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Astrophotography and Sketching >> Sketching

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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Uranus 20" DK. new [Re: Achernar]
      #5472676 - 10/16/12 02:09 AM

Thanks Taras.
Unfortunately this is not a matter of aperture mainly.
The question of scope is mainly lead by the D-d factor combined by the PTV level (or RMS for others). This implies an equivalent perfect diameter that own more or less the properties of this perfect diameter in terms of FTM curve.
For the featuring size on this planet this enough representative.
This way is interresting for comparison between diameters or scope in use. Otherwise we are discussing of the sexe of the trade names long time and useless. In other this means a potential that is degraded by the seeing level.
For instance my C8 equals the mewlon 180 that i owned and solded, now the 250 cassegrain equals almost the Meade 12 which is not bad.
Now when equivalent perfect diameters are defined the comparison between sizes is surprisingly not so crucial because the contrast is lead by this formula well verified with the FTM curve evaluations: c=c' * w/(w+5/6*D)
D the perfect dia, w width of the feature (elongated), c' the contrast actual on the planet.
If you get 1 or 2% on the planet, at the exit of the scope you will have less. Normal because a scope is a low pass filter. Sothat between 10 and 15" the gain is not null, not 1.5x but say 15% more in term of contrast.
Presently for capturing we need contrast.
Also it is necessary to know the scope characteristics (d, PTV actual) for getting the efficiency. Diffraction limited
commercial means only PTV3.3 (just for seeing the airy disk and the 1st diffraction ring well blurred in fact). In that case the strehl ratio is substantially less than 80% sothat the 15" may be less performing than the 10".
Just the idea because the intrinsic scope data are not identified. The 250 in use here equals 168mm perfect.
Hope this may be clear for you and an help.
Stanislas-Jean


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Paul G. Abel
sage


Reged: 01/28/10

Loc: UK
Re: Uranus 20" DK. new [Re: Achernar]
      #5472751 - 10/16/12 05:15 AM

Greetings all,

Many thanks for all the comments, I feel I should addressed some of the points raised here. First of all the telescope used to make the observations;

This was a a 20" Dall Kirkham. No star diagonal was used so North (as indicated by WINJUPOS) is at the bottom of the picture. To confuse matters, NASA have a different defintion of north than the IAU, the NASA definition puts north at the actua pole, the IAU definition (used here) puts north where the northern magnetic pole is located.

No star diagonals were used in this drawings, the image is drawn as it came out of the eyepiece. The markings on Uranus are always vague and ill-defined, even with a telescope as large as this, considerable care has to be taken since the disk is so small and one can easily be decieved. Nevertheless, I am reasonably sure the bright spot and brighter EZ were present.

Now the comparison with Stanislas-jean. Given the curvature of the EZ, some sort of star-diagonal must have been used. It looks as though this drawig was made some 12 days later, and is at a different CM longitude, so unless this is a long lived spot which has drifted in longitude (which I think unlikely), it must be a different feature.

Finally, the comparison between imaging and visual. This argument is probably one of the most pointless arguments in astronomy. Each method has its pros and cons, and in the end both processes require the input human brain (with all its subjectetivity). While it is true some imagers are content just to take pretty pictures, this is also true of some visual observers, I have seen a few drawings which are simply artistic interpretations, not scientific ones.

What we have to do is catagorise between observers, and artists. The observers should represent the scientific area of amateur astronomy. That is not to say the artistic way is any less valid, if that's what gives people pleasure out of astronomy then so be it.

I think most of what Stanislas said has been lost in translation, but I agree: your survey of imagers is wrong, I know quite a few British imagers and they are first class dilgent observers. Don't make sweeping statements which are blatently not true.

Best wishes,
-Paul.


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Paul G. Abel
sage


Reged: 01/28/10

Loc: UK
Re: Uranus 20" DK. new [Re: Paul G. Abel]
      #5472754 - 10/16/12 05:20 AM

stanislas-jean: I don't think that has anything to do with it! It's quite simple, the disk size of Uranus is small and requires a reasonable amount of magnification. If the seeing is poor, high magnification cannot be used.

Taras, you also have to look for some time, you can't just take a quick look, I spend about 15 minutes examining the disk of Uranus at various powers to try and build up a picture of what's there. Of course it maybe that you are sensitive to different colours (this is why people who are sensitive to red have difficulty seeing the markings of Venus).

Best wishes,
-Paul.


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Uranus 20" DK. new [Re: Paul G. Abel]
      #5472773 - 10/16/12 05:52 AM

Paul, your assessment about the visual results are OK.
This is not the same feature. The sketch I performed here in past are performed with a diagonal mirror.
But what I note is brightenings are on this temperate belt.
Last evening I checked again the Uranus at CM 280°, 260° last report here, and the brightening is still here.
Now regarding magnifications for use on this planet the higher the better indeed but considering the seeing level on the moment. 30min arc represent the full moon size with the nake eye, it's a pretty size for collecting features with 500x on good nights, with 20min of arc (still enough) it can be catched still something, under this is problématic with regards to contrast levels. This means here 333x which is enough with a 200mm of good facture.
At final in spite of these conditions the features are not strong and illness indeed but here, as you reported also.
I donot spend more than few minuts for drawing what is reported, I spend time after in order to confirm or try to confirm what was at the first look. This verification stage take some long minuts.
Stanislas-Jean


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Uranus 20" DK. new [Re: stanislas-jean]
      #5474462 - 10/17/12 03:34 AM Attachment (26 downloads)

The sketch of 15th evening showing the same brigthening for CM280° (initially for CM260° last 12th).
Stanislas-Jean


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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Uranus 20" DK. new [Re: stanislas-jean]
      #5476044 - 10/17/12 10:55 PM

Paul,

Im particularly interested in how you reason or interpret the color filters having visibility, or lack on its moon[s].
It would seem blue should transmit the most light because of the spectrum pass in low light for the retina, and the red shoulve yielded nothing instead.

Im tempted to want to think the moon was reddish and so the filter passed it well, but this isnt the case. Perhaps the red helped the seeing steady by passing the sharper less affected light wavelengths. Blue is known dispersal more so in seeing less than optimum.

Pete


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stanislas-jean
Carpal Tunnel


Reged: 10/22/08

Re: Uranus 20" DK. new [Re: azure1961p]
      #5476276 - 10/18/12 02:34 AM

What was in use
WF without filter
W12 deep yellow filter
Refer to the previous post with the clouds absorption bands showed on the spectrum graph for the selection of the filter.
There is no matter of blue filter here, except for showing nothing visually with the use of such filter (mainly the W82) that cut red light but keep the green light. The use of the blue filter is interresting to be more confortable for the uranus report. IF nothing is reported with it is in a certain way a demontration. If something is this has to be controversial. But when showing the italian observer reports of the alpo site the blue filter brought the collection of the dark collar bordering the polar cap. Interresting, no? An artefact, probably not, in spite of the "authorised" views contracditory.
But the contrasts are fluctuing by periods it seems.
Stanislas-Jean


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