Tried visual astronomy yesterday - challenged seeing the Cassini division

[g__day (Matthew)]

Hey folk,

Yesterday I tried some visual astronomy for the first time in ages on the smaller of my 3 scopes (80mm Williams Optics Megrez refractor and 127mm Skywatcher MAK). I wanted to have a look at Saturn and check my mounts alignment after little use in the past 6 months.

I focused my smaller OTAs using a Bhatinov mask on Spica, then locked on Saturn. I was using a 8mm Vixen LWV eye piece on the Megrez (60x magnification) and a 22mm then 13mm LWV eyepiece on the MAK (68 and 115x magnification respectively).

My C9.25 is set up for imaging - so I didn't touch it. Now I noticed Saturn was reasonably sharp for 20 - 30 minutes until the clouds rolled in - so seeing was only average and we had a 1/3 moon. But focused correctly neither scope (or my eyes) would see the Cassini division.

So to my question - is it most likely that this is:

1. Seeing related
2. The angle Saturn is inclined at present makes it hard to split the Cassini division in poor seeing at low magnification
3. An optical alignment issue with my smaller OTAs
4. My eyes - getting old!

Many thanks for your help.

Matthew

[mental4astro (Alexander)]

Answer: 1, seeing.

You might be hard pressed to make out the Casini division in the 80mm, but in the Mak it should be a cake-walk. My C5 never had a problem with it at its current angle, except in poor seeing.

Try the Mak again, but with the 8mm. If seeing is good you should have no problem with it. It had sufficient resolution to make it out ok. I had a C9.25 up until a month ago, and it too struggled, but only when seeing was poor.

[g__day (Matthew)]

Thank you! How big in arc seconds is our view of the Cassini gap at its widest point right now - anyone know?

[dannat (Daniel)]

I have never really appreciated what it's like for the city folk, now I'm using the melb obs scoes, 8" refrac & 12" reflect I see the massive effect light pollution has. They are quality scopes but you need real good nights to see it

I don't feel bad though when I walk out my back door, pull out the 80 mm f15 let it cool for 2mins then immed go above 150x, does it quickly & easily

[alocky (Andrew lockwood)]

Originally Posted by g__day

Thank you! How big in arc seconds is our view of the Cassini gap at its widest point right now - anyone know?

I have often seen a figure of .7" (arc seconds) quoted for Cassini's division. Of interest is the difference between resolving a point source and a line, as that is below the Dawe's limit for a 4", yet I have no trouble seeing the division in any of my 4" scopes. Encke's division has eluded me in anything under 10", in comparison.
Cheers,
Andrew.

[sopticals (Stephen)]

Cassini discovered the division in 1675 using 90x magnification in a 2.5" refractor. Quite amazing considering the division is only around 0.5" arc. I find for me 200x-300x using my binoviewer, is the ideal for easily seeing the CD, and the pastel coloured bands on the planet itself. I really think that for most of us with normal/near normal eyesight 120x plus is needed to magnify the CD enough to clearly pull it out of the bright background of the rings. At the present "orientation" it should be an easy target for a 6" to 8" reflector at 150x.(seeing allowing).

[ColHut (Colin)]

...when seeing allows

You got it. Seeing was awful for me last night, and it certainly wasn't visible to me in my 10" F5 at x125 or x250. The local seeing was ghastly.

[HCR32 (Peter)]

Saturn is a planet that is in a sense forgiving in bad seeing conditions, but the division is one part of the planet that needs seeing to be fair. Your scopes aperture would add to the challenge. Planetary work comes to life with at least a four inch.

[mental4astro (Alexander)]

I just came across this old thread, and in going over it today, there's a few things that I can add to this that will help a lot, especially with resolution.

I too have seen the Cassini Division in a modest ED80. I too was very surprised considering its angular size.

In the last few years I've also been noticing a few other items that seemed to defy the "resolution" limits of various scopes I've used. At the same time, I've also failed to see the same features with other scopes despite the same aperture!

Seeing is one factor, but certainly NOT the only one. I'll leave eyesight out of this post.

First, optical quality. Not all scopes are created equal, even from the same factory and design. I've looked through many different 8" SCTs, new and old, and their optical quality varies TREMENDOUSLY! From superb (very few), to mediocre and unfortunately crap. Age has nothing to do with it as the worst I've seen was a brand new one!!! I will never buy a new or used SCT without star testing it first, and NO retailer will let that happen

But this doesn't explain why an 80mm refractor can show the Cassini Division when its angular size suggests it is too small for an 80mm scope.

