Seeing Saturn

Of course I saw it I am talking about the 'Seeing' & Saturn

I had miraculously been able to squish my XT12i's base through the pre renovation 22" wide back door to the small dark rear yard on Saturday but the transparency was lousy even with the NGC4088 Supernova at Zenith. I may or may not have seen the SN with averted.

My point is though, that whatever weather system caused the lousy transparency on Saturday brought magnificent seeing on Sunday. I had no thoughts about bringing the scope out front or out anywhere all of Sunday. I finished work at 9pm and walked outside to put something in the wheely bin and just on a split second whim said 'feck it', and imediatly lugged out the scope and equatorial platform out front for a quick look at Saturn.

Needless to say that no sooner had I set up the scope when the sky did its usual. Not a wisp of a cloud in the sky for the whole weekend and just like clockwork as soon as I lifted my head after setting up...."OMG, Saturn is gone...arrrggghhh!!"

Thankfully I discovered after whipping out the Bino's and looking further south from whence the cloud came that there were indeed stars visible, so this was just a patch of cloud rather than a bank of cloud.

20 minutes later the sky was 100% clear again. So ultimately I was observing within about 20 minutes of starting to set up.

What did I see? Well I wouldn't have started with my 3.5mm at 428x normally but this was the first EP I put in the focuser last night because I really wanted to fine tune my RACI and Telrad alignment.

Absolutely pin sharp!!.......70% of the time!! with only a small amount of blurring for the remaining 30%. This is only 20 minutes after the scope was inside a heated room. This gentleman and ladies is why good mirror cooling and boundary layer air scrubbing is so advantageous. No leaving the scope out hours in advance for the mirror to cool and waiting hours for the mirror to start delivering its best views. This...is why I went to all the trouble of modding my XT12i to such an extent. It started delivering pin sharp views of Saturn at 428x as soon as I had my first chance to look in the EP after my 20 minute setup and RACI and Telrad fiddling time.

I've heard it too many times. Atmospheric seeing is lousy tonight lads. Oh contraire, your mirror seeing is probably lousy. We can't do anything about atmospheric seeing, but we can do something about mirror 'boundary layer' seeing. With a proper mirror cooling and boundary fan system in place at least you will know for sure that if the seeing appears bad its the kind of seeing you can do nothing about and not the kind that you 'Should' have done something about.

So many of my mods were and are about making the scope as easy and as quick to setup and to collimate to a high degree of accuracy and to deliver the best views it is capable of within the shortest period of time. Here in Ireland we don't have the advantage of 300 days of Arizona clear skies. We have got to take advantage of every gap in the clouds. We do not have the luxury of setting the scope up 4 hours in advance to start naturally cooling to ambient.

One other comment on the mods before I get back to the observing report. SOme of you may have heard me talk about the street lights out front of my house. Where I set up last night in the yard is a lot closer to an amber streetlight than where I set up out on the road for my solo 100hours event. Here, my full tube flocking and long SCT dewshield used as my dob lightshield really came into their own. Pitch black sky around saturn in the FOV despite almost being under that street light. It was only at the very end of the session as Saturn was about an hour past the meridian that the light from the street light had traced down the inside of 1ft lightshield almost to the focuser drawtube. Had I had no lightshield or flocking the streetlight would have been shining into the drawtube from the very beginning of the session.

Anyway, back to the report.....

N & S equatorial bands, pin sharp ring shadow, gap between the disc and the rings, visible tilt of rings disappearing behind the planet, slim shadow of disc on the back of the rings now that Saturn is past opposition. Splendid view! Had to back off to my 5mm at 300x, not due to seeing but to get all the moons into the FOV. Rhea, Titan to the left (newt inverted FOV) and Dione to the right. After checking out SNP I realised that what I thought was something too far out of the ring plane to be a moon and thus must be a background star was in fact Iapetus and what I thought where background faint stars near the rings were in fact Mimas and Enceladus.
6 Saturnian Moons. Woohoo!! Think 5 was my previous record.

So anyway, I figured I have to take advantage of this seeing for a permanent record, so in I went to get the webcam and laptop. Here my efforts in aligning the RACI to the 3.5mm EP paid off as it really helped me re-aquire Saturn reasonably quickly when I'd lose it out of the FOV of my 3x Barlowed SPC900 webcam on the laptop screen.

What else did I learn? That the effort in modding the ALT and AZ scope motions with ebonystar and teflon were worth the effort as the previous inherent scope stiction made re-aquiring the planet or re-centering the planet in the tiny webcam FOV a chore. Still a bit fiddly but much easier than before. Proper counterbalance really paid off too. I recently spent a lot of time finding the right weight and balance point along the OTA axis and on the exact opposite side of the OTA to the focuser and finders. This meant that I did not need to use the ALT brake to hold the OTA at the correct altitude. It stayed where I put it without the brake. What this meant was with the brake disengaged completely there was no brake induced stiction and I could move the scope accurately in ALT with the tip of my finger, ie much finer adjustments to re-centre.

