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Image Scale vs Focal Length - camera and scope FOV combos

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#1 santafe retiree

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Posted 22 May 2019 - 10:14 AM

I currently image with an AT 8RC and a ZWO ASI071. At native FL of 1625mm my image scale is .61 arc seconds/px. With a 0.67 reducer the FL becomes 1088mm and the image scale is .91.

 

I find both of these focal lengths to be very pleasing and give me the framing that I like the best.  I also have an AT65EDQ for wide field imaging.

 

The 8RC has its challenges (collimation and field curvature) and I am considering a Skywatcher Esprit 150 ED. Up to now my hesitation has been that I would lose the image scale/framing that the longer native FL of the 8RC gives me.

 

I started looking at different cameras and came across the ZWO ASI183.  Using that camera with the Esprit 150 would give me an image scale of .47 with a pixel size of 2.4mu and framing very close to the framing of the 8RC/ASI071combo.

 

Using the ASI071 with the SW150 would give me an image scale of .94 with a framing very close to the 8RC/071/0.67 reducer combo.

 

Below is a screenshot of the FOV that I could expect from these various camera/scope combinations.

 

I realize that the ASI183 has much smaller pixels than the ASI071 and that the Esprit150 has 2” less of aperture than the 8RC, both of which mean longer exposures.  But my CEM60 regularly gets less than 0.5 RMS on exposures longer than 6 minutes so I am not as concerned as I might otherwise be.

 

I would end up with the ASI071 and the ASI183 and the SW150.  I would use the 183/150 combo to replace the 8RC at native FL and the 071/150 combo to replace the 8RC with reducer.

 

Am I missing something here or would I end up with a good replacement?

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Edited by santafe retiree, 22 May 2019 - 10:23 AM.


#2 OldManSky

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Posted 22 May 2019 - 10:40 AM

I got the ASI183MM Pro a few months ago, and have been very pleased with the results.  Though it has smaller pixels, it has a nice, high QE.

Just a note -- a smaller aperture won't mean longer exposures.  Focal ratio is what matters there.  At f/7, the 150ED will give you *shorter* exposures to get to the same signal level than your 8RC.



#3 rgsalinger

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Posted 22 May 2019 - 10:57 AM

A few things occur to me that you might want to think about:

 

1. The 071 is a pretty big chip. I suspect that you will always need a flattener to use it. I can't use mine on my AP155 F7 without one. I can use a smaller chip, my ASI1600. If that reducer is also a flattener then you may or may not be OK with the results. These F/R's really vary in quality and you seem to know that as the image scale goes up the problems multiply. 

 

2. There's no reason to image at .47 arc seconds per pixel unless you have 1.5 arc second or better seeing. I suggest binning will get you a better result and will reduce your exposures by a factor of 4. So, again, I'm not sure that you really need that reducer (but you will still need a flattener IMHO for the 071).

 

3. I assume that your thinking of using the color version. If not, then research the back focus requirements carefully and I think that you're going to need an OAG for best results with that long tube. 

 

4. The ASI has less 11 stops of dynamic range at unity gain. That camera is really designed for guys with very wide fields of view - short focal lengths. Still, framing is framing. And you're going to be taking very short exposures with that camera and stacking them.  

 

Rgrds-Ross


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#4 TinySpeck

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Posted 22 May 2019 - 12:05 PM

I'd add to @OldManSky's advice: lower focal ratio does indeed lead to better light-gathering for extended objects like nebulae & galaxies, but aperture provides better light-gathering for star brightness and faint star detection.  So it depends on what you're after.


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#5 santafe retiree

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Posted 22 May 2019 - 12:20 PM

I'd add to @OldManSky's advice: lower focal ratio does indeed lead to better light-gathering for extended objects like nebulae & galaxies, but aperture provides better light-gathering for star brightness and faint star detection.  So it depends on what you're after.

Gerrit, Thanks for your addition to the conversation -- I always believed that given the same focal length and F ratio, the larger aperture will always win whether for visual or AP -- I know it is an oft debated topic but if FL and f/ are the same, then the largest light bucket will prevail 


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#6 santafe retiree

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Posted 22 May 2019 - 12:26 PM

A few things occur to me that you might want to think about:

 

1. The 071 is a pretty big chip. I suspect that you will always need a flattener to use it. I can't use mine on my AP155 F7 without one. I can use a smaller chip, my ASI1600. If that reducer is also a flattener then you may or may not be OK with the results. These F/R's really vary in quality and you seem to know that as the image scale goes up the problems multiply. 

 

2. There's no reason to image at .47 arc seconds per pixel unless you have 1.5 arc second or better seeing. I suggest binning will get you a better result and will reduce your exposures by a factor of 4. So, again, I'm not sure that you really need that reducer (but you will still need a flattener IMHO for the 071).

