Barnard 150 Esprit vs RASA

[gorann]

I had forgot about this Esprit data that I collected back in October with my double Esprit rig on the Mesu, probably because I collected data with my RASA on another rig the same night and got occupied with processing it. Maybe also because I was a bit disappointed that it was not as bright and colorful as a previous image I had taken of Barnard 150 with the RASA. It is striking how different these dark nebula can look and a lot of integration time, or a low f-value is needed to bring out colors. I can now see that my Esprits did not do so badly with the lightness and colour after all. They may win in detail but are behind in brightness and colour, and my guess is they need 20 - 30 hours to catch up with the RASA.

Here is a comparison of the results, both processed in PI (mainly stacking) and PS.

On top: Esprit 150 at f/7 with an ASI6200MM collected the Lum (52 x 4 min) and Esprit 100 at f/5 with an ASI071MC collected the RGB (56 x 5 min). Totally 8.2 hours (on 19th October 2020).

Bottom one: RASA 8 at f/2 with ASI2600MC 19 x 10 min, so totally 3.2 hours (on 20th September 2020).

 

 

 

Edited December 10, 2020 by gorann

[vlaiv]

I don't know - I sort of prefer the top version.

It is much less aggressively pushed and it has much more detail / more resolution.

Look at these groups of stars as an example:

These are almost joined into single structure with RASA - they are not resolved.

Here in Esprit image, they are very nicely resolved as individual stars.

[gorann]

4 minutes ago, vlaiv said:

I don't know - I sort of prefer the top version.

It is much less aggressively pushed and it has much more detail / more resolution.

Look at these groups of stars as an example:

These are almost joined into single structure with RASA - they are not resolved.

Here in Esprit image, they are very nicely resolved as individual stars.

Yes, a good 6" apo-refractor like the Esprit beats the RASA in fine resolution. It could be partly due to the fact that the Esprits 150 had an OAG and was sitting an a Mesu mount while the RASA was on a NEQ6 with guidescope, so guiding was better for the Esprits (cannot remeber by how much). But you have previously pointed out that RASAs are primarily light buckets that are probably not diffraction limited. FL is also different (1050 mm vs 400 mm).

[Phillyo]

Both lovely images. Tough one really, I guess if you had hundreds of clear nights and you wanted perfection then the APO might be the one to go with for more resolved images. However, if you only get a couple of hours here and there and you still want to make lovely images the RASA can collect that data in ~ 1/3 the time and get an image that is about 95% as good? Living in the UK, I know which I would choose based purely on the above images alone!

[gorann]

4 minutes ago, Phillyo said:

Both lovely images. Tough one really, I guess if you had hundreds of clear nights and you wanted perfection then the APO might be the one to go with for more resolved images. However, if you only get a couple of hours here and there and you still want to make lovely images the RASA can collect that data in ~ 1/3 the time and get an image that is about 95% as good? Living in the UK, I know which I would choose based purely on the above images alone!

Thanks Phillyo! I fully agree. Actually, the RASA theoretically collects photons 12 times faster than an f/7 scope. Since I will use the RASA mainly for wide field targets the Esprits also have their place at long clear nights here, whenever those will happen next. Have not seen any stars so far in December, and only clouds and rain are forcasted😥

[tooth_dr]

Nice comparison.  I agree that is quite a result for just over 3 hours with the RASA8 scope, the colour has really been captured.   What about using the luminance of the Esprit with the colour of the RASA?  

[gorann]

10 minutes ago, tooth_dr said:

Nice comparison.  I agree that is quite a result for just over 3 hours with the RASA8 scope, the colour has really been captured.   What about using the luminance of the Esprit with the colour of the RASA?  

Thanks Adam!

That is an interesting idea - will have to try it.

[gorann]

Meanwhile I have made a slightly brigter version of the Esprit image as just suggested by @Rodd on my Astrobin post.

[Rodd]

6 minutes ago, gorann said:

Meanwhile I have made a slightly brigter version of the Esprit imag

And the moose comes alive, trudging through the taiga.

[gorann]

1 minute ago, Rodd said:

And the moose comes alive, trudging through the taiga.

And across the Atlantic!

[Rodd]

5 minutes ago, gorann said:

And across the Atlantic!

So THAT was the reef we hit the other day....swamped us

[Rodd]

1 hour ago, gorann said:

FL is also different (1050 mm vs 400 mm).

