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Celestron Edge and HyperStar over RASA

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#1 EVS

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Posted 13 August 2021 - 08:49 AM

Is a RASA just a single use over Edge that can be used f/10 to f/1.9? Like to hear some points on RASA from the users that went the this direction over Edge conversions. Thanks. 



#2 carolinaskies

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Posted 13 August 2021 - 09:34 AM

RASA is a fast astrograph with sole intent is imaging.  Generally speaking the RASA is a better overall performer than a Hyperstar equipped aperture of the same size.  This is because the unit is fixed in place and optimized whereas a Hyperstar unit is a compromise of sorts.  The factory does not test a specific Edge telescope with a specific Hyperstar to see if that particular combination of that OTA and that Hyperstar have the best performance.  As mirrors can vary in quality to some degree the performance may be close to a RASA or may be measurably deficient.  For those wanting the compactness of an SCT but with the extremely fast focal ratio, the RASA offers that ability in spades.

The Hyperstar capable OTAs offer the 'all-in-one' approach for people who may not have the same resources to use a dedicated astrograph or who prefer a fork mount to an equatorial mount. If you include part-time imaging, occassional special purpose use, etc, all are reasons someone might choose to go with an Edge over a RASA.  That's not including visual use, though there are many individuals who only do imaging and don't care about visual at all and still buy an Edge due to the flat field. 



#3 rmollise

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Posted 13 August 2021 - 09:55 AM

Is a RASA just a single use over Edge that can be used f/10 to f/1.9? Like to hear some points on RASA from the users that went the this direction over Edge conversions. Thanks. 

It depends on the observer's goals. Certainly a  standard Edge scope is far more versatile.



#4 Starlancer

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Posted 13 August 2021 - 01:54 PM

I have a Non edge C9.25 that I recently acquired a Hyperstar for. I wish Celestron made a 9.25 Rasa as I think the 8 is to short a focal length and the 11 is a big jump in price, so I made my own.

I have no issues with the Hyperstar, flat edge to edge, no odd shaped star, just a great image and I can convert it to a long focal length if needed.
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#5 JethroXP

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Posted 15 August 2021 - 12:10 AM

I've been exploring the idea of a RASA (I already have a HyperStar, but it is a bit more "fiddly" than I'd like, and after many nights of swapping between f/10, f/7, and f/1.9 I can see the value of having a scope more-or-less permanently setup without having to reassemble different adapters for different focal length configurations.

 

My conundrum now is RASA 8 or RASA 11.  

 

The RASA 8 has a focal length of 400mm, but you can't put a full frame camera on it as it only has a 22mm image circle (they claim effective out to 32mm but I'm not quite sure how that works).  So APS-C is really about as big as you'd go, and 4/3 actually seems more ideal.  Those sensor sizes have a 1.5x and 2.0x crop factor respectively.  Assume you used APS-C, your effective focal length is now 600mm.

 

The RASA 11 has a focal length of 620mm and a 42mm image circle, ideal for Full Frame cameras (see where I'm going here?).

 

So a RASA 8 with an APS-C sensor and a RASA 11 with a Full Frame sensor will have effectively the same FOV.  You can see for yourself using the FOV Calculator on astronomoy.tools

 

AstronomyToolsFOVCalcRASA8v11
 
So with the appropriate sized camera for each the resulting FOV is virtually identical.  What advantage does a RASA 11 have over a RASA 8?

Edited by JethroXP, 15 August 2021 - 12:14 AM.

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#6 choward94002

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Posted 15 August 2021 - 12:50 AM

You will never regret getting too much signal; you will always regret not getting enough signal.  Signal is always about aperture, period.  You should always get as large of an aperture as possible then scale everything else around it ...

 

My next purchase for my corral is going to be an 11" RASA ... and I would get the 14" RASA if only it didn't cost as much as an entry level car ...

 

The 11" RASA will give you more signal, which will make everything better ... get the 11" ...



#7 JethroXP

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Posted 15 August 2021 - 01:33 AM

You will never regret getting too much signal; you will always regret not getting enough signal.  Signal is always about aperture, period.  You should always get as large of an aperture as possible then scale everything else around it ...

 

My next purchase for my corral is going to be an 11" RASA ... and I would get the 14" RASA if only it didn't cost as much as an entry level car ...

 

The 11" RASA will give you more signal, which will make everything better ... get the 11" ...

Am I reading your signature correctly, you have 12 telescopes, including 8 each C14s?  Isn't your C14 w/ Hyperstar better than a RASA 11?

 

And while I intuitively know what your are saying is true, I'm looking for some qualitative measure of that.  For example I can see that a RASA 8 paired with a ASI2600 has a pixel resolution of 1.94 arcseconds/pixel compared to 1.25 for the RASA 11 with an ASI6200 (both cameras have the same pixel size).  So the RASA 11 produces 35% better resolution.  After having spent an afternoon searching on AstroBin and comparing images of the same targets (and keeping in mind that individual processing workflow is a huge variable) I'm not sure I could pick which image of the same target was from a RASA 11 or a RASA 8.

Another conundrum I'm working through, which I may just decide is a non-issue, is the mount I'm getting is designed for two scopes at the same time, but the load capacity is 70 lbs.  So I can have a EdgeHD 8" (my current scope) and a RASA 8 or 11, or an EdgeHD 11 and a RASA 8 or my current EdgeHD 8" with HyperStar.  But the weight of the EdgeHD 11 and RASA 11 combined is too much.  If I go with the 11" version of each I'll just mount one at a time, thus the potential for non-issue.



#8 choward94002

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Posted 15 August 2021 - 02:44 AM

Am I reading your signature correctly, you have 12 telescopes, including 8 each C14s? 

 

Yep!  Field of scopes, flitting around like sky roomba's ... :)

 

Isn't your C14 w/ Hyperstar better than a RASA 11?

