markse68

That’s the bit i’m not sure about too- if it were a perfect elastic collision then momentum and Ke would be conserved but they call the effect of ejecta production “momentum enhancement” in that the transfer of momentum is higher than the momentum of the projectile alone so presumably it shifts the target more than a perfectly elastic collision would?

https://www.sciencedaily.com/releases/2022/11/221101100742.htm

@saac Jim, I think your water wheel example shows that by scooping the blades the water is deflected backwards transferring more momentum to the wheel than if it were deflected sideways by a flat blade which makes sense but if there were no ejecta at all and the projectile somehow survived the impact and bounced straight back off the target, would that be less efficient than what we see here in deflecting the target? The ejecta has more mass than the projectile but presumably must be moving at lower velocity than the projectile to conserve momentum? 

It’s all very interesting anyhow

Mark

edit: thinking about it some more, is the time factor the key? I could imagine in a perfectly elastic collision the impact would be instantaneous and the bodies would dissipate some energy as their bodies deformed and resonated elastically- a bit like this tennis ball:

whereas with the inelastic collision that happened the impact duration is much longer and the energy channelled into the rearward firing ejecta jets leading to more efficient momentum transfer?

Amazing work Vroobel!

Mark

Misprint maybe but they are all wrong. Maybe a miscalculation across the range. Anyway, hopefully they see this and correct it.

Mark

23 minutes ago, Don Pensack said:

FLO, like APM and Altair, merely copied the numbers from the mfr, which are the actual FS diameters, not the virtual field stops that matter to us.

Effective field stops ARE usually bigger than the internal physical stop but not always.  Sometimes it depends on the aperture of the next lens up or the type of distortion in the field.

No Don, they really got them wrong. For the 18mm, manufacturer says 23.4mm. you say 21.7, Flo says 27mm. They’re all wrong I think 

Mark

1 hour ago, Don Pensack said:

Eyepieces with negative field lenses have a physical field stop in the form of an iris between the negative lens and the positive set higher in the eyepiece.

One thing that happens is that the negative lens moves the focal point up in the eyepiece, away from the physical field stop to a "virtual" field stop in between the lenses.

Since the image is expanded by the negative lens, this virtual field stop will be a different size than the physical stop itself.

The confusion comes from the listing from the manufacturer of the actual physical field stops, which no other manufacturer quotes in negative/positive designs.

All the other manufacturers quote the "virtual" field stop figures, because THEY are what determines the actual true field you see, not the physical stop in the eyepiece.

 

I find it useful to think of the virtual field stop as merely a magnified view of the physical stop that is below it.

 

The measured virtual field stops are (lab, not field measurements):

30mm--36.3mm (though a couple people have seen 36.4mm, an inconsequential difference)

24mm--27.6mm (though the edge is a bit vague, and some have measured 27.3-27.5mm)

18mm--21.7mm

15mm--18.2mm

10mm--(actually a 10.5mm FL)--11.2mm

Yes for some reason i thought the effective fs would be larger than the actual but it makes sense that a diverging negative lens would result in the efs being smaller than the afs. Anyway @FLO’s numbers are wrong.

Mark

it’s confusing huh. So i’m guessing the effective field stop should be used to determine vignetting? And do you know Louis if the field stop diameters Televue list in their specs are also the actual rather than effective? Could explain the vignetting i’m getting with my new finder and a nagler 16mm

Mark

Edit: TV do say the quoted fs is the effective one

I don’t really understand the physics of why it should deflect more with ejecta than with perfect transfer of kinetic energy like a newtons cradle, but doesn’t this mean that should we really need to use this technique we are still in the dark as it’s effect will be totally dependent on the makeup of the asteroid? Which we might not know in time

Mark

Question for @FLO - in your descriptions of your SL UFFs you list field stop diameters but they don’t agree with the specs on the schematic document you show eg the 18mm is listed as 27mm field stop which seems wrong but the schematic says it’s 23.4mm which sounds more likely?

Mark

Now it’s really TOTT. Found a 16mm Nagler so now i have 5deg fov at 15x. Had to shorten the tube by 1mm to reach focus but glad i did- I found the end of the tube was not square to the tube! I have the cell screwed hard up against the tube end so that was forcing the lenses to tilt slightly- not ideal.

The Nagler has a pretty flat field and combined with the ff gives an acceptably sharp image right to the field stop, with a bit of distortion as to be expected, and strangely slight vignetting which i don’t understand- it shouldn’t according to the specs. But it’s fine.

