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 1559 times

With Luminance created from RGB data.

LRGB.png (581.17 KiB) Viewed 1559 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 1559 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 1559 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

[timh]

Nice images Dave and an interesting follow up discussion on optimal f number and the rule of 5 versus wavelength. I've been trying at f/8 (2X barlow) and 1.45 um pixels which would be fine in theory except that such small pixels probably don't have enough well depth. An IR filter looks a sensible option given that really good seeing is rare.

[timh]

Hi Robin,

just picking up on your quote below ...

"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..."

That is the camera that I have but the planetary experts at RAS (Reading Astronomical Society) have commented that the well capacity is too low. My experience is that - using sharpcap - I can keep an eye on the histogram and occasionally adjust the gain back down to avoid overexposure -- but that is a bit 'manual' and overexposure can be a problem. A fair bit more expensive but another alternative to barlows may be to go with powermates which are said to be better in terms of loss of contrast and distortion (as well as keeping the optical train shorter)? Interested to know if you or Dave have any experience of these?
Tim

[turfpit]

Tim

No experience of PowerMates here.


Another process of the RGB data, a bit more adventurous with the colour boost.

Saturn_2026_10-07_RGB.PNG (59.4 KiB) Viewed 131 times

Dave

[admin]

I like the extra colour - I tend to boost saturation by 50 to 100% on most of my planetary images

My understanding of Barlows vs Tele-extenders (Powermates, etc) is that the Barlow is just a diverging lens (well, a group of 2 to reduce chromatic issues) wheras a tele-extender is more complex with 4+ lenses. Barlows give different magnification levels depending on their position in the light path, so the '2x' barlow is just an estimate of the magnification, whereas tele-extenders really do give close to their stated magnification.

For planetary work, you are always going to be pretty much in a tiny area in the center of the field of view of the telescope, so edge effects (which might be better in tele-extenders) don't matter. What is going to be important is light transmission and lack of chromatic aberration - a good barlow might beat a cheap tele-extender in that case. Personally I use an Altair 2" 2x 'Lightwave', which is a 4-element tele-extender. My goal was mainly the 2" format though, rather than it being a tele-extender.

I don't quite get the issue with full well for planetary - you tend to run at high gain for planetary imaging, so the full well that you actually have at those settings is far less than the maximum full well of the camera at minimum gain. As an example, I have some planetary videos captured with the IMX571 sensor (max FWD 51000e or so). Looking back at the gains I used, when I was imaging Jupiter, the gain led to a maximum full well of about 500e, and imaging Saturn (higher gain), the full well would have been about 200e. Effectively, you pick your full well value when planetary imaging by picking the gain (up to the sensor maximum full well). It looks like the IMX715 goes up to 10000e at minimum gain, so there is plenty of headroom there.

Of course there is the issue that you get less actual photons hitting the sensor per pixel for small pixel cameras like the IMX715, but the number per pixel per second for the IMX715 (1.45 micron) with no barlow is going to be very close to the rate for a 2.8 micron sensor with a 2x barlow...

cheers,

Robin

[timh]

"Effectively, you pick your full well value when planetary imaging by picking the gain (up to the sensor maximum full well). "

Thanks very much for that logical reply Robin. I didn't understand the well capacity argument either but its importance seemed to be established lore amongst folk who produce some amazingly good planetary images so I don't feel in a position to argue.

I feel encouraged to continue with the current set up and - balancing gain versus exposure and frame rate - maybe get some images of Saturn if the weather lets up

Tim