RASA8 - One imaging scope to rule them all?

[tomato]

I was looking at imaging scales the other night and noticed that my current small galaxy camera, the ASI 178 with a RASA8 gives an imaging scale of 1.24 arcsecond per pixel binned 1x1. So last night I put it on the RASA and went after NGC 891. I scaled 94 mins of subs with the same integration taken with the Esprit 150 and the same camera binned 2x2 so imaging at 0.94 arcsecond per pixel. After the same process workflow, here are the results:

RASA8

and the Esprit 150 version:

OK, the 6" APO clearly wins (phew!) but wait, as usual with my comparison posts, there is a catch, the retrofitted cooler on the camera is providing quite an additional central obstruction on the RASA:

Also I have just noticed that there is now a superior alternative to the 178 available, the ASI678 which has smaller pixels (equivalent to 1.03 arcseconds per pixel), lower read noise and no amp glow. It is only available in OSC, but using a mono camera on the RASA to get a colour image would be tedious to say the least. I think I will take a punt on one of these cameras and see how good small galaxy imaging could be with the RASA.

I always thought that if I had to go down to just one imaging scope it would be the Esprit 150, but now I'm not so sure...

[ollypenrice]

The 150 wins - on this target - but would it beat a larger RASA?

I often drop old TEC 140 data onto RASA 8 widefields in order to tweak up the resolution in regions of interest, but what I find pretty shocking is how shallow the high res data are. Even in 20 hour TEC runs I have a black background sky while, in the RASA with 3 hours, I have structured faint dust.

Olly

[skybadger]

This sounds like a reason to do osc on the rasc and mix in mono luminance on the refractor. 

Not that I have a spare asi178 cool mono available to try with...

[tomato]

Just looked at the images again, where did the small background galaxy at 3 o’clock disappear to on the RASA image?🤔

[Elp]

Auto software? For raw sharpness you can't beat a refractor, I often supplement my HS data with my apo for this reason, even though the refractor is over half the aperture.

[vlaiv]

Look at spot diagram for RASA8 to see where the lack of sharpness comes from.

It has nothing to do with additional central obstruction (which impact will be negligible given other factors - it acts on order of magnitude smaller stuff - like for planetary where normal F/ratio is over F/10 not F/2).

Small pixels used with RASA8 are a waste. Ideal pixel size for this scope is about 9-10um if I'm not mistaken. Consult spot diagram for details.

[Mandy D]

4 hours ago, tomato said:

Just looked at the images again, where did the small background galaxy at 3 o’clock disappear to on the RASA image?🤔

There is a blurred smudge right where it should be, so I guess that is what we get from this device.

[gorann]

20 hours ago, tomato said:

Just looked at the images again, where did the small background galaxy at 3 o’clock disappear to on the RASA image?🤔

If you used StarXT it could possibly explain it.

[ollypenrice]

On 24/08/2023 at 12:08, vlaiv said:

Look at spot diagram for RASA8 to see where the lack of sharpness comes from.

It has nothing to do with additional central obstruction (which impact will be negligible given other factors - it acts on order of magnitude smaller stuff - like for planetary where normal F/ratio is over F/10 not F/2).

Small pixels used with RASA8 are a waste. Ideal pixel size for this scope is about 9-10um if I'm not mistaken. Consult spot diagram for details.

This doesn't square with what I actually find in our RASA data. You and I have debated many times, over the years, you taking the theoretical position and I the empirical one.  No reason to stop doing so now!!

This is a close crop of the Bubble nebula extracted from a RASA 8 widefield. The image comes from a focal length of just 400mm. Pixel size is 3.76 microns in a Bayer Matrix. (I don't know how you feel the Bayer needs to be factored in, if at all.) I think that this Bubble is exceptionally well resolved for a focal length of 400mm. Dammit, it is bloody good for 400mm! 

