Saturn and Jupiter Saturday 24 September 2022

[Averton]

We were having gaps in the clouds of about 2-4 minutes as they were streaming up from the south. Just enough time to capture a few videos of Saturn and Jupiter before a huge cloud bank shut things down just after 11pm. The seeing was variable but with some good patches. Jupiter shows the GRS and Ganymede looking big and bright (as captured) about to pass behind the planet. Saturn's moons are shown brighter and larger than actual.

Images taken with 6" SCT, 1.8x barlow, ASI178MC and ZWO ADC.

[Saturnine]

Hi P& C

Really impressive images for an 6" SCT , apart from the seeing , does the ADC make for much improvement. Would be interested in seeing before and after processed images with and without the ADC .

[Averton]

Thanks Jeff.
Unfortunately we can't show you an image with and without the ADC as we only took video with the ADC fitted to the optical train. It is a bit of a nuisance swapping it in and out. One thing for certain is that the ADC makes a huge difference regardless of the altitude of the planet. A member of our local astronomy group did a test recently with and without an ADC every 5 degrees of altitude from around 30 to 60 degrees and at every step there was a noticeable difference. Obviously the greatest difference occurs at the lowest elevations. Also it helps with or without the ADC to tick "RGB align" in AutoStakkert!3 when stacking.

[Dave882]

Great images! Glad you had some good seeing with the GRS in transit.

[Crater101]

They've come up really well, considering that you only had some gaps in the cloud to work with. Great images!

[Mickoid]

These look great. I'm amazed at how consistently good your shots turn out. They're better than I get with my 8 inch scope so I'm wondering if shooting at f25 is actually reducing the resolution I can achieve with the 8 inch. Either that, or your ADC really makes that much a difference and I need to buy one of these! I always do a RGB align in Registax. I might try some shots at around f15 and see if I can get better detail. :question:

[evltoy]

Ditto what Michael has said.. "I'm amazed at how consistently good your shots turn out". You also seem to have your post works sorted.... well done P&C

@Mickoid I did read some where that the bigger the scope the better the sky has to be. The same goes for barlows - I have a ES x2 and TV x3 and most of the time I can only use the ES x2. I may even have to drop down to a x1.5 (f15) on my C8 with the crappy skies we have most of the time.

The ADC will help as it is always better to start post work with good data, but saying that you can always get away with RGB align in Registax which I still do even though I use an ADC.

[Averton]

Thanks David :)

[Averton]

Thanks Warren. These days gaps in the cloud are about as good as it gets :)

[Averton]

Thanks Michael.

We have done a lot of experimenting and optimising of our optical set up and processing just trying to get the best results possible. So this is some of the things that we do which we are sure you most likely already know, but there just might be something of use.
The ADC is not a silver bullet and only fixes atmospheric dispersion and that's it. Attached are the only images we kept from a with and without experiment of the ADC on Venus in April. Venus does show AD very well. Having said this, we would always use the ADC on planets.

You might have noticed we quote our barlow magnification as 1.8. It's recommended to be at >F10 when using an ADC. This is so the beam is not too convergent and therefore introducing other aberrations. We therefore use the barlow first, then the ADC, then the camera. This introduced some other issues as the magnification of common barlows change with the focal distance between the barlow elements and the sensor, so adding the ADC in there increases the magnification significantly. You can change the barlow magnification a little by varying this length but it does introduce spherical aberration. Telecentric barlows like the Powermates overcome this but the only one suitable for us is 2.5x which is pushing the magnification of the scope and would require excellent seeing. So we removed the barlow element from a x2 barlow, and screw it directly to the nose tube of the ADC which we had shortened by 15mm in a lathe. We then take the nose tube off the camera and the draw tube off the front of the ADC and then use a 42mm male to male thread adapter to attach the camera directly to the front of the ADC. This gives a focal length slightly shorter than the actual barlow body, which was what we were trying to achieve. It calculates to be roughly 1.8 times however, in measurements it is probably closer to 1.9x. This is because the camera sensor is already further back than the focal point using an eyepiece in the barlow body.

Having said all this, a person we know has experimented with using the barlow after the ADC (so the ADC is at F10) and reports that it works OK. This would solve the added focal length with a barlow. We haven't tried this so can't comment ourselves.
Increasing magnification with barlows does require better and better seeing. Also as you increase your focal ratio, for a given camera, you are increasing oversampling. We have tried F25 and F20 and definitely get better results at F18 for our set up.

We use AutoStakkert (AS!3) to stack the images and the RGB align in AutoStakkert. We have tried to use RGB align in Registax with unsatisfactory results. According to the Planetary Imaging book, AS!3 is the way to go for the stacking and then go to Registax for the wavelet sharpening which is what we do and have found to give the best results for us.

[Averton]

Thanks Wayne.
Totally agree. More often than not less is better with the seeing we get.

[evltoy]

Where did you read about the ADC having a sweet spot at f10?

I would be able to do this as my ES Barlow is a 2" :(

[Averton]

Not a sweet spot, just greater than F10 so your x2 is a winner. Good ADC information can be found in the Planetary Astronomy book page 77.
"The corrector must be used with a focal ratio F/D greater than 10".

[evltoy]

Thank you for that valuable information... I'll look into that book.

[Averton]

No problems Wayne. We thought most people already knew about this book. It is an absolute sensational book with so much information. It's also not that old, so the info is up to date.

https://www.planetary-astronomy-and-imaging.com/en/planetary-astronomy-the-book/