I picked up some ADM tube rings- bit bulky but they are so nice to adjust- really lovely.

Mark

Really good seeing tonight and pretty cold! Sorrels are doing a grand job though  Mars of course!

Mark

Very nice Geoff! I think the seeing form here in London at least is stunning tonight and you have some really nice detail there so must be there too. Interesting the ice cap/clouds match what I thought I was seeing- it must be an optical illusion I think but the brighter white part to the right of your image seems to bulge slightly from the disk! Oh I prefr the RH one- seems clearer

Mark

Mars is very sharp here in London tonight (as was Jupiter) Maybe the haze is helping- better than last night anyway. Feel like I'm on the verge of making out the finger protrusions near the Valles Marineris, and Acidalia Planitia further north is quite clearly defined with a bridge reaching up to I guess Margaritifer Terra. The cloud/ice at the pole seems to be extending above the the disk a little making it slightly non-circular- bit of a bulge? Is that even possible?

Mark

Ditto- I have a tri-finder setup but have come to the conclusion that a double would probably be fine- I don’t really use both rdf and starsense at the same time- one or the other really, and the only reason i use the rdf really is that i’m using a very old iphone for the starsense and it needs an external power bank as its battery is on its last legs and I can’t be arsed sometimes.

Mark

Hi nfazuk, a good way of gauging the values is searching the completed classifieds on here and on Astrobuysell- they go back years and you should be able to find similar items that have sold in the past and a range of values

Hope that helps and sorry you're giving up

Mark

Funny but the Pleiades seems to suit the finder view more than my scopes- in the 8" F8 I cant really fit it all in and it really needs the black surroundings I think, and in the 6" f5 I can get it all in but without a coma corrector it looks a bit rubbish  I'm sure a short refractor would suit it better

Mark

Congratulations Gary!

I'm just back inside thawing my toes in front of a fan heater after about an hour in "feels like -5". Back out in a bit. Gonna be full thermals tomorrow night for a Blackheath common Flamsteed meetup if the forecast holds 🥶!

Great seeing so far tonight- caught Ganymede as it passed onto the disk by chance and just noticed GRS should be central now so better hurry up and thaw out!

Mars looking very good too- need to spend a bit of time on it tonight as Valles Marineris is bang central right now

Right- I'm off back out

Hi Doug, welcome 🤗 

Mark

“ ceramic bearings have load ratings 25 to 35 per cent lower than steel bearings. As the material is much harder, it is also more brittle. Therefore, when put under too much pressure, ceramic material may crack while steel is more likely to suffer indentations.”

that’s where I’d look

Mark

It’s just a blue parts crate i found nearby to the desk i wanted to rest the tube on Magnus- nothing special or modified at all. I just wanted to raise the tube off the deck so a fan heater could be used efficiently to dry the paint quicker (and pre-warm it before spraying) Anything of suitable height would have done.

Mark

Did you change the worm bearings? They would take a lot of axial force when the mount jammed. Would click too if damaged

Mark

So they must have been prepared and  had a solar filter I guess? We often have big broadcast box lenses at work- very tempting to rig up an ep and try one on 🌙  

Mark

43 minutes ago, Captain Scarlet said:

Maybe take the opportunity to check the centredness of the spot while it’s out?

M

It's back in I put the spot on when I got the scope so I'm fairly confident. Made me think about stuff though- obsessing about collimation using autocolimators like the Cat'sEye but that relies on the centre spot being perfectly central. And not central to the mirror blank but central to the paraboloid. How would you get that perfect?

Mark

Not in action but out of- but it has been snowing hard and it was dark so decided on a little maintenance ready for Wednesday- mirror clean and matt black tube respray- has made a significant difference that will hopefully improve contrast on Mars. Really lovely little scope to work on this Tal-150p. Also realised I had been collimating wrong as the secondary has no offset 🤦‍♂️ Should improve things too!

Before:

After

Mark

Next weekend Valles Marineris should be centre stage- a very interesting feature to tease out of the pinkness 

Mark

13 minutes ago, Neil H said:

Hi Mark what type of filters 

I tried a Baader contrast booster last night which helped a bit though i have to say i prefer viewing without- it takes more time though. I think the Baader Neodymium can help too as can basic coloured filters- red orange or blue- if you have a set try them out- different colours reveal different features I think

Mark