Olly

[ollypenrice]

On 23/08/2023 at 23:57, skybadger said:

This sounds like a reason to do osc on the rasc and mix in mono luminance on the refractor. 

Not that I have a spare asi178 cool mono available to try with...

No, if you use luminance from a slower system you will simply darken out the wonderful faint nebulosity which the fast system is so good at finding.

What is worth doing is using high resolution data (probably as luminance) to enhance regions of interest which the RASA cannot resolve to the same level. I find that the best way to do this is to register the high res, with stars, to fit the widefield and then de-star it. It can then be applied as luminance in Ps which allows you to manipulate its curve while it's in situ over the widefield. A seamless blend is possible. When you re-apply the stars, use only the widefield stars so they will be consistent across the image.

Olly

[vlaiv]

32 minutes ago, ollypenrice said:

Dammit, it is bloody good for 400mm! 

But is it as sharp as can be produced by 8" optics at "400mm" of focal length (or rather at that resolution)

 

[ollypenrice]

2 minutes ago, vlaiv said:

But is it as sharp as can be produced by 8" optics at "400mm" of focal length (or rather at that resolution)

 

That's narrowband! And there is more information in ours everywhere except in the little strand from centre to 4 o'clock. Nope, you're not winning this one so far!

lly

[vlaiv]

34 minutes ago, ollypenrice said:

That's narrowband! And there is more information in ours everywhere except in the little strand from centre to 4 o'clock. Nope, you're not winning this one so far!

lly

left is 6" + 4" data - which should be less "resolving" than 8" data.

 

[vlaiv]

Olly, if you really want to see if RASA can be compared to even 80mm scope - point it to the moon and try to match 80mm in resolution of the image on the moon.

 

[ollypenrice]

9 hours ago, vlaiv said:

Olly, if you really want to see if RASA can be compared to even 80mm scope - point it to the moon and try to match 80mm in resolution of the image on the moon.

 

Rather than test the RASA on objects I'm not going to image, I'd rather test it on the objects I am going to image. That's my whole point: used as intended it performs extremely well on nebulosity.

In the two images you post above, I would describe the resolution as very similar, the processing being different. Mine came from a  widefield mosaic for which I needed dynamic range at the bottom end, in other parts of the image, leaving me with less to play with around the Bubble. I'm not concerned with comparing aperture with aperture. Its perfectly true that the RASA doesn't resolve as an 8 inch aperture would be able to do. What matters is how it resolves as an instrument with a 400mm focal length. As such, which is how it is intended to be used, I think the resolution is excellent.

Olly

[wimvb]

On 23/08/2023 at 20:46, tomato said:

I was looking at imaging scales the other night and noticed that my current small galaxy camera, the ASI 178 with a RASA8 gives an imaging scale of 1.24 arcsecond per pixel binned 1x1. So last night I put it on the RASA and went after NGC 891. I scaled 94 mins of subs with the same integration taken with the Esprit 150 and the same camera binned 2x2 so imaging at 0.94 arcsecond per pixel. After the same process workflow, here are the results:

RASA8

and the Esprit 150 version:

Also I have just noticed that there is now a superior alternative to the 178 available, the ASI678 which has smaller pixels (equivalent to 1.03 arcseconds per pixel), lower read noise and no amp glow. It is only available in OSC, but using a mono camera on the RASA to get a colour image would be tedious to say the least. I think I will take a punt on one of these cameras and see how good small galaxy imaging could be with the RASA.

I always thought that if I had to go down to just one imaging scope it would be the Esprit 150, but now I'm not so sure...

Nice images, but if you used any form of sharpening, the comparison isn't relevant anymore. (The image taken with the Esprit looks sharpened.) The amount of sharpening during deconvolution eg, depends on snr. Better would be to show the images with just an stf stretch applied.

Btw, with the youngest generation of sony sensors, one may start to question the need for cooling, or the use of calibration frames, for that matter. It'snjust a shame that ZWO put the power/data connectors on the wrong side for their non-cooled cameras.

