe that a larger aperture has no bearing or other than focal length. If the larger aperture is gathering more light, i would  have thought it would concentrate more photons onto the camera chip  or to saturate the pixels on the chip quicker.

[SamAndrew]

F ratio is the "speed"; if you want to reduce exposure time you need to increase the speed, the same way you would for a normal camera lens. To increase speed you need to add aperture while maintaining the same focal length.

A 12" scope with 1000mm focal length would be 44% quicker than a 10" scope with 1000mm focal length. The 12" scope would be F3.3 in this case

[alacant]

5 hours ago, SamAndrew said:

sub lengths will be the same length if the F ratio is the same, the 12

Nope

If the object fits into the camera chip, the 12" will record it in about half the time.

HTH

 

[Northernlight]

yeah this is my understanding, if the aperture is larger for the same focal ratio, then the larger aperture will reduce time needed to collect the same amount of data

[alacant]

6 minutes ago, Northernlight said:

if the aperture is larger for the same focal ratio

Even better... If the aperture is larger for the same focal ratio irrespective of focal ratio....

On my dslr for example, a 12" f5 would take about half the time to produce an image of m33 of comprable size to that which a 10" f4 produces. A 10" f6 would take the same time as the 10" f4.

HTH

 

[Northernlight]

Hi Alacant, yes this was my understanding -  lets use a small galaxy as a good example - assuming that the galaxy fits into the fov of the sensor for both scopes then the 12" would be a lost faster and have the bonus of a slightly larger image scale than the 10" - so it seems like a no brainer for the fickle english weather

Well, it sounds great but as many have said  you need a high end mount. whilst i do have an EQ8 i do worry, that i might end up throwing out more subs

So whilst i would like a 12" - I'm going to let common sense prevail and go with a 10", but i also spoke to Tommy at Lacerta and they should have their new scope ready by the end of the year - so should be worth the wait.

 

 

Edited August 26, 2020 by Northernlight

[SamAndrew]

3 hours ago, alacant said:

Nope

If the object fits into the camera chip, the 12" will record it in about half the time.

HTH

 

No because it's spread over more pixels, so still takes the same amount of time to get the same signal to noise ratio. 

[Northernlight]

Well, i'm in agreement with alacant . We need a 3rd opinion on this  - Olly where are you lol

 

[alacant]

10 minutes ago, Northernlight said:

We need a 3rd opinion on this

@vlaiv maybe able to do it in numbers for you:)

 

[alacant]

20 minutes ago, SamAndrew said:

the same amount of time to get the same signal to noise ratio. 

To get the images the same size, you would have to enlarge the 10" image. Does that help explain?

In the end, the only way to prove it is to do it. Not necessarily with 10" and 12" telescopes though!

Cheers

 

[vlaiv]

Ah the old speed thing again

You are both right. Really.

It depends on how one looks at things and what remains constant.

Speed makes sense when one keeps pixel size constant. Mind you - with AP speed does not increase like in regular daytime photography because we operate in "photon starvation" mode - very few photons reach us and there are plenty of noise sources that impact result - some of them are not related to aperture (dark current noise and read noise) and some are (LP noise and shot noise). Depending on ratio of these - we will approach "speed rule" from daytime photography - that F/stop up/down exposure up/down thingy that I never remember

Speed makes no sense at all if one keeps sampling rate constant. This either means using different pixel size camera with other scope or using binning to some extent.

If one keeps sampling rate fixed then aperture is what determines "speed" rather than F/speed or F/ratio.

Is it worth getting 12" over 10" - depends if you have a choice of camera to pair either with and what do you want in terms of sampling rate and FOV.

 

[SamAndrew]

Keeping the sampling rate the same means keeping the focal length the same. Adding apature while maining the same focal length will reduce your sub length. Adding appature and focal length in the same ratio (e.g. keeping the f ratio the same) results in a bigger image of the object you're capturing, but the sub length will be the same.

[vlaiv]

31 minutes ago, SamAndrew said:

Keeping the sampling rate the same means keeping the focal length the same. Adding apature while maining the same focal length will reduce your sub length. Adding appature and focal length in the same ratio (e.g. keeping the f ratio the same) results in a bigger image of the object you're capturing, but the sub length will be the same.

Not necessarily.

