F ratio - again!

[vlaiv]

25 minutes ago, wimvb said:

Yes,

but (p/F)^2 is still the same for all three. Noah and Jemima adjusted the F ratio, each in their own way. The third person adjusted the second parameter: pixel size.

We agree on that, problem I was trying to point out is that people often don't think of aperture at resolution and only think in terms of F/ratio - and conclude that "faster F/ratio" scope is better option than "slower F/ratio" scope for imaging.

This is why I pointed out your sentence "But they both can get away with shorter exposure times because of the gain in F-ratio".

[Jessun]

-

 

 

 

 

Edited May 15, 2021 by Jessun

[Icesheet]

I wanted to resurrect this discussion as I have a real world example of this and was curious as to what’s best to do.

I have limited time to image, weather, family, work etc. I have an Esprit 100ED and ASI1600mm but have became frustrated with partial datasets due to unpredictable weather and I always felt I wanted a bit more in FOV. 
 

A Rasa 8/ 2600MC combo seems to produce great images with relatively short integration times (particularly from @gorann). So, when a good deal on a 2600MC appeared I snapped it up, with the intention of swapping out the Esprit for a RASA at some point. Then I learned about the Starizona Apex-L. It would give me a slightly wider, but comparable FOV to the RASA at f3.6. Then I stumbled upon this f-ratio myth and realised I already had an other option available to me and that’s the Esprit at native focal length with a modded Canon 6D (picked up on a whim last year). See snaps from astronomy tools below for FOV comparisons 👇. 
 

Rasa combo = 1.94”/pixel

Esprit & Apex = 2.17”/pixel

Esprit & 6D = 2.45”/pixel

 

Obviously, the RASA is the best option on paper. My maths (please correct me if I’m wrong) shows the Rasa is 3.2x more efficient than the Esprit and Apex and 2.5x more efficient than the Esprit and 6D. 
 

I thought doing this would make the choice clear. I suppose it does but the results, particularly with the 6D have cast doubts in my mind. For instance if I imaged a target for 4 hours with all systems would there likely be a noticeable difference in the images when considering SNR and image resolution? Would I actually be better saving my money by not buying a reducer or a RASA and actually put money in my pocket by selling the 2600MC?

I think I’m actually more confused than I was before 😆

What would you do??
 

Edit to add: I was really surprised at the results using the Esprit natively with the 6D. The bigger pixels of the 6D made more of a difference than the reduction of f ratio in the Esprit! I know it’s not as simple as that as lots of other things will come into play. However, it’s showed me there is more than one way to skin a cat and it doesn’t necessarily need to empty your wallet further 😂

 

Edited August 19, 2021 by Icesheet

[michael.h.f.wilkinson]

38 minutes ago, Icesheet said:

I wanted to resurrect this discussion as I have a real world example of this and was curious as to what’s best to do.

I have limited time to image, weather, family, work etc. I have an Esprit 100ED and ASI1600mm but have became frustrated with partial datasets due to unpredictable weather and I always felt I wanted a bit more in FOV. 
 

A Rasa 8/ 2600MC combo seems to produce great images with relatively short integration times (particularly from @gorann). So, when a good deal on a 2600MC appeared I snapped it up, with the intention of swapping out the Esprit for a RASA at some point. Then I learned about the Starizona Apex-L. It would give me a slightly wider, but comparable FOV to the RASA at f3.6. Then I stumbled upon this f-ratio myth and realised I already had an other option available to me and that’s the Esprit at native focal length with a modded Canon 6D (picked up on a whim last year). See snaps from astronomy tools below for FOV comparisons 👇. 
 

Rasa combo = 1.94”/pixel

Esprit & Apex = 2.17”/pixel

Esprit & 6D = 2.45”/pixel

 

Obviously, the RASA is the best option on paper. My maths (please correct me if I’m wrong) shows the Rasa is 3.2x more efficient than the Esprit and Apex and 2.5x more efficient than the Esprit and 6D. 
 

I thought doing this would make the choice clear. I suppose it does but the results, particularly with the 6D have cast doubts in my mind. For instance if I imaged a target for 4 hours with all systems would there likely be a noticeable difference in the images when considering SNR and image resolution? Would I actually be better saving my money by not buying a reducer or a RASA and actually put money in my pocket by selling the 2600MC?

I think I’m actually more confused than I was before 😆

What would you do??
 

 

 

If we assume the fields of view are similar, quite clearly the RASA will be gathering roughly 4x the the number of photons. The 6D will most likely have a lower QE than the ASI2600, which may offset the slight increase in photons per pixel, compared to a camera with smaller pixels. To match the performance of the RASA at 4 hours integration, you would need to nearly quadruple the exposure time on the Esprit.

[Icesheet]

46 minutes ago, michael.h.f.wilkinson said:

If we assume the fields of view are similar, quite clearly the RASA will be gathering roughly 4x the the number of photons. The 6D will most likely have a lower QE than the ASI2600, which may offset the slight increase in photons per pixel, compared to a camera with smaller pixels. To match the performance of the RASA at 4 hours integration, you would need to nearly quadruple the exposure time on the Esprit.

Yes, I didn’t make it clear enough. On paper the RASA is better in all aspects. Although based on my calculations using @wimvb formulas it wouldn’t need 4x exposure time, it’s more like 3. I presume this is also partly because the Esprit options are under-sampling somewhat. This 6D is modded but I take your comment on board, it’s not going to be as sensitive as the 2600MC. 
 