[tomato]

Both images were run through the same workflow which included BXT. We all know the F7 6” APO should be better on detail than the RASA, the point I was trying to make is the RASA can do better than I would expect on small targets so if you had to go for just one scope for your DSO imaging, the RASA could be the best choice:

RASA 11 £4315

Esprit 150 £4999

 

[Elp]

2 hours ago, wimvb said:

ZWO put the power/data connectors on the wrong side for their non-cooled cameras

You can mitigate the diffraction spike by using a 90 degree usb connection cable, it made a big difference on my HS and I use uncooled cameras for DSO often.

[wimvb]

1 hour ago, tomato said:

just one scope for your DSO imaging, the RASA could be the best choice

Couldn't agree more.

[vlaiv]

1 hour ago, tomato said:

just one scope for your DSO imaging, the RASA could be the best choice:

Ok, let's do a comparison of say RASA 8" and EdgeHD8" or RC8" scopes.

In either case you sacrifice something, question is - what you want to sacrifice.

RASA8" simply can't do high resolution work properly. Other two options can.

RASA8" is fast, but other two scopes are equally fast - so this is a tie (speed is not F/ratio - speed is aperture at resolution and all scopes are 8" so it is only the question of matching resolution between them - RASA is 400mm and other scopes are 2000 and 1600mm respectively so matching resolution is really a piece of cake - one just needs to use correct bin factor).

RASA8" can do wide field - while other two can't. While this is technically true - one can go around it. If you want to make wide field image with EdgeHD 8" or RC8" you can and it won't be lower quality than one produced with RASA8".

So what is the score:

Tied on one point, RASA leads on one point - but other two scopes can match it if used in certain way with only time as being cost to do so, and RASA looses on one point - but this time without possibility to match other two scopes - no matter how much you try - you simply need to accept lower resolution of that optics.

Given what I've said above - how is RASA possibly the best choice?

[andrew s]

12 minutes ago, vlaiv said:

only time as being cost to do so

I think this is the key point. The RASA has a higher etendue than the others.  It can capture more light than the others for while its capture area is the same it captures it over a wider field. Great if you have limited clear skies.

In the end it depends what you want  from your system.

We are just lucky to have such a wide choice of telescopes and modern CMOS detectors to choose from.

Regards Andrew 

 

[ollypenrice]

7 hours ago, wimvb said:

Nice images, but if you used any form of sharpening, the comparison isn't relevant anymore.

One caveat, though: the deeper the data the more effectively it can be sharpened.

Olly

[saac]

Does anybody know what the problem is with new RASA 8  units being released by the company?  I committed funds early on in the year  but ended up withdrawing due to difficulties getting hold of one. FLO are still advising "contact us".  In the end I updated my ED 80 with an Esprit 120 and I'm still itching to put it through its paces.  I guess the RAS8 will just need to wait a while longer. 

Jim 

[tomato]

6 hours ago, vlaiv said:

RASA8" can do wide field - while other two can't. While this is technically true - one can go around it. If you want to make wide field image with EdgeHD 8" or RC8" you can and it won't be lower quality than one produced with RASA8".

Take Olly and Paul's 3 panel RASA8 mosaic of the Ghost and Iris Nebula region:

If I tried to get as deep and wide with this with an Esprit 150/IMX571 set up here in the UK I could forget imaging small galaxies for a whole season , but each of these RASA panels is only 55 minutes integration. Wide and deep images from an RC or Edge HD are as rare as hen's teeth for a reason, folks aren't prepared to devote the clear sky time to them. But IMHO the RASA can produce reasonable images of small objects (though never as good as an RC or SCT) so that's why I would chose a RASA11 if I could only have one scope.

[vlaiv]

2 hours ago, ollypenrice said:

One caveat, though: the deeper the data the more effectively it can be sharpened.

Olly

Up to a point - it really fast gets into territory of diminishing returns.