 

The C14 with Hyperstar is better than an 11" RASA from a signal standpoint, yes ... however, as folks have mentioned, it's "fiddly" ... it's a solid 3hr to get everything collimated and adjusted up (1hr with the Hotech during the day to get it collimated, 2hr at night with CCD Inspector getting the tilt dialed in) so it's pretty much permanently mounted.  The RASA also has a flatter field than the HyperStar just due to the optics of the C14 itself, and the whole point of a wide field scope is, well, a flat wide field.  Once I crop off the corners I effectively have turned the C14 into a C11 for the field (but the C14 still has more signal).

 

I don't do a whole lot of wide field stuff so the C14 with HyperStar is usually an F/11 scope and every time I think about the 3hr grind to get set up for that wide field object I start thinking "... do I really want that object? ..." which shouldn't be the question I'm asking.  If I had a RASA then all I would need to do is a 10 min swap out of a C11 for the RASA and I'm done ...

 

And while I intuitively know what your are saying is true, I'm looking for some qualitative measure of that.  For example I can see that a RASA 8 paired with a ASI2600 has a pixel resolution of 1.94 arcseconds/pixel compared to 1.25 for the RASA 11 with an ASI6200 (both cameras have the same pixel size).  So the RASA 11 produces 35% better resolution.  After having spent an afternoon searching on AstroBin and comparing images of the same targets (and keeping in mind that individual processing workflow is a huge variable) I'm not sure I could pick which image of the same target was from a RASA 11 or a RASA 8.

 

Signal always wins, period.  Aperture always gathers more signal, period.  Pixel resolutions and such are important but that will be much more affected by seeing by anything else and that changes nightly if not hourly ... I would use those pixel resolutions as a determiner if you should bin, not to pair up optics ...

Another conundrum I'm working through, which I may just decide is a non-issue, is the mount I'm getting is designed for two scopes at the same time, but the load capacity is 70 lbs.  So I can have a EdgeHD 8" (my current scope) and a RASA 8 or 11, or an EdgeHD 11 and a RASA 8 or my current EdgeHD 8" with HyperStar.  But the weight of the EdgeHD 11 and RASA 11 combined is too much.  If I go with the 11" version of each I'll just mount one at a time, thus the potential for non-issue.

 

I have never seen someone complain about the mount being too smooth and the tracking too precise and they need to add more weight to overcome that, but I see nightly complaints about the reverse. 

 

I would strongly advise against overloading the mount with two scopes, even if it claims it's "designed for two scopes" which likely just means you've got a dual saddle ... I would forget about this "dual scope" thing and mount up either the EdgeHD 11 or the RASA 11 (or get a second mount and start your own corral of scopes :) ) ...


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#9 JethroXP

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Posted 15 August 2021 - 08:35 AM

 

 

I have never seen someone complain about the mount being too smooth and the tracking too precise and they need to add more weight to overcome that, but I see nightly complaints about the reverse. 

 

I would strongly advise against overloading the mount with two scopes, even if it claims it's "designed for two scopes" which likely just means you've got a dual saddle ... I would forget about this "dual scope" thing and mount up either the EdgeHD 11 or the RASA 11 (or get a second mount and start your own corral of scopes smile.gif ) ...

 

Yup, dual saddle.  One of their promo photos shows an EdgeHD 11 and a RASA 8 mounted together which is what got me thinking about the idea ;-)

m-due-04-grey.jpg


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#10 choward94002

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Posted 15 August 2021 - 03:28 PM

Yup, dual saddle.  One of their promo photos shows an EdgeHD 11 and a RASA 8 mounted together which is what got me thinking about the idea ;-)

m-due-04-grey.jpg

Oy, that design is just a disaster waiting for a victim ... let's break this down a bit, and I'm going to assume that there's a counterweight bar that goes through the hole next to the grey bracket, otherwise that's just another issue ...

 

The biggest mount issue that AP folks face is tracking; keeping an object from not moving during the time of the exposure is the name of the game, and there are endless reasons why folks don't succeed at that.  Most of those failed tracking attempts will come down to one of two reasons: the tracking isn't smooth (so you get squiggles) or there is flexure (so you get oblong eggs). 

 

Let's look at this mount first from the tracking issue:  for smooth tracking you want to keep the center of gravity of the stuff that is getting moved in line with the thing doing the moving.  Think of a ceiling fan, now imagine that you tape a rock to the end of one of the fan blades, what will happen?  The fan will wobble, as you changed it's "moment arm" [https://www.cloudyni...rm-for-balance/]. The same will happen with your scope, if the moment arms on both sides of the motor are not identical then it will wobble ... not by much, but AP is a game of microns. 

 

In this mount we have two movement motors, the DEC motor between the two scopes and the RA motor next to the grey bracket.  Look at the RA motor where it rotates around the [missing] counterweight bar, now look at the scopes and DEC motor assembly ... for the RA motor to track without wobbling, the moment arm of the "stuff on the left", which is the C11 and a bit of the DEC mount, needs to match the moment arm of the "stuff on the right" which is the DEC motor, the brackets, the RA motor assembly and the RASA.  Do they match?  I think not, and tracking will suffer mightily as a result as it sweeps across the sky.  You're going to need to do a *lot* of counterweighting on the C11 side, which all counts toward the payload limit ...

 

Let's look at the flexure issue next, which is basically a change in what the guiding mechanism is pointing at versus what the imaging mechanism is pointing at.  There are a lot of issues here, let's start with the biggest one ... what will you use to track this thing?  No matter how you track this, because the two scopes are linked through the DEC motor (which by definition has flexure, otherwise it would never turn) one scope will ALWAYS have flexure issues.  Moving down the scope, see that grey bracket that's held to the mount and arm with four bolts, one on each side?  That bolt at the middle of the "L" is going to act as a pivot point for all of that mass of the scope assembly and lever that right into the only bolt that is holding everything together, the one at the end of the short "L" and I will guarantee that bolt is going to flex ... again, game of microns.  Imagine how the load is going to change on those two bolts as the scope assembly rotates from vertical (shown) to horizontal to inverted ... that's all flexure, and it's all going to cause oblong stars ...