The problem here has to do with the accepted notion of "resolution" for scopes, and we need to rethink this.

First we need to understand how we get the Dawe's limit and Rayleigh limit in relation to Airy Disks. The key is the Airy Disk itself. It is an actual disk, not a pinpoint of light. It has a dimension. And the Dawe's limit is the smallest separation that can be made out between two stars of equal magnitude. And this separation is not a complete separation with a black gap between the two, but one that resembles a dog biscuit, two distinct lobes, that the observer can say with confidence that they see these two lobes. Curiously enough, the smallest gap that can be see between two stars is still smaller than the Airy Disks themselves, yet no one seems to think about that! And that is a clue that no one picks up on...

When looking at extended objects, such as the Moon & planets, things are VERY DIFFERENT. Here there is no Airy Disks and the diffraction pattern of a point source of light is totally disrupted. The result is the resolution limit of any scope is actually much smaller. More than 1/10th smaller than the Dawe's limit for a given scope. This goes for both visual and photo.

This is why a 7" scope can resolve the Encke Division in Saturn's rings, a division that has an angular size of 0.05 arcsec, more than 1/10th smaller than the Dawe's limit for a 7" scope.

Remember, this assumes very good optical quality as some scopes are not even able to resolve to the resolving power of their aperture.

I've seen the Encke Division in a 7" Skywatcher Mak. And this explains my seeing the Cassini Division in an 80mm refractor & why the Cassini Division was discovered by a 2.5" scope. Also a major testament to the superb quality of that 2.5" refractor considering when it was made and the quality of eyepieces available then too!

I've also seen 8" SCTs struggle to show the Cassini Division. And of course the Encke Division was out of the question...

Alex.

[Prickly]

Hi

Don’t forget too to allow the scope some time to adjust to ambient temperature. Tube currents can be a factor and maks can take a bit of time to cool down. Could be a combination of seeing, altitude and scope cooling possibly. You would think you should be able to pick this feature up definitely in the mak.

LVWs are great eyepieces. Really impressed with mine. No problems there.

Enjoy the planetary viewing. Mars is still very impressive.

Cheers
David

[mental4astro (Alexander)]

David, this is going to make your wheels spin!

Instead of letting your Mak cool, rather DON'T let it cool!

The problem that cooling introduces is the metal tube cools quickly, leaving the air inside the tube to create a temperature gradient between the metal tube and the baffle/primary mirror, and this produces the tube currents.

By insulating the OTA, you don't allow the metal tube to cool too quickly, there by not creating a temperature gradient between the tube and the internals of the OTA, and hence no tube currents.

A few of us have started doing this to our Maks. The insulation can be as simple as a black yoga mat! And the rolled up mat is made such that it extends over the corrector lens just like a dew shield, making it duel purpose. And the result is immediate with no waiting time between set up to observing. This has been done from 6" Maks to 9" Maks.

Alex.

[Wavytone]

Cassini's division is indeed 0.69 arcsec at its widest point as seen from earth, though with the rings tilted on a shallow angle, if you can see it following around much of the ring plane then you are certainly seeing a dark band considerably smaller than this, possibly 0.2 arc seconds wide. And as Alex indicated its a good test of small scopes.

+1 insulating your mak will stop the internal tube current, so its ready for use from setup. Keeping the heat in the OTA also means the corrector will not cool quickly, so it stays dew-free for much longer (quite possibly all night) - and without the need for a heater.

I use a homemade plastic dewcap that extends back over much of the OTA see https://www.cloudynights.com/topic/6...tube-currents/ though at a pinch even corrugated cardboard, foam plastic, foam rubber or a picnic rug work fine. Experimentation indicates its not necessary to cover the entire OTA nor the back, and IMHO there is some merit in leaving the back exposed.

Same applies to SCTs by the way.

Simple engineering consideration of the thermal issues imply that thin-walled dark metallic tubes are about the worst possible choice (Meade, Celestron, take heed). Carbon fibre isn't much better as it is thermally conductive - and black. A good coat of white paint does make a difference - and is why refractors were traditionally white - and likewise telescope domes.

[Prickly]

Sounds like excellent advice to me. I'm fairly new to sct use and have never used maks so please take the cooling advice with a grain of salt. I will give the insulation idea a go too with my c9.25.

To me insulating the tube makes great sense. Thermal currents may in part explain the difficulties perhaps and this could be something to experiment with for sure.

