Saturn near opposition, RGB filters

[turfpit]

Captured 8th October 2026 (planet was at opposition on 7th Oct). Poor seeing with Low and High fronts battling it out above me. Good to see the rings are back.
Equipment: Celestron AVX mount, Celestron C8, Celestron R, G, B filters, manual filter wheel, JMI Motofocuser, Skyris 236M, Masuyama 1.5X Barlow.
Software: SharpCap, BiggSky, GIMP.
Capture: Sets of 2000, 3000, 5000 frames with 640x480 capture area. Centre or Lock used to hold ROI steady. Exposure for each filter adjusted to put histogram at ~75% saturation.
Processing: Stack + Denoise (100/100), Decon. Combine RGB sets (R used for alignment), create Luminance (from RGB) for LRGB. GIMP for UnSharpMask. BiggSky was only stacking ~30% of frames, I regularly see 70-90% of frames stacked - a good indicator of the conditions.

For R filter: SER, MONO8, 640x480, gain 408/480, exposure 12ms, 5000 frames @ 82 fps.

RGB.png (540.67 KiB) Viewed 573 times

With Luminance created from RGB data.

LRGB.png (581.17 KiB) Viewed 573 times

Finished session with Altair 183M, UV/IR-cut filter, using SER, MONO8, 640x480, gain 2514/4999, exposure 23ms, 5000 frames @ 35fps.

2026-10-07-2240_1-Saturn4_Stack2_F1521_Decon_View_16b.png (144.34 KiB) Viewed 573 times

10 years ago using a newly purchased Nexstar 6SE and 2Mp Solar System Imager I managed this image. It took me 3 nights to get the planet on the camera sensor!

01_48_21_pipp_g6_ap20_conv.png (33.3 KiB) Viewed 573 times

Dave

[admin]

Hi Dave,

thanks for sharing -those are great (particularly the second one with the synthetic luminance - seems to have more 'pop' and 'sharpness' to it).

The 1.5x barlow/3m focal length obviously works out well for you - have you tried pushing things a bit further with a 2x barlow (or even 2.5?).

cheers,

Robin

[turfpit]

Robin

Thanks. I have a 2X Barlow but the results have always been disappointing - could be user error. However using critical sampling theory with 5 x sensor pixel size, I have

C8 scope f/10, with 1.5X Barlow makes f/15
Skyris pixel size = 2.8µm, 5 x 2.8 = 14
which makes for a good match, if I understand this correctly.

Dave

[turfpit]

Robin

I found some useful Barlow info, see section 6.1 in
https://www.cloudynights.com/forums/top ... t-12404914

According to the article, my 1.5X Barlow is a decent match in good seeing.
For the 2X Barlow to be a good match, I would need excellent seeing. 7 x 2.8 = 19.6 with the C8 at f/20.

Explains why viewtopic.php?t=7344 worked better with the 2X Barlow. ASI120 sensor is 3.75µm.
5 x 3.75 = 18.75 on an f/20 scope.

The seeing will always be issue if I want good results with the 2x Barlow. The 1.5X Barlow will give me a better match over a wider variety of atmospheric conditions.

Dave

[admin]

Hi Dave,

thanks for sharing that post - following back from the first CN link leads to a really good post explaining where the 'rule of 5' comes from and flagging some of the caveats associated with it (https://www.cloudynights.com/forums/top ... ch-barlow/). Some of the nuances are lost if you just take the rule of 5 as given.

Things that the post with the calculations implies but doesn't explicitly mention include

* You should consider a higher f-no for the blue channel where you are around 450nm not 550. Similarly, imaging in the Red/IR you would get away with smaller f-no in theory.
* Higher f-no that specified doesn't hurt you unless you start to get to the point where the image brightness decreases and puts you in trouble with exposure and hitting required frame rates. You do however get more oversampling, bigger files to process, etc.

I tend to do planetary with my f/12 classical cass + 2x barlow, so f/24, and I use a pixel size of 3.75um, so 6.4x - comfortably on the high side so if there is any very good seeing for short periods the setup should handle it. At times I have looked at the images and felt there is more to be had and wondered about a 2.5x barlow. I don't know if anyone even makes one of those in 2" though!

You can also see the attraction of small pixel cameras here - something like the IMX715 sensor with 1.45um pixels would go nicely with an SCT with *no* barlow - regardless of how good your barlow is, it will introduce some optical distortion, loss of contrast, etc...

cheers,

Robin