 

... now let's compare this to a "real" dual mounting system [https://www.cloudyni...by-side-setup/]

 

post-15367-14072912806674_thumb.jpg

 

First, let's look at moment arms first through the DEC motor.  Notice how the two telescopes are centered for the moment arms; the side by side saddle allows as much adjustment as needed and you could hang some weights anywhere along that saddle if needed.  The center of gravity of saddle is also centered on the RA shaft (no L shaped arm) so again, the moment arm of the assembly matches the moment arm of the counterweights.

 

The scopes are hard locked into the saddle which is mounted firmly into the RA motor shaft, so nothing to flex there ... because there is no possibility of flexure between the two scopes (note how the saddle is locked into the ADM OTA dovetail plates) I would be confident with only one scope providing the tracking for both scopes ...

 

The scope you showed is very artistic and would look very retro, especially if paired with a Sesto Sento 2 focuser, but in real life I would bin both of them and get a solid mount for the RASA (AP's are good) ...


Edited by choward94002, 15 August 2021 - 03:57 PM.


#11 JethroXP

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Posted 15 August 2021 - 10:41 PM

We are getting way off topic, but to clarify, there is no counter-weight bar.  This isn't a GEM.  You can learn more about it here:
https://www.avalon-i...rgo2-pro-detail

And I'll definitely share my experiences with it (initially with just a single EdgeHD 8") when it arrives next month.


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#12 555aaa

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Posted 17 August 2021 - 02:58 PM

I haven’t used the Hyperstar but the RASA 11 has good correction out to the corners of a full frame chip. If you were using a smaller chip the 8 is probably a better bang for your buck or the Hyperstar because it is versatile. The 11 is very popular in the research community. I am working with one team that has 40 RASA 11s and another team that has 12. I have four myself. None of them are using the Hyperstar.

Edited by 555aaa, 17 August 2021 - 02:59 PM.

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#13 JethroXP

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Posted 27 August 2021 - 06:20 PM

 

And while I intuitively know what your are saying is true, I'm looking for some qualitative measure of that.  For example I can see that a RASA 8 paired with a ASI2600 has a pixel resolution of 1.94 arcseconds/pixel compared to 1.25 for the RASA 11 with an ASI6200 (both cameras have the same pixel size).  So the RASA 11 produces 35% better resolution.  After having spent an afternoon searching on AstroBin and comparing images of the same targets (and keeping in mind that individual processing workflow is a huge variable) I'm not sure I could pick which image of the same target was from a RASA 11 or a RASA 8.

 

Signal always wins, period.  Aperture always gathers more signal, period.  Pixel resolutions and such are important but that will be much more affected by seeing by anything else and that changes nightly if not hourly ... I would use those pixel resolutions as a determiner if you should bin, not to pair up optics ...

 

 

If I may, I'd like to keep drilling down on this point.  I hear what you are saying.  And on another forum where I asked a similar question I got a similar reply, along the lines of "Bigger is always better" regarding aperture.  I'm just trying to understand why, how that is quantifiable, and how that benefit manifests in my workflow (shorter or few subs) or in my results (more detail and clarity). 

For a scope like an EdgeHD with a long focal length I absolutely get it, because the pixel size of modern cameras results in images with the number of arc seconds / pixel being smaller that than he diffraction limit of the scope.  So if you can increase the aperture and reduce that diffraction limit you should end up with more detail (assuming you aren't also seeing limited).

 

But for short focal lengths, such as with the RASA, I'm struggling to understand the benefit.  An 11" RASA has the same diffraction limit as an 11" EdgeHD because it's based on aperture size.  However at the focal length of a RASA, the arc seconds / pixel of modern cameras are much larger than the diffraction limit, so reducing the diffraction limit doesn't add any value.

Looking at the unobstructed surface area of the corrector plates for each, the RASA 11 has double the surface area compared to the RASA 8.  The RASA 8 operates at f/2 while the RASA 11 is at f/2.2 which feels close enough to be insignificant.  Given the similar f-ratio and doubled light gathering surface area of the RASA 11 does that mean photos of similar detail and clarity would take twice as long (or perhaps twice as many subs) from the RASA 8 (assuming same camera and similar seeing conditions, moon phase, etc.)?  I suspect it’s not that simple.  Or maybe it is?

 

It's coming down to roughly a $6,000 decision.  $1,800 for a RASA 8" using the ASI2600MC Pro I already own.  Or $3,700 for a RASA 11" plus $4,000 for a ASI6200MC Pro full frame camera.  Like I said, I can wrap my head around "more signal always wins" and "bigger is always better" for long focal lengths but I'm trying to understand how this applies to the RASA to determine if that is worth the extra $6,000.

 


Edited by JethroXP, 27 August 2021 - 06:25 PM.


#14 carolinaskies

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Posted 27 August 2021 - 07:48 PM

If I may, I'd like to keep drilling down on this point.  I hear what you are saying.  And on another forum where I asked a similar question I got a similar reply, along the lines of "Bigger is always better" regarding aperture.  I'm just trying to understand why, how that is quantifiable, and how that benefit manifests in my workflow (shorter or few subs) or in my results (more detail and clarity). 

For a scope like an EdgeHD with a long focal length I absolutely get it, because the pixel size of modern cameras results in images with the number of arc seconds / pixel being smaller that than he diffraction limit of the scope.  So if you can increase the aperture and reduce that diffraction limit you should end up with more detail (assuming you aren't also seeing limited).

 

But for short focal lengths, such as with the RASA, I'm struggling to understand the benefit.  An 11" RASA has the same diffraction limit as an 11" EdgeHD because it's based on aperture size.  However at the focal length of a RASA, the arc seconds / pixel of modern cameras are much larger than the diffraction limit, so reducing the diffraction limit doesn't add any value.

