78 replies to this topic

[Kevin Barker]

Hello Kelvin, interesting observation. Here is a comparison of your formula with the magnifications I'm using for observing Jupiter:

 

Lens           CB    A     Mmax   |  My usual mag  (in parenthesis what I use to confirm certain features in good seeing)

 

C63/840       1,9   1,76   111x    |  105x  (up to 120x)

AS80/1200     1,4   1,86   149x    |  120x  (up to 150x)

ATC82/1670    0,89  1,96   160x    |  111-134x (up to 170x)

ED100/900     ~1,4  1,86   186x    |  150  (up to 180x)

AS110/1670    1,9   1,76   194x    |  165x

 

Basically, there as a good agreement. But since all of my telescopes are of low CB, I guess I can't  provide any input/check for the slope dependence on CB.

Thank you for your observations. You have a great collection of low CB refractors. That As110/1650 must be great to use. I owned an APQ100/1000 for ten years. I used to use the 6 mm Orthoscopic for the best details on Jupiter's belts which led to 166X. Very similar to your "usual mag" for the 110.

 

I do think my achromat formula might be more appropriate to achromats CB from 2-9.

 

Now if you plotted CB  versus your "usual mag"/D and call this A with error bars for uncertainty  It would be interesting to see if we get a linear relationship.  I might try and do this today although I do not know the uncertainty in your "usual mag". Perhaps +/- 5...........  

 

I certainly like seeing the reality that really high mag is not great for the subtle fine detail we see on Jupiter!!

 

Kevin

Edited by Kevin Barker, 24 October 2014 - 03:19 PM.

[Kevin Barker]

Hello Kelvin, interesting observation. Here is a comparison of your formula with the magnifications I'm using for observing Jupiter:

 

Lens           CB    A     Mmax   |  My usual mag  (in parenthesis what I use to confirm certain features in good seeing)

 

C63/840       1,9   1,76   111x    |  105x  (up to 120x)

AS80/1200     1,4   1,86   149x    |  120x  (up to 150x)

ATC82/1670    0,89  1,96   160x    |  111-134x (up to 170x)

ED100/900     ~1,4  1,86   186x    |  150  (up to 180x)

AS110/1670    1,9   1,76   194x    |  165x

 

Basically, there as a good agreement. But since all of my telescopes are of low CB, I guess I can't  provide any input/check for the slope dependence on CB.

I note your exit pupil for your usual mag is usually between 0.6 mm and 0.66 mm!!!

 

For well corrected long focus semi apo's you are using an exit pupil around 0.60-0.70 mm.

 

For my current  apo and close to semi apo scopes I would use 195 X with my Zeiss APQ130 and 121 X for my AS80/840...with the 0.66 mm exit pupil.

[Sasa]

Hello Kelvin,

 

    yes, this is what I what I meant, my table is basically just showing that I prefer exit pupil around 0.65mm for observing Jupiter. Also be aware that my experience is not that great, I'm observing planets regularly for no more than 4 years. My taste was definitely changing, for example in the beginning my usual magnification on Jupiter in (former) ED100 was 186x because I had no 6mm eyepiece, next step was 7mm (128x). Later on, when I learn how to see more things on Jupiter, I actually liked more observing through 7mm than through 5.1mm. My feeling was, once I got 6mm eyepiece, that this was near my optimum (150x). It also answers Stanislas-Jean comment (I don't think my ED100 was a bad telescope, I considered it great one; contrast on planets seemed in direct comparison to be on the same level as in TEC140 for example, of course, the scale was smaller; but it is true AS110 is providing yet another level for the richness of the view, not sure how much of additional 1cm in diameter helps in here, could be a lot).

 

I'm also afraid that precision of +/-5 in mag is way to small, given the 1mm steps in focal lengths of my eyepieces, it would translate to about 10% precision for f/15 scopes, and 15-20% for f/9. Now for f/20 refractor I have more finer steps, here I definitely tend to prefer 15mm eyepiece (111x) over 16mm, power 104x is simply too small. As for 134x versus 111x, I have no strong preference. I usually do the sketch at 111x and check some features at 134x. I rarely find something new at 134x but the large scale helps to place the observed features into the sketch with somewhat better precision. This gives you feeling how precise my numbers are...

 

My only high CB data point would be from SkyWatcher 120/600 refractor, but here Jupiter was horrible even at 72x...

 

                       All the best,

 

                                    Alexander

[Kevin Barker]

 

 

 

 

My only high CB data point would be from SkyWatcher 120/600 refractor, but here Jupiter was horrible even at 72x...

 

Alexander

The 120/600 is probably going to have a lot of spherochromatism and other aberrations. I calculate the CB to be about 8. So a mag of 66 might be my M_{0 ^{(assuming a delta F of 1/1850 F})}

 

I have a similar scope a crudely made 123/830 which should give m about 85 X. Unfortunately I have yet to get the collimation spot on and i need to redesign the lens cell mounting. I do however agree with this power being optimum to see what Jupiter can deliver. From one night when the seeing was excellent it did show me pleasing images at 85X using a Zeiss 10 mm ortho. That is a 1.45 mm exit pupil. It did quite well above 100X this night but it was not showing me new features which were not visible before.

 

Not enough repeated observing to confirm this though and the collimation is off a bit. It does not show star  F in Theta Orionis for example.

 

So as we get more CB we end up not being able to usefully maintain a high contrast planetary image with that optimum exit pupil diameter around the 0.65 mm area.  As the CB gets above say  4-5?? then the exit pupil for optimal Jovian planetary viewing rises well above 0.65 mm.

 

And yes we are limited by the eyepieces we have. Unfortunately Zoom eyepieces tend to not perform as well as fixed focal length eyepieces.

 

Would'nt it be great if there was a zoom which operated like a top notch planetary eyepiece across a range of focal lengths.

 

Kevin

Edited by Kevin Barker, 23 October 2014 - 06:40 PM.