 

Rgrds-Ross

One of the reasons I like the SW 150ED is that it comes with a dedicated flattener specific to the scope.  The FR I currently have is from Astro-Physics and I use it solely on the 8RC.  It is not a flattener and the off axis astigmatism I am currently getting is definitely due to the scope but it may also be contributed to by the FR.  I would NOT be using that FR on the SW.

 

I usually image only if my seeing is 2" or better but the idea of binning is something I had not considered.


Edited by santafe retiree, 22 May 2019 - 12:27 PM.


#7 Joe G

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Posted 22 May 2019 - 01:15 PM

Gerrit, Thanks for your addition to the conversation -- I always believed that given the same focal length and F ratio, the larger aperture will always win whether for visual or AP -- I know it is an oft debated topic but if FL and f/ are the same, then the largest light bucket will prevail 

They are not the same focal ratio.  Your AT has a focal length of 1625mm and an aperture of 200mm for a focal ratio of F8.  The SW 150 has a focal length of 1050mm and an aperture of 150mm for a focal ratio of F7.

 

The SW will gather more light faster.  You would need 30% longer exposure with the AT to be equivalent -- (8/7)^2.

 

Personally I doubt you will notice the difference in aperture in your photos. 

 

The advantage of the refractor with the smaller pixel camera is you get similar image scale without the need to worry about collimation and field curvature you mentioned.

 

I have the SW carbon fiber version of the Esprit150.  It is a very nice scope and the included field flattener does indeed flatten the field over a full frame sensor so it would be no problem with your ASI 071.


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#8 OldManSky

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Posted 22 May 2019 - 01:16 PM

Gerrit, Thanks for your addition to the conversation -- I always believed that given the same focal length and F ratio, the larger aperture will always win whether for visual or AP -- I know it is an oft debated topic but if FL and f/ are the same, then the largest light bucket will prevail 

The larger light bucket will prevail in resolution, and will put more energy into true point-sources (stars in good seeing)...but not for extended objects :)

 

At any rate, isn't your RC8 f/8 while the 150 is f/7?  And the focal length is also different?



#9 bmhjr

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Posted 22 May 2019 - 02:00 PM

 .... and the 071/150 combo to replace the 8RC with reducer.

 

This combo would put the image scale roughly the same, but the bigger aperture wins with the 8RC.  Either way you should have some nice options.


Edited by bmhjr, 22 May 2019 - 02:01 PM.


#10 santafe retiree

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Posted 22 May 2019 - 02:13 PM

The larger light bucket will prevail in resolution, and will put more energy into true point-sources (stars in good seeing)...but not for extended objects smile.gif

 

At any rate, isn't your RC8 f/8 while the 150 is f/7?  And the focal length is also different?

The comparisons would be:

 

1) For the narrow FOV:

 

RC8/ASI071 vs SW150/ASI183

 

1625mm/f8/4.78um vs 1050mm/f6.9/2.4um

 

Here, the RC8 seems to win due to larger aperture, longer FL, and larger pixels even tough the SW has faster f/

 

then --

 

2) For the wider FOV

 

RC8/FR 0.67/ASI071 vs SW150/ASI071 for the wider FOV

 

1088mm/f5.36/4.78um vs 1050mm/f6.9/4.78um

 

Here the RC8 seems to win again due to larger aperture and faster f/

 

Hence the question, how much am I giving up if I swap out the RC8 for the SW150/ASI183 ???

 

Or am I victim of faulty reasoning/misconceptions about the optical principles involved?

 

Regards,

 

Tom 


Edited by santafe retiree, 22 May 2019 - 02:18 PM.


#11 santafe retiree

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Posted 22 May 2019 - 02:17 PM

They are not the same focal ratio.  Your AT has a focal length of 1625mm and an aperture of 200mm for a focal ratio of F8.  The SW 150 has a focal length of 1050mm and an aperture of 150mm for a focal ratio of F7.

 

The SW will gather more light faster.  You would need 30% longer exposure with the AT to be equivalent -- (8/7)^2.

 

Personally I doubt you will notice the difference in aperture in your photos. 

 

The advantage of the refractor with the smaller pixel camera is you get similar image scale without the need to worry about collimation and field curvature you mentioned.

 

I have the SW carbon fiber version of the Esprit150.  It is a very nice scope and the included field flattener does indeed flatten the field over a full frame sensor so it would be no problem with your ASI 071.

Joe,  I replied to OldMan Sky before I saw your post -- I would appreciate your thoughts on my post #10 above -- Regards, Tom 



#12 bmhjr

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Posted 22 May 2019 - 02:21 PM

Jon and Tim have provided a nice, simple equation to calculate a "performance factor" that allows you to compare the difference between various combinations of scopes, pixels, QE

 

https://www.cloudyni...-2#entry9061068

 

Maybe this will help with seeing the differences.