Looking at the unobstructed surface area of the corrector plates for each, the RASA 11 has double the surface area compared to the RASA 8.  The RASA 8 operates at f/2 while the RASA 11 is at f/2.2 which feels close enough to be insignificant.  Given the similar f-ratio and doubled light gathering surface area of the RASA 11 does that mean photos of similar detail and clarity would take twice as long (or perhaps twice as many subs) from the RASA 8 (assuming same camera and similar seeing conditions, moon phase, etc.)?  I suspect it’s not that simple.  Or maybe it is?

 

It's coming down to roughly a $6,000 decision.  $1,800 for a RASA 8" using the ASI2600MC Pro I already own.  Or $3,700 for a RASA 11" plus $4,000 for a ASI6200MC Pro full frame camera.  Like I said, I can wrap my head around "more signal always wins" and "bigger is always better" for long focal lengths but I'm trying to understand how this applies to the RASA to determine if that is worth the extra $6,000.

 

First there is mechanical difference you didn't mention between the R8 and R11.  That is sensor size compatibility and ability to use a filter wheel.  The 8 is limited in maximum sensor area and single filter; the 11 can use larger sensors and a filter wheel especially helpful if shooting Mono. 

   Per Celestron and the 11... "Large 43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52mm for larger format sensors"

   Per Celestron and the 8... "Flat field free of optical aberrations like field curvature, coma, astigmatism and chromatic aberration across an entire APS-C sensor. Unlike the larger RASAs that work with DSLR and large CCD cameras, the 8” model was designed with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras in mind. The telescope does not work with standard DSLR cameras.

Next concerning aperture effects on imaging the same target, the 11 has a longer focal length meaning the maximum field of view is a bit smaller 620 vs 400mm but has about 2/10 arc second greater resolution which doesn't sound significant, but if you're imaging for detail with significant hours of data that is detail that will sift out as signal.  The larger aperture means more signal per unit of time to fill the pixel well given near identical focal ratios. 

Looking at images on Astrobin is in someways useful but in other ways useless because the data collected more often has gone through levels of processing rather than being a single unaltered image.  That processing has the tendency to blend detail to the aesthetic pleasure of the individual.  A more helpful resource would be raw single image data of the same target through the 8 & 11 using comparable imaging system to match pixel scale.  This would identify better the reality of the difference between 8 & 11.  

In general...
If I were to state the reasons for choosing the 11 over the 8 they would be as follows:

  • greater signal per unit of time
  • versatile options for sensor and filter

If I were to state the reasons for choosing the 8 over the 11 they would be as follows:

  • budget friendly
  • can be used on wider array of mounts

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#15 JethroXP

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Posted 27 August 2021 - 09:19 PM

 

First there is mechanical difference you didn't mention between the R8 and R11.  That is sensor size compatibility and ability to use a filter wheel.  The 8 is limited in maximum sensor area and single filter; the 11 can use larger sensors and a filter wheel especially helpful if shooting Mono. 

   Per Celestron and the 11... "Large 43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52mm for larger format sensors"

   Per Celestron and the 8... "Flat field free of optical aberrations like field curvature, coma, astigmatism and chromatic aberration across an entire APS-C sensor. Unlike the larger RASAs that work with DSLR and large CCD cameras, the 8” model was designed with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras in mind. The telescope does not work with standard DSLR cameras.

Next concerning aperture effects on imaging the same target, the 11 has a longer focal length meaning the maximum field of view is a bit smaller 620 vs 400mm but has about 2/10 arc second greater resolution which doesn't sound significant, but if you're imaging for detail with significant hours of data that is detail that will sift out as signal.  The larger aperture means more signal per unit of time to fill the pixel well given near identical focal ratios. 

Looking at images on Astrobin is in someways useful but in other ways useless because the data collected more often has gone through levels of processing rather than being a single unaltered image.  That processing has the tendency to blend detail to the aesthetic pleasure of the individual.  A more helpful resource would be raw single image data of the same target through the 8 & 11 using comparable imaging system to match pixel scale.  This would identify better the reality of the difference between 8 & 11.  

In general...
If I were to state the reasons for choosing the 11 over the 8 they would be as follows:

  • greater signal per unit of time
  • versatile options for sensor and filter

If I were to state the reasons for choosing the 8 over the 11 they would be as follows:

  • budget friendly
  • can be used on wider array of mounts

 

On the mechanical difference, great point, I sort of glossed over that.  Though that did factor into my "$6,000 difference" as I'm assuming that to take full advantage of the RASA 11 I'd want to pair it with a full frame camera and I'm using the ZWO ASI6200MC Pro as a reference for price.

The RASA 8, paired with my existing ZWO ASI2600MC Pro (APS-C Sensor, so 1.5x crop factor) will have a FOV equivalent to a 600mm focal length.  This is almost identical to the field of view of the RASA 11 with a full frame camera taking full advantage of its native 620mm focal length.

I do want to question the point you made about the resolution advantage the larger aperture RASA 11 has over the RASA 8.  Both the ASI2600MC Pro and ASI6200MC Pro have pixel sizes of 3.76um.  So at the focal length of the RASA 8 they would capture 1.94 arcseconds/pixel while the focal length of the RASA 11 would be 1.25 arcseconds/pixel.  That's well above the diffraction limit of either scope (0.57 and 0.41 arcseconds respectively) so the fact that one is better than the other is a moot point with those pixel sizes.  I think a camera with pixels 3x smaller would be needed before that would matter (and I don't know if such a camera exists in the consumer market).  If we were talking about the EdgeHD variants this would be significant, because at those focal lengths these same cameras are capturing 0.38 and 0.27 arcseconds/pixel respectively for the 8" and 11" versions of the EdgeHD.

I do like your reasons for choosing one over the other, except I feel like I'm still trying to understand the greater signal per unit of time thing.  I think if we were talking EdgeHD 8" vs 11" that makes sense and manifests in the improved resolution of the 11" at long focal lengths.  But at short focal lengths and current gen cameras, I think that advantage is negated.  So if I don't plan to do mono, I'm finding myself leaning toward the RASA 8 as the best option.