F/ratio is ratio of two quantities - aperture and focal length.

Sampling rate is again ratio of two quantities - pixel size and focal length.

Keeping sampling rate the same means keeping ratio of pixel size and focal length the same.

Change focal length and pixel size and you will maintain your sampling rate.

2.4um pixel at 600mm is equal to 3.8um pixel at 950mm - here we changed focal length and we kept sampling rate the same.

This is very valuable thing with integer multiplications of pixel size - as this is easily done either as hardware or software binning (only difference between two bin methods being amount of resulting read noise).

For this reason F/ratio of telescope won't tell you how fast it is - you did not say what camera you'll be using. It is the reason why "slow" scope can be faster than "fast" scope - even with same camera (if you bin in case of slow scope to maintain or exceed sampling rate and aperture of slow scope is greater).

Three variables play part in all of this - pixel size, focal length and aperture size.

You can fix any two of them and play with third one to get different results (pixel size can be changed either by binning or by changing camera, focal length can be changed either by using barlow or focal reducer and aperture can be changed by using aperture stop. Alternatively aperture and focal length can be changed together - but not necessarily by same amount - if you change the scope).

When choosing a telescope / camera  combination - I have found that it is better not to look at F/ratio of telescope first, but rather set resolution and FOV that you want to work with, then choose camera (or use one you have) and from that determine focal length you need.

In the end - get the largest aperture with that focal length that will both - illuminate with corrected field whole sensor that you'll be using and that you can afford and mount.

[Northernlight]

My camera is a QSI 683 with 5.4 micron pixels,  so it's a fairly decent match to both the 10" & 12" 

10" - 1.11" per pixel   /  12" - 0.93" per pixel

[vlaiv]

Looking at your signature - you seem to have 8" GSO RC?

What sort of FWHM do you get in your subs (in arc seconds) with that scope and said camera on EQ8 mount?

 

[ollypenrice]

45 minutes ago, vlaiv said:

Not necessarily.

F/ratio is ratio of two quantities - aperture and focal length.

Sampling rate is again ratio of two quantities - pixel size and focal length.

Keeping sampling rate the same means keeping ratio of pixel size and focal length the same.

Change focal length and pixel size and you will maintain your sampling rate.

2.4um pixel at 600mm is equal to 3.8um pixel at 950mm - here we changed focal length and we kept sampling rate the same.

This is very valuable thing with integer multiplications of pixel size - as this is easily done either as hardware or software binning (only difference between two bin methods being amount of resulting read noise).

For this reason F/ratio of telescope won't tell you how fast it is - you did not say what camera you'll be using. It is the reason why "slow" scope can be faster than "fast" scope - even with same camera (if you bin in case of slow scope to maintain or exceed sampling rate and aperture of slow scope is greater).

Three variables play part in all of this - pixel size, focal length and aperture size.

You can fix any two of them and play with third one to get different results (pixel size can be changed either by binning or by changing camera, focal length can be changed either by using barlow or focal reducer and aperture can be changed by using aperture stop. Alternatively aperture and focal length can be changed together - but not necessarily by same amount - if you change the scope).

When choosing a telescope / camera  combination - I have found that it is better not to look at F/ratio of telescope first, but rather set resolution and FOV that you want to work with, then choose camera (or use one you have) and from that determine focal length you need.

In the end - get the largest aperture with that focal length that will both - illuminate with corrected field whole sensor that you'll be using and that you can afford and mount.

Hooray! 😁

Here's another spanner for the works: you do have control over image size simply by downsizing an object image from a longer FL/larger aperture to the size of the same object from the smaller scope. That way you get the extra light concentrated onto the same number of pixels - ish. It may not be as good as real binning. Vlaiv knows more about how to calculate this correctly than I do but I know from experience that reducing the display size of an image has a massive effect on its signal to noise ratio when it comes to stretching. For a couple of years I was working with a big scope at an unrealistic 0.6"PP. Not to worry, we never tried to present an image at 100%. 66% still gave a big image and the smoothness was the benefit from the big aperture/generous supply of light. This is a very close crop from an Eagle Nebula image shot by the oversampling rig but reduced. Note that it's essentially a broadband LRGB image enhaced by Ha and OIII, not a NB-only image as produced by those cheats at Hubble. 🤣 Yes, this is a joke!

Olly