My thinking here is what difference will I see in my final images from typical night of imaging and would I get the value for the extra money I pay out? I’m sure there is diminishing returns in SNR as integration time increases too? So that 3x extra exposure time and resolution may not translate completely to the final image. Having said that it will have a greater impact on those nights weather cuts a session even shorter than normal which was part of the reason I looked at the RASA in the first place 🙈

Been an interesting exercise for me and I will definitely be considering these things more in the future. The Esprit offers me a bit more flexibility as (cough, cough) I’m partial to popping an eye piece in now and again

Edited August 19, 2021 by Icesheet

[wimvb]

39 minutes ago, Icesheet said:

I presume this is also partly because the Esprit options are under-sampling somewhat.

If you undersample, aperture is your only measure for light gathering. If all the light from an object (star) is captured by one pixel, regardless of which size that pixel is, then the light falling on that pixel in a unit of time, is proportional to the area of the scope aperture. This is the most extreme case of undersampling. In the most extreme case of oversampling, a uniform patch of sky, the formula I gave holds true. For real stars and details in galaxies, the truth lies somewhere in the middle of course, but for most nebulae, you can apply the formula I gave.

But this is only the geometry of photon capture. In a real system you need to factor in quantum efficiency/sensitivity and noise. As for Göran's results, he has the advantage of imaging from a very dark site (mag 21.5 or better). His images have no light pollution, other than from weak natural sources, and therefore have very low noise to begin with. Jerry Lodriguss wrote an article about the effect of light pollution, and came to the conclusion that for every unit of magnitude you lose due to LP, you need to increase your total integration time by a factor of 2.5. In other words, if Göran were to move his RASA/ASI2600 to my site (mag 20.5), he would need 2.5 times longer integration time to get the same final result. Imaging from a dark site is more effective than getting a more sensitive camera or buying aperture.

[Icesheet]

Thanks for the info! Yeah my skies are similar to yours, so I realised I wouldn’t necessarily achieve Göran like results. In fact I doubt I could regardless of how long I integrate and how good the skies are. Improving my processing would probably make a bigger difference than all of this.

 

23 minutes ago, wimvb said:

For real stars and details in galaxies, the truth lies somewhere in the middle of course, but for most nebulae, you can apply the formula I gave.

I’m curious then. Do you think the formula applies here and do you arrive at the same result as me? I did expect the RASA to be even further ahead than it was but there’s every chance my math is wrong!

[ollypenrice]

I wouldn't underestimate the contribution of a cooled astro camera...

Olly

[michael.h.f.wilkinson]

2 hours ago, Icesheet said:

Yes, I didn’t make it clear enough. On paper the RASA is better in all aspects. Although based on my calculations using @wimvb formulas it wouldn’t need 4x exposure time, it’s more like 3. I presume this is also partly because the Esprit options are under-sampling somewhat. This 6D is modded but I take your comment on board, it’s not going to be as sensitive as the 2600MC. 
 

My thinking here is what difference will I see in my final images from typical night of imaging and would I get the value for the extra money I pay out? I’m sure there is diminishing returns in SNR as integration time increases too? So that 3x extra exposure time and resolution may not translate completely to the final image. Having said that it will have a greater impact on those nights weather cuts a session even shorter than normal which was part of the reason I looked at the RASA in the first place 🙈

Been an interesting exercise for me and I will definitely be considering these things more in the future. The Esprit offers me a bit more flexibility as (cough, cough) I’m partial to popping an eye piece in now and again

I notice clear improvements up to 13h of exposure time with my Meade SN6 Schmidt-Newton (6" F/5) and (non-cooled) ASI183MC on M27 using an Optolong L-eNhance filter which gives quite a dark background. I am certainly interested in getting a RASA 8, but as stop-gap will go for a Starizona Night Owl 0.4x reducer on my C8, giving me roughly the same focal length at F/4.

[Peter Drew]

lly, "Devil's advocate".   🙂

Edited August 19, 2021 by Peter Drew
typo

[wimvb]

52 minutes ago, ollypenrice said:

I wouldn't underestimate the contribution of a cooled astro camera...

Olly

Yes, low noise AND high sensitivity.

[Icesheet]

44 minutes ago, michael.h.f.wilkinson said:

I notice clear improvements up to 13h of exposure time with my Meade SN6 Schmidt-Newton (6" F/5) and (non-cooled) ASI183MC on M27 using an Optolong L-eNhance filter which gives quite a dark background. I am certainly interested in getting a RASA 8, but as stop-gap will go for a Starizona Night Owl 0.4x reducer on my C8, giving me roughly the same focal length at F/4.

I think I’m heading towards the RASA 8 too. Just trying to find ways to convince myself otherwise and save another hit to my pocket!

[wimvb]

1 minute ago, Icesheet said:

Just trying to find ways to convince myself otherwise and save another hit to my pocket!

Doomed to fail. 😉

[gorann]

Sorry if I am partly responsible for this RASA8 hype, but it is an amazing scope for wilde field imaging and my feeling is that it shows that aperture rules and cannot be overestimated. But then I am, as @wimvb says, at a dark site. I could in no way get as deep into faint nebulosity with my Esprit 150, but then I could never get as close to the finest detials with my RASA8 as with my Esprit 150 or 14" SCT, so it is all about what you are aiming for and if you want it all you need more than one scope.

Edited August 19, 2021 by gorann

[Icesheet]

11 hours ago, gorann said:

Sorry if I am partly responsible for this RASA8 hype

Celestron should have you on commission! I’m pretty sure your images have resulted in a good few RASA purchases!

[gorann]

4 hours ago, Icesheet said:

Celestron should have you on commission! I’m pretty sure your images have resulted in a good few RASA purchases!

I even convinced myself to buy a second one and I got no discount (but I didn't ask🥴)

[wimvb]

Like I said before, any attempt to save money in AP is doomed to fail.