Edited by JethroXP, 27 August 2021 - 09:20 PM.


#16 555aaa

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Posted 27 August 2021 - 10:10 PM

The RASA optical design is not diffraction limited. It has substantially worse resolution than the C11 or the Meade 10” SCT. It’s also worth remembering that the diffraction spot size in microns only depends on the focal ratio and at f/2 that’s around 2 microns. The spot diagrams are in the Celestron RASA white paper.

Edited by 555aaa, 27 August 2021 - 10:14 PM.

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#17 choward94002

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Posted 27 August 2021 - 11:20 PM

@JethroXP ... I think you're missing the forest by staring at the trees ...

 

Step back and ask, "why do I want to get a RASA in the first place, of any size?"  People do that for one of three reasons ...

 

a) They are in light polluted skies that limits the exposure time and don't want to solve that problem with narrowband, and instead go with a faster scope

b) They want to do large field objects and dont' want to fiddle with mosaics

c) They want as flat of a field as possible for as large of an area as possible (see reason b)

 

In all cases you want as large of an aperture as possible, both for gathering as much "good signal" during a unit time as possible while the "bad signal" is coming in and (as has been pointed out) you can attach a filter wheel or big camera without obstructing too much of the view with a larger aperture

 

If you can say "don't care" to all of the above reasons (and I'd say two would be enough) then you won't be happy with the RASA and I'd point you to a HyperStar.  I personally have a HyperStar for a C14 as well as for a C11 because I just don't do enough wide field to justify another scope with the cost of a used C14, I don't really need to deal with light pollution at my dark site and the tiny little fuzz balls I shoot don't take up too much of the field so I don't care about coma anywhere but in the center ...

 

In your position I would go with the 11" because it gives you the flexibility to mount filter wheels or whatever, gives you the signal gathering ability to go for objects like the Veil and has a flat enough field that you can do that Veil without needing to mosaic things ... you can fiddle with camera matching and pixel scales and diffraction and such if you want, but again remember the fundamental reason(s) you want a RASA and focus on those ..


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#18 JethroXP

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Posted 28 August 2021 - 01:06 AM

The RASA optical design is not diffraction limited. It has substantially worse resolution than the C11 or the Meade 10” SCT. It’s also worth remembering that the diffraction spot size in microns only depends on the focal ratio and at f/2 that’s around 2 microns. The spot diagrams are in the Celestron RASA white paper.

 

Thank you!  This is exactly the education I was looking for!

 

 

@JethroXP ... I think you're missing the forest by staring at the trees ...

 

Step back and ask, "why do I want to get a RASA in the first place, of any size?"  People do that for one of three reasons ...

 

a) They are in light polluted skies that limits the exposure time and don't want to solve that problem with narrowband, and instead go with a faster scope

b) They want to do large field objects and dont' want to fiddle with mosaics

c) They want as flat of a field as possible for as large of an area as possible (see reason b)

 

In all cases you want as large of an aperture as possible, both for gathering as much "good signal" during a unit time as possible while the "bad signal" is coming in and (as has been pointed out) you can attach a filter wheel or big camera without obstructing too much of the view with a larger aperture

 

If you can say "don't care" to all of the above reasons (and I'd say two would be enough) then you won't be happy with the RASA and I'd point you to a HyperStar.  I personally have a HyperStar for a C14 as well as for a C11 because I just don't do enough wide field to justify another scope with the cost of a used C14, I don't really need to deal with light pollution at my dark site and the tiny little fuzz balls I shoot don't take up too much of the field so I don't care about coma anywhere but in the center ...

 

In your position I would go with the 11" because it gives you the flexibility to mount filter wheels or whatever, gives you the signal gathering ability to go for objects like the Veil and has a flat enough field that you can do that Veil without needing to mosaic things ... you can fiddle with camera matching and pixel scales and diffraction and such if you want, but again remember the fundamental reason(s) you want a RASA and focus on those ..

Great feedback, and great approach.  I do already have a HyperStar on my EdgeHD 8".  I'd say I'm looking for some of all three a, b, and c which is why I originally got the HyperStar.  I have shot M31, The Veil, and the Elephant Trunk Nebula with it, however I've found it very temperamental to use and the coma in the corners does bug me, particularly on large targets that do fill the whole frame.  I am only interested in OSC as well due to frequent bad weather and limited opportunity to image.

Interesting to note that the FOV of the RASA 8 with an APS-C camera is nearly identical to the FOV of a RASA 11 with a full frame camera.  So with a RASA 8 I can use the camera I already have and maintain the FOV I've gotten used to with the HyperStar.  To match that FOV with the RASA 11 I'd need to spend another $4,000-ish for a full frame camera.  I think it's that added expense that is driving me to understand the performance difference and if it matters for my situation.

 



#19 choward94002

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Posted 28 August 2021 - 12:08 PM

Thank you!  This is exactly the education I was looking for!

 

 

Great feedback, and great approach.  I do already have a HyperStar on my EdgeHD 8".  I'd say I'm looking for some of all three a, b, and c which is why I originally got the HyperStar.  I have shot M31, The Veil, and the Elephant Trunk Nebula with it, however I've found it very temperamental to use and the coma in the corners does bug me, particularly on large targets that do fill the whole frame.  I am only interested in OSC as well due to frequent bad weather and limited opportunity to image.

Interesting to note that the FOV of the RASA 8 with an APS-C camera is nearly identical to the FOV of a RASA 11 with a full frame camera.  So with a RASA 8 I can use the camera I already have and maintain the FOV I've gotten used to with the HyperStar.  To match that FOV with the RASA 11 I'd need to spend another $4,000-ish for a full frame camera.  I think it's that added expense that is driving me to understand the performance difference and if it matters for my situation.

 

OK, got it ... so if we ranked the three things, it looks like a flat FOV would be first.  That said, then there are two questions: what advantage (for you) does the 11" have over the 8" and is that advantage enough to compensate for drop in FOV?  And if that advantage is enough to justify a drop in FOV, is it worth dropping $4K on getting the FOV up again?