So anyway, I did a couple of 5 minute planetary imaging runs. This is where I ran into a bit of trouble with some hardware I could do nothing about. My EQ platform which is superb for Visual magnifications but at 800x + with the webcam it starts to highlight some drift. I just do not have the patience to do a drift align of the platform. Saturn drifted very slowly left/right but reasonably quickly up/down. While my ebonystar/teflon and counterweight mods made recentering a lot easier than previously it was still a bit fiddly re-centering. While before the mods last year, I'd lose Saturn out of the FOV due to backlash, stiction and overshoot and I'd have to be very very lucky to get it back before the end of the 5 minute imaging run, now, with the mods I would never lose it completely and could quickly re-aquire but a lot of frames were wasted with Saturn too close to the edge or vibration blurring as I re-centered.

During my attempts last night at processing PPM wouldn't or couldn't centre a significant proportion of the frames for Registax and registax just couldn't keep alignment. Going to try re-processing again tonight and manually sort through the 5000 odd frames and only feed PPMcentre the frames where Saturn was reasonably centered to begin with.

Thats not going to be an issue for Knightrider though Will probably be finished too late for this Saturn season but will be ready for Jupiter in the late Summer/Autumn although I don't know whether its worth imaging the Jovian giant at only 22º this year.

The reason EQ platform drift won't be an issue with Knightrider is that I'll have fitted the JMI ALT AZ motor system to Knightrider. While this system with its user adjusted ALT AZ tracking speed is good for 10-15 minutes tracking before user re-adjustment and thus for Visual and Outreach work, it is however no Servocat or DobdriverII with computer controlled automatic full sky sidereal tracking. No good for imaging then. Then I realised that DOH! No, I would not sell the EQ platform with the Orion XT12i I would need to keep it for imaging. But what the JMI ALT AZ motor did bring to the table was this. I get the non field rotating rough tracking from the platform and don't need to go mad on the platform alignment and where with the old scope I had to manually adjust ALT AZ to re-centre which was still a bit fiddley even with the bearing mods, now with the TNT I can smoothly and finely and accurately recentre with the JMI ALT AZ motors with out risk of overshoot or introducing image vibration. Like I said, doing it by hand meant vibration. Vibration meant lots of wasted frames, can't constantly re-centre because I just wast more frames due to vibration, can't let it drift too close to edge of FOV because PPM centre can't centre frame in processing but that means more vibration inducing re-centering. Rock and a hard place! the JMI TNT in combo with the platform means the best of all worlds and being able to put 16" apetures worth of resolution on a planet, keeping the non field rotating Saturn smoothly centred and at a fraction of the cost of a 16" scope on an uber mount.

Luckily I already have the other vibration inducer under control. While I have the Lightbridges GSO dual speed focuser fitted to last nights scope and this showed up how hard it is to focus manually on a planet in the tiny webcam FOV when you are stuck at the other end of the scope feet away from the laptop screen, when every touch of the focuser risks knocking Saturn totally out of the high mag FOV, where the very act of focusing by hand vibrates the image on screen meaning you can't judge the bloody focus. ie another Rock and a hard place Well thankfully I already have my Moonlite focuser with Rigel Motofocus fitted to Knightrider. So when imaging with Knightrider with the Motofocus and the platform and the JMI TNT, I won't ever have to touch the scope at all during an imaging run.

These are some very good reasons why I am so hardware orientated with an eye on functionality, value for money and price performance ratio. I am not such a hardware mod and gadget whore just for the sake of it. Its all about delivering what I want, when I want it, for a price that doesn't break the bank given the functionality and apeture that I am getting.

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THE END

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Man,that must go as one of the longest posts on the boards, but I do understand what you're on about.
I routinely find that when I use the shroud (mimicks a solid tube) on the obsessionator that the views of Saturn are seriously inferior conpared to when I dont use it, it either causes tube currents or traps currents from the primary mirror, either way its a big nono for me when planetary viewing/imaging.
Also, I had to reprogram the slower slew speeds on the scope for centering the planets on the webcam chip, the wrong slew speed here is a nightmare, thankfully stiction/overshoot is not an issue with the scope.
When it comes to astronomical equipment, cutting corners and trying to save a few euros always comes back to bite you in the ass.
From experience, I've arrived at the philosophy of buying it once and buying it right.

Dave_Lillis wrote:

From experience, I've arrived at the philosophy of buying it once and buying it right.

Or "buy cheap, buy twice" as a colleague once told me. And how true it is...

I have a shroud on the Lightbridge too. The disadvantage over a solid tube I think is that with a non uniform inner surface with rocker box lips or tube endrings, truss poles and billowed shroud inner surface and then another lip into the UTA you have a multitude of deflection points for the tube currents running up the inside of 'the tube' to be deflected back into the lightpath. With a solid tube you have a uniform surface for the tube currents blowing out by the fans to hug the inner smooth walls and remain out of the lightpath. Thats why common wisdom is, to have the OTA tube an inch or two wider than the actual mirror. To give the currents within the tube space to flow up and out without impinging into the lightpath.