[JeniSkunk (Jenifur)]

My eyesight isn't brilliant, badly shortsighted in my right eye, and slightly long sighted and lazy eye in the left.
I was only able to see the Cassini Division when I used my 6.5mm Saxon Cielo HD with a 2x Barlow on my Sky-Watcher Heritage 130P.
Using my 12mm Saxon Cielo HD with the 2x Barlow, I could see the rings, but not the Cassini Division.

I generally try to let my scope adjust to ambient temperature for about 30 minutes before I'm going to use it.

[Outcast (Carlton)]

I use closed cell foam as a dew shield on my LX90 8" SCT. I usually have it sitting about 3 - 4" onto the OTA, almost to the fork arms. I didn't do it this way to intentionally insulate the OTA, it's just the way I installed it after I made it & the way I set it up each time to give the foam rigidity so that it doesn't collapse in on itself at the far end & restrict my light gathering.

Edit: actually looking at my scope, it's probably like 5 - 6" down the OTA, the piece of foam I use as a dew shield is 18" long.

Is this sufficient insulation to achieve what you are indicating below?

Or should I be looking to make some modifications to have it extend further down the OTA?

Cheers

Carlton

Originally Posted by mental4astro

David, this is going to make your wheels spin!

Instead of letting your Mak cool, rather DON'T let it cool!

The problem that cooling introduces is the metal tube cools quickly, leaving the air inside the tube to create a temperature gradient between the metal tube and the baffle/primary mirror, and this produces the tube currents.

By insulating the OTA, you don't allow the metal tube to cool too quickly, there by not creating a temperature gradient between the tube and the internals of the OTA, and hence no tube currents.

A few of us have started doing this to our Maks. The insulation can be as simple as a black yoga mat! And the rolled up mat is made such that it extends over the corrector lens just like a dew shield, making it duel purpose. And the result is immediate with no waiting time between set up to observing. This has been done from 6" Maks to 9" Maks.

Alex.

[Wavytone]

Originally Posted by Outcast

Is this sufficient insulation to achieve what you are indicating below?

Simple - if it shows a tube current or the corrector dews, cover more of the OTA.

[Outcast (Carlton)]

No dew forming on the corrector plate I'm pleased to say but, how do I distinguish between tube currents & upper air currents causing any seeing issues?

Originally Posted by Wavytone

Simple - if it shows a tube current or the corrector dews, cover more of the OTA.

[astro744]

I've seen it through my Tele Vue 60 with Nagler zooms often. Requires good seeing and reasonably wide open rings. The TV-60 cools very fast; I take it out on a Telepod and go inside to make a cuppa. By the time I return it is at ambient temperature. I really like this little 'scope for the fast high quality planetary views it gives.

[mental4astro (Alexander)]

Originally Posted by Outcast

No dew forming on the corrector plate I'm pleased to say but, how do I distinguish between tube currents & upper air currents causing any seeing issues?

Carlton, I've just found a great Youtube video that shows tube currents in a C14 over the course of a few hours. Clip is a couple of minutes long, and it shows the C14 cool over this time and the tube currents that afflict it. Towards the end the author puts a warm hand in from of the scope, and the effect is quite dramatic.

Click here to load the video

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You'll notice that the plumes of heat pretty much follow one direction, and is a totally different effect from poor seeing with a defocused star.


Gosh, the Cassini Division in a 60mm scope! Blooming awesome!

Alex.

[Outcast (Carlton)]

Thanks Alex,

Having watched the video, the next time I am out I will look for this effect on a defocused star to determine whether I am seeing tube currents or whether the viewing is just affected by thermal currents in the atmosphere.

To date, I can't say I have paid that much attention; other than looking at globular clusters & centring stars when aligning (I defocus as part of the centring process in the absence of a reticule eyepiece) I haven't really looked that closely at stars.

Planning to start looking at splitting binary stars soon & having a look at the star testing of my optics...

My passion for astronomy has taken off since upgrading to an 8 inch SCT & my thirst for knowledge has deepened so as to get as much out of my viewing as possible. I've learned more in the past month from some of your & others posts than I have learned in the past 5 years of very casual observing through an 80mm refractor & I love it.

Regards

Carlton

Originally Posted by mental4astro

Carlton, I've just found a great Youtube video that shows tube currents in a C14 over the course of a few hours. Clip is a couple of minutes long, and it shows the C14 cool over this time and the tube currents that afflict it. Towards the end the author puts a warm hand in from of the scope, and the effect is quite dramatic.

You'll notice that the plumes of heat pretty much follow one direction, and is a totally different effect from poor seeing with a defocused star.


Gosh, the Cassini Division in a 60mm scope! Blooming awesome!

Alex.