 

It's also worth noting that of all of the components in an optical system, the OTA is the hardest to upgrade.  If I want to move from camera A to camera B it's very easy to post the camera on the sale boards, get probably 80% of my money back, box it up and ship it out, get camera B and I'm done.  An OTA is much harder to price, sell and ship especially with larger OTA's.  I shuffle 8" scopes all the time, but the big 14's and even 11's require an onsite handoff or some very specialized shipping containers.  There is absolutely nothing stopping you from getting the 11" RASA and using it with your current camera, then upgrading that camera to a larger one in the future ...

 

It really comes down to what kind of objects you're shooting and where you're shooting from.  I'm all about the galaxies and small planetaries, that's my focus (pun intended) and that's what guides my optics purchases.  It's nice to do wide field sometimes, and there are some really nice wide field objects but again, focused on small fuzzies.  I have the HyperStar's if I *really* want to get a wide field of something, but that's pretty rare for me

 

For some people it's all about the wide field objects, especially if you're making an mosaic series of the Orion Molecular Cloud, and that's what guides those optics purchases and you look to be in that camp.  Knowing that I would suggest getting first the 11" RASA, use your existing camera and then later upgrade the camera to a (probably cheaper) full frame camera ...


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#20 JethroXP

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Posted 28 August 2021 - 09:02 PM

OK, got it ... so if we ranked the three things, it looks like a flat FOV would be first.  That said, then there are two questions: what advantage (for you) does the 11" have over the 8" and is that advantage enough to compensate for drop in FOV?  And if that advantage is enough to justify a drop in FOV, is it worth dropping $4K on getting the FOV up again?

 

It's also worth noting that of all of the components in an optical system, the OTA is the hardest to upgrade.  If I want to move from camera A to camera B it's very easy to post the camera on the sale boards, get probably 80% of my money back, box it up and ship it out, get camera B and I'm done.  An OTA is much harder to price, sell and ship especially with larger OTA's.  I shuffle 8" scopes all the time, but the big 14's and even 11's require an onsite handoff or some very specialized shipping containers.  There is absolutely nothing stopping you from getting the 11" RASA and using it with your current camera, then upgrading that camera to a larger one in the future ...

 

It really comes down to what kind of objects you're shooting and where you're shooting from.  I'm all about the galaxies and small planetaries, that's my focus (pun intended) and that's what guides my optics purchases.  It's nice to do wide field sometimes, and there are some really nice wide field objects but again, focused on small fuzzies.  I have the HyperStar's if I *really* want to get a wide field of something, but that's pretty rare for me

 

For some people it's all about the wide field objects, especially if you're making an mosaic series of the Orion Molecular Cloud, and that's what guides those optics purchases and you look to be in that camp.  Knowing that I would suggest getting first the 11" RASA, use your existing camera and then later upgrade the camera to a (probably cheaper) full frame camera ...

I really to appreciate your input and the thoughtful discussion, so thank you!

 

Yes, flat FOV is important, which even beyond ease of use is driving me toward a RASA vs. my current HyperStar.

 

That question, what advantage does the 11 have over the 8 is the big one I'm trying to figure out.  If we were talking EdgeHD 8" vs. 11" I don't think I'd have this quandary.  The  EdgeHD 11" reaches farther and resolves more detail than the 8".  The fact that the RASA 11 reaches farther than the RASA 8 is actually a negative in my opinion, but one that can be overcome by using a full frame camera.  So the big question is, does it also resolve more detail?  I think intuitively the answer is yes, and I hear and understand that bigger aperture means more signal and more signal means better data.  But I don't know how to visualize that, or compare it to the RASA 8 to determine if its a difference that matters enough for me to justify the additional expense.

As for objects I'm shooting, things like M31, North America Nebula, Elephant trunk Nebula, The Veil Nebula, and I have asperations of doing the Squid some day (Ou4 within SH 2-129).

 

I like your advice, very logical and reasonable.  You are right, I don't need to get the Camera and the OTA at the same time, and adding the camera later will come in handy when I upgrade my EdgeHD 8" to 11" as well, though the APS-C camera I currently have will be a nice pairing with that too.



#21 carolinaskies

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Posted 29 August 2021 - 08:24 AM

If you want to try to grasp the difference between the 8" and 11"  aperture consider the following thought experiment... 

A mirror is a bucket to hold light.
Aperture is the diameter of the bucket.
Light is water flowing into the bucket.
Focal ratio is the pressure at which the water flows into the bucket over time.
Signal is the total amount of water collected for a unit of time. 

We can use these parameters to discuss the differences quite easily.

The two differences between an 8 & 11 are the aperture and the focal ratio.  The latter, focal ratio difference is ~10%, so that the 8" has slightly faster accumulation.  The former, aperture has an area difference of 50.3 vs 95.03, meaning the 11" collects almost double the signal for a unit of time.  If we express the actual difference it would be (area11@F/2.2) - (area8@F/2)= total signal gain.  In the world of photography F/stops are the unit of comparison.  Because the 11" ~doubles the area that is equivalent to gaining ~full f/stop. That means comparatively the 11" is gathering closer to F/1.2 amount of light signal vs the 8" F/2.  

How does this relate to astrophotography then?  It means that if you want to measure actual signal as equivalent, the 11" can expose about 1/2 the amount of time of the 8".   Or conversely, you can ~double the signal in the same exposure time.   This is what aperture gets you and why scientific research is conducted with the biggest mirrors possible, because signal is king.   In practical application it means you can image more targets in a night, gather more signal on a dim target, etc.   If you don't have lots of time to image that makes the larger aperture more productive.  


 


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#22 Rasfahan

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Posted 29 August 2021 - 08:35 AM

I really to appreciate your input and the thoughtful discussion, so thank you!

 

Yes, flat FOV is important, which even beyond ease of use is driving me toward a RASA vs. my current HyperStar.