 

Bill


Edited by bmhjr, 22 May 2019 - 02:21 PM.

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#13 santafe retiree

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Posted 22 May 2019 - 02:29 PM

Jon and Tim have provided a nice, simple equation to calculate a "performance factor" that allows you to compare the difference between various combinations of scopes, pixels, QE

 

https://www.cloudyni...-2#entry9061068

 

Maybe this will help with seeing the differences.

Bill,

 

I truly appreciate the reference because I think it will most likely answer myquestion -- but being "maths challenged" (polite term for me being functionally math illiterate) can you explain the A^2 * S^2 part of the equation

 

 

P = A^2 * S^2 * q

 

Regards,

 

Tom


Edited by santafe retiree, 22 May 2019 - 02:33 PM.


#14 bmhjr

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Posted 22 May 2019 - 02:33 PM

Bill,

 

I truly appreciate the reference -- but being "maths challenged" (polite term for me being functionally math illiterate) can you explain the A^2 * S^2 part of the equation

 

 

P = A^2 * S^2 * q

 

Regards,

 

Tom

 

Performance = Aperture squared * Imagescale squared * QE.   

 

The aperture is in mm and image scale in arcsec/pixel.   The result will give you a number that you can compare between different combinations and see how large of a difference certain parameters may or may not matter.


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#15 santafe retiree

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Posted 22 May 2019 - 03:04 PM

Performance = Aperture squared * Imagescale squared * QE.   

 

The aperture is in mm and image scale in arcsec/pixel.   The result will give you a number that you can compare between different combinations and see how large of a difference certain parameters may or may not matter.

Bill,

 

Again,many thanks!  The references to Jon Rista's posts and the explanation of the maths terms you offered are VERY helpful!

 

So for the 8RC/071 combo:

 

(200 ^)  * (.61 ^)  * .50 = 7442

 

and the SW150/183 combo:

 

(150 ^) * (.47 ^) * .84 =  4175

 

I am assuming the bigger number means "better" ????

 

Regards,

 

Tom



#16 bmhjr

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Posted 22 May 2019 - 03:05 PM

You got it.  One caveat, if you have a central obstruction in the OTA, you may want to reduce the aperture by the appropriate percentage so it keeps the comparison equal.  


Edited by bmhjr, 22 May 2019 - 03:12 PM.

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#17 Joe G

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Posted 22 May 2019 - 03:10 PM

Joe,  I replied to OldMan Sky before I saw your post -- I would appreciate your thoughts on my post #10 above -- Regards, Tom 

Tom,

 

The link Bill gives above is good in explaining the differences, much better than I can do.

 

But in practice, most imagers that prefer a "longer" focal length refractor do so for more basic reasons that you alluded to in your original post, i.e. collimation, FC, etc.

 

It used to be with larger pixel cameras that to get higher resolution image scale you needed to bump up focal length, and to do that you needed to go to a RC or SCT.  For a long time the RC scopes were very expensive and now with the AT scopes the price point has been brought down.  But many don't like to fuss with the negatives of the RC/SCT type scopes.

 

What has more recently changed is there are now smaller pixel cameras which coupled with "regular" refractors give the high resolution image scale tradeoff you are contemplating.  I say "regular" because some might not consider an expensive 6" refractor "regular."

 

But the ASI 183 with its small pixels now allows someone with a longish focal length refractor to get the image scale that you could only obtain with your much longer AT 8RC.  So now a 150mm refractor can be useful for the small stuff, at least some of it.

 

There are differences between the two cameras, but in general, they both have relatively low read noise versus the older CCD cameras.  My guess is that in practice, by taking a "normal" amount of subexposures and stacking, etc, the difference in setups is likely marginal in the sense that you can get good results either way, especially in Santa Fe skies.

 

The deciding factor in my view is the hassle of the RC versus the refractor.  How well does the RC perform across the larger ASI 071 chip?  I know the SW 150 will have no problems giving a nice flat field with that chip and certainly will have no issues with the smaller ASI 183.  As you pointed out in your original post, the RC setup gives an image scale of 0.61"/pix versus 0.47"/pix for the refractor.  The FOVs are close with the RC setup slightly wider.  Are you happy with your RC/ASI 071 images?  If the refractor mitigates any "unhappiness" then maybe it is worth it to you.  If your images are good across the frame with the RC setup then maybe not?

 

Do you just want a large, nice refractor?

 

The refractor also makes for a great visual scope if you like visual.

 

Hope that helps.

 

Joe


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#18 Joe G

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Posted 22 May 2019 - 03:19 PM

One last gasp...

 

I assume you live in the hills above town and have decently dark skies right in your backyard?  You have a big Dob and a small refractor.

 

We have the same mounts.  I'm not a refractor versus Dob snob, but I do like the view through a 5" or 6" refractor versus the smaller ones visually.  There really is more pop to the planets and the brighter stuff, all that the refractor lovers appreciate.