I think this fact is key.  the resolution of the Esprit was much higher (arcsec/pix), requiring less of a crop to zoom into the stars.  Aslo--this was a RASA 8--with a large obstruction--so its not really 8" vs 6"   Its more 7" to 6" (or even closer).  We might see a different story with the RASA 11 (if seeing supports it).  But, I have felt for some time that large aperture is somewhat wasted on widefield shots.  Yes, the aperture speeds up the image  as it collects more photons (one reason big scopes like the LT need very short exposures), but brightness (signal strength) is worthless on its own.  

[tomato]

And with both scopes, mounts, cameras and observatories available, you can take your pick.😎

When  my Esprits go back on the mount, I will try collecting L with the KAF 8300 on one, and RGB on the other with the 268c.

[Laurin Dave]

Both very nice Goran although I have to say I do prefer the top one..  I do think the f number thing is misleading ... your Esprit150/Esprit100 system captures photons/per unit area of sky significantly faster than your RASA/ASI2600 system does - 2.25 times if my calcs are correct although most of this is luminance with the colour data only being captured at 25% of the rate of the RASA (if you ran the Esprit100 with a mono plus filters it would double its speed)..     which I think you can see clearly by comparing the richness of the colours..    

Dave

 

[Adam J]

I actually have a preferance for the top version too. More natural far better resolution.

[wimvb]

Apples and pears, I'm afraid. Atmospheric conditions are also at play here, and may have caused a difference in apparent resolution. Plus, as Rodd said, the central obstruction (camera) of the RASA, decreases the effective aperture. Unfortunately, that makes the RASA not quite f/2. Pixel size difference has also to be taken into account. The effectiveness of an optical system to collect photons is proportional to the square of pixel size divided by the square of f-ratio, unless there is (severe) undersampling.

[AstroNtinos]

Wow this image is very stunning. Great job.

[gorann]

19 hours ago, Laurin Dave said:

Both very nice Goran although I have to say I do prefer the top one..  I do think the f number thing is misleading ... your Esprit150/Esprit100 system captures photons/per unit area of sky significantly faster than your RASA/ASI2600 system does - 2.25 times if my calcs are correct although most of this is luminance with the colour data only being captured at 25% of the rate of the RASA (if you ran the Esprit100 with a mono plus filters it would double its speed)..     which I think you can see clearly by comparing the richness of the colours.   

Dave

 

Thanks Dave! Yes, I think your calculations are right, but it feels like the RASA somehow creates a much brighter image in a shorter time, but thinking about it now that may be an effect of the colour rather than the luminance. For the luminance the Esprit 150 also have the advantage of having a mono camera so all pixels collects luminance. When it comes to the colour, then the Esprit rig cannot compete as long as I collect RGB with the Esprit 100 that has a front lens area that is only about 30% of the RASA 8. Clearly also the RASA wins in FOV even if I have a full frame APS on the Esprit 150 and an APS-C on the RASA . I would need to make a two panel mosaic with the Esprits to cover the same area.

[Laurin Dave]

6 minutes ago, gorann said:

Thanks Dave! Yes, I think your calculations are right, but it feels like the RASA somehow creates a much brighter image in a shorter time, but thinking about it now that may be an effect of the colour rather than the luminance. For the luminance the Esprit 150 also have the advantage of having a mono camera so all pixels collects luminance. When it comes to the colour, then the Esprit rig cannot compete as long as I collect RGB with the Esprit 100 that has a front lens area that is only about 30% of the RASA 8. Clearly also the RASA wins in FOV even if I have a full frame APS on the Esprit 150 and an APS-C on the RASA . I would need to make a two panel mosaic with the Esprits to cover the same area.

The total area of glass collecting light is nigh on identical at 25529mm2 for the Esprit duo and 25057 mm2 for the RASA (taking into account the 90mm diameter camera).   You could speed up the colour capture by going mono on the Esprit100 or as Adam has suggested use RASA colour data with Esprit150 luminance... be very interested to see that..

Dave

ps... my ASI2600MC and Askar200 should arrive tomorrow... what is the world coming to!

 

[gorann]

21 hours ago, Rodd said:

I think this fact is key.  the resolution of the Esprit was much higher (arcsec/pix), requiring less of a crop to zoom into the stars.  Aslo--this was a RASA 8--with a large obstruction--so its not really 8" vs 6"   Its more 7" to 6" (or even closer).  We might see a different story with the RASA 11 (if seeing supports it).  But, I have felt for some time that large aperture is somewhat wasted on widefield shots.  Yes, the aperture speeds up the image  as it collects more photons (one reason big scopes like the LT need very short exposures), but brightness (signal strength) is worthless on its own.  