 

That question, what advantage does the 11 have over the 8 is the big one I'm trying to figure out.  If we were talking EdgeHD 8" vs. 11" I don't think I'd have this quandary.  The  EdgeHD 11" reaches farther and resolves more detail than the 8".  The fact that the RASA 11 reaches farther than the RASA 8 is actually a negative in my opinion, but one that can be overcome by using a full frame camera.  So the big question is, does it also resolve more detail?  I think intuitively the answer is yes, and I hear and understand that bigger aperture means more signal and more signal means better data.  But I don't know how to visualize that, or compare it to the RASA 8 to determine if its a difference that matters enough for me to justify the additional expense.

As for objects I'm shooting, things like M31, North America Nebula, Elephant trunk Nebula, The Veil Nebula, and I have asperations of doing the Squid some day (Ou4 within SH 2-129).

 

I like your advice, very logical and reasonable.  You are right, I don't need to get the Camera and the OTA at the same time, and adding the camera later will come in handy when I upgrade my EdgeHD 8" to 11" as well, though the APS-C camera I currently have will be a nice pairing with that too.

My thoughts on this:

 

Your problem here is your thinking in terms of focal length as „reach“ - this comes from terrestrial photography and is essentially incorrect there, too.

 

In long-exposure DSO photography you want to think about the pixel scale (arc-seconds per pixel), the field of view (does my object fit?) and the resolution limit of your optics compared to your seeing and pixel scale.

 

„Resolution“:

The RASA 11 has a published spot size of 4.5 microns across the full frame field, that equates to about 1.3“ (but it is hard work to get it so). If you have a seeing of 2“, all other sources of blurring disregarded, that would come to a spot of about 2.4“, so about 7 microns. In that case you are slightly undersampled with a camera with 3.75 micron pixels. The EdgeHD 11 has a diffraction-limited spot size of 7 microns (but published specs are 20 microns across the field… I am not sure how these two go together). Seeing of 2“ will basically blurr this to about 2“ (27 microns). To be well-sampled you will want to bin your 3.75 micron pixels 2x. So: Due to the (slightly) better optical performance (and a lack of, say 2 micron pixeled cameras with large sensors) you will have better peak resolution in the EdgeHD than in the RASA. Of course, it is much more difficult to get the RASA to perform to specs than the EdgeHD. If your seeing is worse than 2“, this will not matter much, though. Note that optics can actually resolve details smaller than their spot size, so this is more of a qualitative comparison.

FOV

The field of view is, of course, determined by sensor size and focal length, assuming the corrected field of the optics is good for the sensor size. APS-C on RASA 8 is about the same as full frame on RASA 11.

Light gathering

Light gathering can be looked at like this: Assuming you have two optical systems with the same pixel scale (arcsec/pixel), the one with the larger area will gather more light per pixel. This relation goes up linearly with the clear aperture. So if your two scopes have the same pixel scale, and the object fits on both chips, the RASA 11 will need about half the total integration time of the RASA 8. If that is worth the price difference, is up to you.

Price

The price calculation looks quite a bit more fortunate for the RASA 11 when you realize that there are not that many targets that do not fit into APS-C at 620mm FL. Of course, full frame (or APS-C on the RASA 8) will fit neither M31 or the full Elephant Trunk but you can just squeeze in the Northern Veil. I‘ve also found M4/3 to be quite enough to handle on the RASA. Fitting an ASI6200MC is tempting, but my game is not quite there yet.

 

Mounting

Looking at the image you posted, the C11 on that M-Uno was fitted for hyperstar. The setup will probably work like this, since you only need about 10-20s subs. But what for? Both scopes serve the same purpose. When asked, Avalon support told me the M-Uno will not be a good match for a C11 at full focal length. Really, with these big scopes you want a much beefier mount. Don‘t save on the mount, it‘s no fun.

 

Combining light gathering, resolution and FOV in one instrument leads to the f/ratio-aperture-FOV-fever as mentioned in another thread. It‘s a pretty serious affliction that costs a lot of money and caused much pain before it heals. Pick the compromise you want, and rather go with two scope/camera combinations than trying to do all with one.


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#23 JethroXP

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Posted 29 August 2021 - 08:47 AM

If you want to try to grasp the difference between the 8" and 11"  aperture consider the following thought experiment... 

A mirror is a bucket to hold light.
Aperture is the diameter of the bucket.
Light is water flowing into the bucket.
Focal ratio is the pressure at which the water flows into the bucket over time.
Signal is the total amount of water collected for a unit of time. 

We can use these parameters to discuss the differences quite easily.

The two differences between an 8 & 11 are the aperture and the focal ratio.  The latter, focal ratio difference is ~10%, so that the 8" has slightly faster accumulation.  The former, aperture has an area difference of 50.3 vs 95.03, meaning the 11" collects almost double the signal for a unit of time.  If we express the actual difference it would be (area11@F/2.2) - (area8@F/2)= total signal gain.  In the world of photography F/stops are the unit of comparison.  Because the 11" ~doubles the area that is equivalent to gaining ~full f/stop. That means comparatively the 11" is gathering closer to F/1.2 amount of light signal vs the 8" F/2.  

How does this relate to astrophotography then?  It means that if you want to measure actual signal as equivalent, the 11" can expose about 1/2 the amount of time of the 8".   Or conversely, you can ~double the signal in the same exposure time.   This is what aperture gets you and why scientific research is conducted with the biggest mirrors possible, because signal is king.   In practical application it means you can image more targets in a night, gather more signal on a dim target, etc.   If you don't have lots of time to image that makes the larger aperture more productive.  


 

Thank you!  This is exactly what I needed! 

 

I love the analogy, and makes perfect sense given I've heard of reflectors in general referred to as "light buckets".  So the 8" bucket at f/2 "fills up" slightly faster than the 11" at f/2.2 and I think I was stuck at the point of "well, both buckets filled in about the same time, why is one better?" while overlooking the obvious fact that one bucket "held" twice as much volume as the other. 