Perhaps a solution to minimise the deflection in a truss with a shroud would be to have a cowling ring around the outside bottom of the UTA to which the top of the shroud attaches. ie air running up the shroud/tube escapes out the cowling instead of being deflected by the bottom ring of the UTA back into the ligthpath. Sure you have a gap in your shroud now but its a gap facing the area of sky your scope is pointing. There is unlikely to be a streetlight in the FOV so your shroud still performs its primary functions of lightshielding the Primary and stopping body heat currents floating into the lightpath while at the same time not introducing deflected tube currents.

My solution is an insulated lightbridge tube to eliminate the tube induced 'tube currents' cased by the metal OTA and UTA, Sealing around the primary so that the rear cooling fan doesn't blow air and currents up the inside of the tube and shroud and a boundary layer removal system where a nozzled plenum cooler blows across the surface of the mirror clearing the boundary layer and instead of this air then rising up the tube I have another set of plenum nozzles sucking this air away and blowing it out back of the mirror via ducts. So there should be no boundary layer or any air flowing up my OTA/Shroud/UTA tube.

Personally I would always use my shroud even at a dark site viewing DSO's. Sure I don't need its lightshielding function but it is still protecting the primary from clumbseyness, its still blocking body heat currents getting into the lightpath.

I know what you are saying in relation to buy once. But thats easy to say when your first dob tracking solution purchase was for for an Obsession!! :laugh: There really is only one choice there. Spend over a grand on a large quality EQ platform for it or spend over a grand on a servocat and another 6 or 7 hundred on an argo if it takes your fancy. I think a servocat and argo is nearly or over 2 grand isn't it. At any rate, your choice was really made for you. The Obsessionator deserved only the best.

In my case my first dob was a 12in Orion and the only tracking option for that was an EQ platform. Its not like I could look into the future and decide I'll buy it 3 sizes too big to account for a hypothetical future 20in Dob purchase. Buy once! I did oversize it 1 step up which did funnily enough take into account a future 16in Dob purchase. Buy once! Servocat was not an option for the Orion. Now servocat was actually an option for the Lightbridge but again its nearly 2000 euro for the SC/argo vs 500 for the JMI TNT and Intelliscope mod on an LB16 I got for 1,100. One also had to factor in to the cost benefit analysis the money I would lose on resale of the platform.

I know in hindsigt that for a grand or two more than my total astro spend to date I could have gotten a 15in Obsession with servocat and argo and all the trimmings. ie the pinnacle of the size and functionality of what I am putting together with a hodge podge of gear with platforms and JMI TNT's and Intelliscopes and cheap chinese 16" Lightbrdges. But who recommends to a beginner that they should buy a 15in Obsession with servocat and argo and all the trimmings as their first scope. No one. So one is always bound to have a legacy of other older cheaper gear who's depreciation on resale has to be taken into account for the cost benefit analysis.

Well thats how I am rationalising my purchase 'mistakes'. :laugh:

Dave_Lillis wrote:

I routinely find that when I use the shroud (mimicks a solid tube) on the obsessionator that the views of Saturn are seriously inferior conpared to when I dont use it, it either causes tube currents or traps currents from the primary mirror, either way its a big nono for me when planetary viewing/imaging.

The shroud does not need to extend down the full length of the truss to the mirror box to be fully effective - I leave a gap of a foot or so between the mirror and the bottom of the shroud to allow warm air to escape out of the light path, and it works pretty well. Keith is bang on about using fans to cool down the optics and break up the boundary layer (look at Bird's planetary photos from Oz and then look at his telescope set up.) I'm incorporating quite a few fans (and a good bit of ducting) in my rebuild of the 20" to tackle this problem

Phil.

With regard to leaving a gap at the bottom of the shroud like you have there Philip, I wonder while it solves the body heat drifting into the lightpath issue, dropping something on the primary issue and your open 'mirror box' obviously means it solves the tube currents issue, does it in fact stop the ambient light hitting the primary issue? Though of course you don't have the streetlight issue that I have so I guess thats the least important shroud function for you.

Its probably actually more suitable for Daves Obsession where his mirror box affords much more lightshielding of the primary. Tis a bit of a DOH moment for me with all my talk of shrouds hooked around a cowling in the UTA to create a ring exhaust for the shroud. Just pull the shroud up a few inches at the bottom!! LOL Thats why Knightrider is taking so long. Kind of like measure twice cut once. All along the way I'd seem to come up with the most complicated solution to a particular problem first and only after pondering it for a few days do I cop onto the KISS solution Its taking so long because I am not doing the mods straight away but giving myself some time to brainstorm a more KISS solution. You have the knack of seeing the KISS idea first try

BTW, heres a processed image. Note, I can't process to save my life.