gorann

In that article there are two cross sections of camera lenses and counting the the optical elements it does not seem to be much of a gain: 14 elements in the flatened image lens and 12 in the curved one... And buying a camera with a curved sensor means that all the lenses you have bough become useless. Interesting idea but I cannot see it take off except in very special instances.

Wow Adam, you got some very good data there - this is a really exciting thread, keeping us all in suspense! I thought I should check how many panels I need for the RASA8, but it is a purely theoretical question right now since no one in Scandinavia has seen the sun or the moon since late November and no clear sky in sight in the 10 day forecasts.....

Thanks a lot Craig!

Thanks a lot Adam, Dave, and Michael, much appreciated!

So acting on your requests (thanks!), I have now made an image with Eprit luminance on top of the RASA RGB. You find it here:

I recently posted a comparison of two images I have taken of this area in Spetember and October. One with the RASA 8 and ASI2600MC OSC (3.2 hours) and one with my double Esprit rig (Esprit 150 with ASI6200MM for Lum and Esprit 100 with ASI071 for RGB, totally 8.2 hours). The conclusion was that the colour signal was stronger in the RASA data while resolution was better in the Esprit data. @tooth_dr and @Laurin Dave then suggested that I should make an image with the Esprit data as luminance for the RASA data, so here it is. Hope you like it. A bit like the best of two worlds.

Thanks Vlaiv! I have actually thought of the possibility of a dual RASA 8 rig, since my iOptron CEM70 that currently holds my RASA 8 should be able to support a second one. But I hold my horses and save my coins until there is a mono APS-C produced and which I could put on the second scope.

Interesting @vlaiv! One comment about the imaging circle of the RASA 8. 22 mm is what they promise to be fully corrected but Celestron say that the usable field is 32 mm with "only minimal performance loss at edge of FOV". That appears to be true at least in my sample. I use it with an ASI2600MC APS-C that has a diagonal of 28.3 mm and there is very litte vignetting and stars are quite ok in the corners. So a comparison of FOV with the RASA 8 and ASI2600 vs RASA 11 and ASI6200 gives identical FOVs. At FLO the RASA 8 system (scope + camera) will cost you 3732 GBP and the RASA 11 (scope + camera) 7838 GBP. Then there is the possible additional cost for a mount that can handle about 20 kg of scope and camera (RASA 11) compared to 10 kg (RASA 8). Here is my first light with the RASA 8 and ASI2600MC, which happened to be 2.6 hours of M31 (and before I realized that I could arrange the camera cables in a circle to avoid star spikes)

Thanks a lot Vlaiv! Then you confirm what Rood argue🥴, that the RASA 11 will resolve more with the same camera due to a larger aperture. But it is at the cost of a smaller FOV. Then the question is if it is noticible, worth the money, worth the smaller FOV and all the photons lost outside the sensor😀.

Thanks Vlaiv, so what is your conclusion regarding RASA 8 vs RASA 11. The RASA 11 has 2 x the light gathering area but spreads it at 2.7 x the imaging circle area. To me that means less photons per pixel (with the same camera) and much smaller FOV for that same camera. Would it still give more resolution?

not sure about that Rodd, you may be right, but the photons collected will be spread out over more pixels as I understand it. Maybe our SGL oracle @vlaiv can explain?

Yes, I think we agree that the RASA 8 and a 6" refractor both have their places and do slightly different things. I think the RASA shines when time is limited. Regarding a RASA 11, it will collect about twice as many photons, but deliver them over a much larger imaging circle (the area in the circle is 2.7 times larger) so if you want to collect a decent part of the photons you need a 24 x 36 mm sensor. With a 24 x 18 mm sensor you will collect most of what a RASA 8 is collecting but only a fraction of what a RASA 11 is collecting, so unless you have a big sensor camera I don't think there is much to gain with a RASA 11. It also weighs nearly three times more (19.5 vs 7.7 kg) and costs the double.

Thanks a lot!

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

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.

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.

And across the Atlantic!

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

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

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😥

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).

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).

Found the tape on a European site so ordered it also from there. They were actually cheaper than the grandado.com. I just have to remember checking out the companies before pressing the buy button - a good site for that is www.trustpilot.com

Thanks Martin. much appreciated!

Forgot to say that those first-light images are impressive, expecially the stars that look great all the way to the cornes!