 

This really helps with my understanding, thank you for taking the time to explain it! 

 

The key take away for me is (to paraphrase what you wrote):  "A larger aperture allows you to be more productive in a given period of time".

 

This also explains what someone said on another thread "Just like with hot-rods, there is no replacement for displacement.  In astrophotography there is no replacement for aperture except time".  I get it now.  I could potentially get equivalent signal in one hour from an 11" that it would take 2 hours to gather with an 8".  I know it won't work out exactly like that, but as an approximation that helps me make the value assessment I was looking for.



#24 JethroXP

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Posted 29 August 2021 - 09:12 AM

My thoughts on this:

 

Your problem here is your thinking in terms of focal length as „reach“ - this comes from terrestrial photography and is essentially incorrect there, too.

 

In long-exposure DSO photography you want to think about the pixel scale (arc-seconds per pixel), the field of view (does my object fit?) and the resolution limit of your optics compared to your seeing and pixel scale.

 

„Resolution“:

The RASA 11 has a published spot size of 4.5 microns across the full frame field, that equates to about 1.3“ (but it is hard work to get it so). If you have a seeing of 2“, all other sources of blurring disregarded, that would come to a spot of about 2.4“, so about 7 microns. In that case you are slightly undersampled with a camera with 3.75 micron pixels. The EdgeHD 11 has a diffraction-limited spot size of 7 microns (but published specs are 20 microns across the field… I am not sure how these two go together). Seeing of 2“ will basically blurr this to about 2“ (27 microns). To be well-sampled you will want to bin your 3.75 micron pixels 2x. So: Due to the (slightly) better optical performance (and a lack of, say 2 micron pixeled cameras with large sensors) you will have better peak resolution in the EdgeHD than in the RASA. Of course, it is much more difficult to get the RASA to perform to specs than the EdgeHD. If your seeing is worse than 2“, this will not matter much, though. Note that optics can actually resolve details smaller than their spot size, so this is more of a qualitative comparison.

FOV

The field of view is, of course, determined by sensor size and focal length, assuming the corrected field of the optics is good for the sensor size. APS-C on RASA 8 is about the same as full frame on RASA 11.

Light gathering

Light gathering can be looked at like this: Assuming you have two optical systems with the same pixel scale (arcsec/pixel), the one with the larger area will gather more light per pixel. This relation goes up linearly with the clear aperture. So if your two scopes have the same pixel scale, and the object fits on both chips, the RASA 11 will need about half the total integration time of the RASA 8. If that is worth the price difference, is up to you.

Price

The price calculation looks quite a bit more fortunate for the RASA 11 when you realize that there are not that many targets that do not fit into APS-C at 620mm FL. Of course, full frame (or APS-C on the RASA 8) will fit neither M31 or the full Elephant Trunk but you can just squeeze in the Northern Veil. I‘ve also found M4/3 to be quite enough to handle on the RASA. Fitting an ASI6200MC is tempting, but my game is not quite there yet.

 

Mounting

Looking at the image you posted, the C11 on that M-Uno was fitted for hyperstar. The setup will probably work like this, since you only need about 10-20s subs. But what for? Both scopes serve the same purpose. When asked, Avalon support told me the M-Uno will not be a good match for a C11 at full focal length. Really, with these big scopes you want a much beefier mount. Don‘t save on the mount, it‘s no fun.

 

Combining light gathering, resolution and FOV in one instrument leads to the f/ratio-aperture-FOV-fever as mentioned in another thread. It‘s a pretty serious affliction that costs a lot of money and caused much pain before it heals. Pick the compromise you want, and rather go with two scope/camera combinations than trying to do all with one.

Thank you!  Indeed, I've been thinking about this wrong, which is why I'm so appreciative of folks like yourself taking the time to share and help me understand.  Thinking about it wrong comes from not knowing what I didn't know.  Grateful for you and others helping to fill in those gaps.

 

Just a point of correction on the image I shared above.  That's actually the newer, and larger, M-Due.  It has a higher payload capacity than the M-Uno, which topped out at 44 lbs, while the M-Due is 55 lbs for a single scope and 70 lbs for dual scopes.  At 43 lbs for the RASA 11 plus camera, guide scope, guide cam, I should be within that single scope load limit.  I could even put the guide scope and guide cam on the other saddle to distribute load better.

What I've learned is helping me net out the price calculation in favor of the RASA 11, since another key factor I have to deal with is that clear skies are rare where I live (Pacific Northwest, famous for rain, and well below the national average for cloudless days).

 

I was trying to decide if I would remain an 8" astro guy, with an EdgeHD 8" and adding a RASA 8". or if I would make the jump to 11" for both.  This discussion really helped with my understanding.  I'm ready to join team 11.  Now if only I can find them!



#25 carolinaskies

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Posted 29 August 2021 - 09:13 AM

Thank you!  This is exactly what I needed! 

 

I love the analogy, and makes perfect sense given I've heard of reflectors in general referred to as "light buckets".  So the 8" bucket at f/2 "fills up" slightly faster than the 11" at f/2.2 and I think I was stuck at the point of "well, both buckets filled in about the same time, why is one better?" while overlooking the obvious fact that one bucket "held" twice as much volume as the other. 

 

This really helps with my understanding, thank you for taking the time to explain it! 

 

The key take away for me is (to paraphrase what you wrote):  "A larger aperture allows you to be more productive in a given period of time".

 

This also explains what someone said on another thread "Just like with hot-rods, there is no replacement for displacement.  In astrophotography there is no replacement for aperture except time".  I get it now.  I could potentially get equivalent signal in one hour from an 11" that it would take 2 hours to gather with an 8".  I know it won't work out exactly like that, but as an approximation that helps me make the value assessment I was looking for.

I'm glad to be of assistance.  The analogy is one I've been working on for a while to help explain different characteristic of telescopes.  In the case of the RASAs it's helpful that the focal ratio's are nearly the same, making the standout point of comparison exactly what you wanted to know.  
 


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