 

So if that factors in I can see how that might sway you.

 

The Esprit refractors are excellent. 


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#19 bmhjr

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Posted 22 May 2019 - 03:33 PM

One last gasp...

 

I assume you live in the hills above town and have decently dark skies right in your backyard?  You have a big Dob and a small refractor.

 

We have the same mounts.  I'm not a refractor versus Dob snob, but I do like the view through a 5" or 6" refractor versus the smaller ones visually.  There really is more pop to the planets and the brighter stuff, all that the refractor lovers appreciate.

 

So if that factors in I can see how that might sway you.

 

The Esprit refractors are excellent. 

 

I agree with this.  Personally, I like big refractors and hope to get one in the not too distant future.  No clue what camera to put on it but I plan to look through it too.  I think you have a solid choice.


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#20 santafe retiree

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Posted 22 May 2019 - 04:14 PM

Bill,

 

Again,many thanks!  The references to Jon Rista's posts and the explanation of the maths terms you offered are VERY helpful!

 

So for the 8RC/071 combo:

 

(200 ^)  * (.61 ^)  * .50 = 7442

 

and the SW150/183 combo:

 

(150 ^) * (.47 ^) * .84 =  4175

 

I am assuming the bigger number means "better" ????

 

Regards,

 

Tom

So if I am interpreting this result correctly, the 8RC/071 combo collects almost twice as much light as the SW150/183 combo in the same time period?  Are my imaging sessions really going to be twice as long with the SW150/183 rig?   Yikes! 



#21 OldManSky

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Posted 22 May 2019 - 04:56 PM

So if I am interpreting this result correctly, the 8RC/071 combo collects almost twice as much light as the SW150/183 combo in the same time period?  Are my imaging sessions really going to be twice as long with the SW150/183 rig?   Yikes! 

Probably not.

Jon's post (from which you did your calculation) lists all the factors that affect the final result:

 

1) Aperture

2) Focal Length

3) Pixel Size

4) Quantum Efficiency

5) Telescope Transmission Rate

6) Filter Transmission Rate

7) Central Obstruction (or lack thereof)

8) Read Noise

 

You accounted for 3 of them.  There are five more.  The 8RC's central obstruction is a big one in this case, reducing that 203mm aperture to something like 150mm (I don't know what the actual obstruction is on the 8RC, I guessed 25% by diameter though I think it's higher).  I'd wager the transmission rate of the refractor is higher than the reflector, too.  If you just account for the central obstruction, the numbers already get a lot closer...:)


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#22 kathyastro

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Posted 22 May 2019 - 05:04 PM

 I know it is an oft debated topic but if FL and f/ are the same, then the largest light bucket will prevail 

Say what?  If both FL and f/ are the same, by definition the apertures are the same too, so neither is larger.


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#23 santafe retiree

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Posted 22 May 2019 - 05:09 PM

Say what?  If both FL and f/ are the same, by definition the apertures are the same too, so neither is larger.

As noted in the original post the 8RC rig has a focal reducer in the optical train  



#24 santafe retiree

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Posted 22 May 2019 - 05:13 PM

Probably not.

Jon's post (from which you did your calculation) lists all the factors that affect the final result:

 

1) Aperture

2) Focal Length

3) Pixel Size

4) Quantum Efficiency

5) Telescope Transmission Rate

6) Filter Transmission Rate

7) Central Obstruction (or lack thereof)

8) Read Noise

 

You accounted for 3 of them.  There are five more.  The 8RC's central obstruction is a big one in this case, reducing that 203mm aperture to something like 150mm (I don't know what the actual obstruction is on the 8RC, I guessed 25% by diameter though I think it's higher).  I'd wager the transmission rate of the refractor is higher than the reflector, too.  If you just account for the central obstruction, the numbers already get a lot closer...smile.gif

Excellent point -- I feel a whole lot better about the calc now -- THX 



#25 santafe retiree

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Posted 22 May 2019 - 05:36 PM

Joe, Bill, and Paul

 

Thank you all for your thoughtful replies -- all very helpful, all very educational  bow.gif    -- and all adding to the paralysis by analysis fog but most importantly all confirming what my gut has been telling me -- I will be happier with the big frac.

 

Joe's point about what I call my "cussing match" with my RC every time I use it is a very compelling reason.

 

Add in the prospect of using the SW150 for visual which I can't do with the 8RC and .... well ..... let's see ... since I originally bought the dob for visual DSO work and now I am doing most of the DSO stuff with either straight AP or some EAA/live stacking and I really just use the dob for planetary stuff ... hmmm ....  I can sell the dob, reclaim some garage floor space, and then justify the big frac as being my new planetary scope as well as my new AP rig ... right????   wink.gif -- 

 

Again, my thanks!  waytogo.gif


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