Interesting points Rodd. The central obstruction is taking up 22 % of the front lens area, so in comparison to a refractor it is an f/2.5 rather than f/2 and when I calulate the light collecting area it is almost exactly that of a 7" reafractor. However, you would have a hard time making a 7" refractor with f/2.5 and a 400 mm FL, so both types of scopes clearly have their pros and cons. The RASA 11 the diameter of the central obstruction is larger so it covers 17% of the fromt lens, so not very different from the RASA 8. So the central obstruction makes the apparent f-value of the RASA 11 rise from 2.2 to 2.65.

[tooth_dr]

1 minute ago, Laurin Dave said:

my ASI2600MC and Askar200 should arrive tomorrow... what is the world coming to!

When did you order the 2600?  I've been mulling it over for a couple of weeks now, and plan to place an order tonight along with the NBX filter.  The plan is to use this on the Epsilon, then buy the mono version in Jan and put the colour version on the second scope, and use the mono on the Epsilon.

 

3 minutes ago, Laurin Dave said:

Adam has suggested use RASA colour data with Esprit150 luminance... be very interested to see that..

I can understand Gorans issue with the colour from the 100.  I use an 80mm to collect colour, and with the luminance from the Epsilon it just cant deliver.

[gorann]

1 hour ago, Laurin Dave said:

The total area of glass collecting light is nigh on identical at 25529mm2 for the Esprit duo and 25057 mm2 for the RASA (taking into account the 90mm diameter camera).   You could speed up the colour capture by going mono on the Esprit100 or as Adam has suggested use RASA colour data with Esprit150 luminance... be very interested to see that..

Dave

ps... my ASI2600MC and Askar200 should arrive tomorrow... what is the world coming to!

 

According to my calculations the RASA 8 with the central obstruction corresponds to a 7" rather than 6" refractor. Have to check it again. Yes, Over the weekend I will have a go at Adam's suggestion🙂

EDIT: Sorry Dave, I missed that you were talking about both Esprits together

Edited December 11, 2020 by gorann

[gorann]

3 hours ago, AstroNtinos said:

Wow this image is very stunning. Great job.

Thanks a lot!

[Laurin Dave]

3 minutes ago, tooth_dr said:

When did you order the 2600?  I've been mulling it over for a couple of weeks now, and plan to place an order tonight along with the NBX filter.  The plan is to use this on the Epsilon, then buy the mono version in Jan and put the colour version on the second scope, and use the mono on the Epsilon.

I ordered it last Monday..  wasn't expecting it till the new year as FLO quote 15-20 working days delivery but DHL just told me that it'll arrive on Monday  

[Rodd]

14 minutes ago, gorann said:

Interesting points Rodd. The central obstruction is taking up 22 % of the front lens area, so in comparison to a refractor it is an f/2.5 rather than f/2 and when I calulate the light collecting area it is almost exactly that of a 7" reafractor. However, you would have a hard time making a 7" refractor with f/2.5 and a 400 mm FL, so both types of scopes clearly have their pros and cons. The RASA 11 the diameter of the central obstruction is larger so it covers 17% of the fromt lens, so not very different from the RASA 8. So the central obstruction makes the apparent f-value of the RASA 11 rise from 2.2 to 2.65.

I was speaking more in terms more of resolution than speed.  Aperture is aperture, regardless of F ratio.   Regarding apertuire alone, I would raher have a diffraction limited 6" refractor than a 7" RASA.  The 11 has more of an obstruction, but it has more aperture--so it will still be greater as far as aperture.  In terms of resolving stars, a fast system with a small focal length will not be as good--even though it will be much faster.  One really can't compare the RASA to a 6" refractor because they do completely different things.   The refactor will resolve stars and details at a longer focal length while the RASA will collect lots of signal over a wider FOV with a shorter focal length.  The real compariosn is between a RASA and a fast refractor (4"), or large refractor reduced (I am not aware of a 6" refractor that has a native FR of 3 or 4).  An apt comparison is the FSQ 106 with .6x reducer (F3)--or the VSX 100 (F3).  I suppose the epsilon would be a good comparison too.--but not your typical 6" APO.   The key point is to compare systems of equal resolution (or nearly) in arcsec/pix.