C6 v C9.25 for Planetary Imaging?

[BlueAstra]

Considering a Celestron C6 or C9.25 for planetary imaging with a CCD camera. At first I thought its obvious, get a C9.25. However, that may be true visually, but is it true with a CCD camera? The Rayleigh limit for the C6 is 0.92 arcsec, and for the C9.25 is 0.59 arcsec. So the C9.25 has better angular resolution. However at the CCD plane the spot size is the same for each scope, 6.7 um (focal length x angular resolution, also proportional to wavelength x F/Number). So the same F/Number gives me the same spot size in the CCD plane. Both scopes are F/10, so both give the same spot size. I guess the C9.25 may be a bit brighter because of the larger aperture, but the C6 samples less 'atmosphere' so may give a steadier image. Both have about the same obscuration ratio. If the C9.25 has a better aberration correction, that may help, but for a price factor of x2 it would have to be good. So, comments please, which one for CCD imaging?

[Stu]

I will confess to not knowing what most of your post means (being a visual observer), but in a simplistic way I can only say that the best planetary imagers in the world use the largest SCTs they can handle, C11 and C14 for instance to get the best resolution possible.

I'm not sure what Damian Peach uses currently but almost certainly a large SCT for planetary imaging.

http://www.damianpeach.com/jup17.htm

So, provided your seeing is up to it, a C9.25 with the optimum camera will certainly give you better images.

[BlueAstra]

The C14 and C11 have slightly lower obscuration ratios, but similar F/Numbers, so would produce similar spot sizes. The lower obscuration would make the central peak of the spot slightly brighter.

[michael.h.f.wilkinson]

It is true that at the same F-number you get the same spot size, but because of the much longer focal length, you get far more spots, so to speak. The image of Jupiter in the focal plane is 1.54x larger (linearly) in the C9.25 compared to the C6, which means 2.38x in surface area. To make full use of the scope's resolution, you need to match the pixel size of a CCD or CMOS chip to the focal ratio of the scope. Usually that means using a Barlow or tele-centric lens (like a PowerMate) to achieve the right focal ratio for a given camera and scope. With my ASI174MM (5.3micron pixels) I usually aim at F/25 to F/30, whereas with my ASI224MC (3.75micron pixels) I get best results between F/16-F/25, and the ASI178MM (2.4micron pixels), needs only F/12-F/15 or thereabouts.  For 6.7 micron pixels F/30 or a bit higher would be required.

[JeremyS]

For some additional considerations from the world's expert on planetary imaging, have a look at this article on the BAA website by Damian Peach concerning the effect of atmospheric dispersion at various apertures: https://britastro.org/node/9058

Jeremy

[Stargazer33]

astroavani who's a member here uses a C14 and he has captured albedo detail on Ganymede using a C14 Edge, ASI224 & a 2x Powermate.

[Mr Spock]

Damian Peach used to use a C9.25 before he moved to the C14. Take a look at his images and you'll be convinced!

C925 Review - Damian Peach

[Peter Drew]

As a "spectator" rather than an imager, the results of planetary photography speak for themselves. The leading imagers seem to have the financial ability to acquire any choice of instrument yet favour, in the main, large SCT's, others have done as well with similar size Newtonians. Reduction in contrast due to central obstructions are negated via processing.   

[Miguelo65]

Makes sense , but in any planetary gallery photo or sketching, you'll see that image resolution details are always better in a say, C 14 in relation to a smaller OTA. (IMO). No experience in my C 9 yet anyway

Regards

[BlueAstra]

So from this analysis, the optimum sampling of the Airy disc is with an F/Number of about 5 x Pixel Size (microns). So from Michael's post, his sensors would work best at 5.3um = F/26.5, 3.75um = F/18.75, 2.4um = F/12, all of which ties in with his experience. However, this just states the optimum F/Number for Airy disc sampling, so on the face of it both the C6 and C9,.25 would work optimally at say F/30 with a 5.6um pixel (DMK), since both work at F/10 (done with a x3 barlow). However, I think the key thing is Michael's comment on image scale. With the longer focal length, the C9.25 will always put x1.5 more pixels on the object than the C6 for a given camera.

[michael.h.f.wilkinson]

2 hours ago, Stargazer33 said:

astroavani who's a member here uses a C14 and he has captured albedo detail on Ganymede using a C14 Edge, ASI224 & a 2x Powermate.

I have actually got some albedo spots on Ganymede with a C8. Not nearly as good as astroavani's work with the C14, but it is certainly possible with a C9.25

The lower of the three moons is Ganymede, and there is definite structure there.

 

[Owmuchonomy]

As Michael says. My simplistic approach is image at 5x pixel size. A longer fl scope will give better image scale at a given f/. Actually, f/30 is more tricky to achieve with nice TV glass ??. I aim for f/25 with my ASI174 or native f/10 with my ASI290, the latter giving spectacular lunar image resolution.

[michael.h.f.wilkinson]

For F/30 I use a Meade Series 5000 3x TeleXtender, which is a tele-centric lens just like a PowerMate. The Meade version is no longer sold, but the Bresser SA Barlows have the same optics (and housing), and the ES Focal Extenders have the same optics in a different housing, it would appear. Siebert Optics also makes a range of tele-centric Barlows (from 1.3x to 3.0x in 1.25" fitting, and from 1.5x to 4.0x in 2").

[CraigT82]

You could always do a search on astrobin for images made with C6s and C9.25s.

One thing to note is that temperature management is critical on SCTs for planetary imaging, and the bigger you go the more challenging it will be to keep the scope at ambient temp

[newbie alert]

7 hours ago, Mr Spock said:

Damian Peach used to use a C9.25 before he moved to the C14. Take a look at his images and you'll be convinced!

C925 Review - Damian Peach

I'm sure Damien could do great images with a milk bottle and a wedding table camera...

[michael.h.f.wilkinson]

I have seen quite a few images made either with a C6 or a SkyMax150 (similar aperture), and whereas they are pretty good, they cannot match what I can do with a C8. By contrast, I have seen quite a few images made with a C9.25, which are a lot better than the best I have managed with my C8. A C9.25 will hit a C6 for six, easily.

[si@nite]

On ‎01‎/‎05‎/‎2017 at 11:18, Stargazer33 said:

astroavani who's a member here uses a C14 and he has captured albedo detail on Ganymede using a C14 Edge, ASI224 & a 2x Powermate.

This lrgb shot showing moon detail, taken with my C9 - 3x barlow - ADC - skyris 618m!