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Use of reflectors for unequal binaries

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#1 WRAK

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Posted 07 May 2013 - 06:25 AM

So far I have only used refractors for double star observing for several reasons - most important ease of use.
This is especially true for apertures up to 6" or slightly above. Larger refractors are too heavy to be considered for easy use.
As for some targets an aperture larger than 150mm seems of interest reflectors have to be considered - still relatively easy to handle but with some obstacles like thermal issues, collimation.
And not to forget the effects of central obstuction on the performance of the scope for resolving unequal binaries compared with refractors.
Before starting using reflectors for this purpose I would like to state my expectations for later verification or probably falsification:
- With the exception of the Schiefspiegler all reflectors have as far as I know a central obstuction thus reducing the photon sampling surface area of the aperture. The calculation is easy enough: Area of aperture minus area of central obstruction gives the comparable area of a refractor. Example reflector 210mm with CO 0f 0.3 corresponds to a refractor with ~190mm when taking the spiders into consideration
- This was the easy part - now come the effects of CO on image quality regarding diffraction pattern: While a refractor samples 83.8% of the total light in the Airy disk (means within the first minimum) a reflector with 30% CO samples here only 68.2% - this means that in terms of brightness of the spurious disk the reflector drops to the performance of a 171mm refractor. This may be not even to notice for the primaries of not this faint doubles but can already be a big effect for very faint companions where a little bit can make the difference
- The Airy diks of stars is somewhat smaller with CO than without - this is an advantage for resolving close equal bright doubles but this is not what we are looking for
- The first diffraction ring with CO is brighter than without - this is a problem for close faint secondaries near or on the first diffraction ring. This means for close unequal doubles scopes with CO are second choice.

Today I got the message that the 225mm inner diameter iris diaphragm is again on its way and I hope this time for a working one.
This seems to get very interesting and to some degree I think that I may have to make a separate RoT model for resolving unequal doubles for reflectors.
Wilfried

#2 fred1871

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Posted 07 May 2013 - 07:43 PM

Wilfried, is this a new version of the topic I started back in January, called "Refractors vs SCTs and Maks for uneven doubles"? :grin:

I'm happy to see it revived in whatever form. I think we have a long way to go in understanding how CO impacts on uneven double stars.

Tomorrow (after I've finished some urgent matters today) I'll have another look at some material I intended to write about earlier, but put aside. In particular, relating to the changing form of the diffraction pattern - and how much energy is transferred into particular diffraction rings - with changing levels of CO. I think it's more complicated than first appears.

Then there's the effect of spherical aberration, often treated as similar to CO in its detrimental effect on images, but the pattern seems to be different. And CO and SA interact. Messy.

On the practical side, I'm intending to re-observe with my 235mm SCT various uneven pairs for which I have observing notes made with my 140mm refractor. I want to see if I can get significantly more difficult doubles with the larger scope despite 0.36 CO, and test this with pairs that have different amounts of delta-m. Does a large delta-m equalize the two scopes? - where a moderate delta-m allows the larger one to do better?

In past observing I don't think I've pushed the SCT as hard as the refractor. Time to find its limits.

#3 WRAK

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Posted 08 May 2013 - 01:43 AM

Fred, so we can do some work parallel. I did not exactly remember your thread but was sure this topic was discussed probably several times on this forum. The reason for starting this again is simply timing with my own observation plans. With the help of an iris I will be able to change not only the size of the aperture but also of the CO ratio.
One of the first things I will check is the TML for different sizes of CO (usually calculated only by aperture but this can not be correct) and another one is the visual behavior of the diffraction pattern with changing CO.
Wilfried

#4 azure1961p

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Posted 08 May 2013 - 10:56 PM

I think a progressive direction in CO affected double star astronomy is in the deliberate use of over sized CO masks to shrink the spurious disc to gain a tighter *pinch* between merged Dawes (or closer) doubles. There's got to be a diminishing return point with something like this where after a certain size the dimming of the diffraction pattern no longer suports a given CO mask percentage. I'm guessing its around 60% .

At anyrate I think the potential is their to achieve something's with an artificially larger CO that outdoes a refractor of the same aperture. I haven't done it yet but it seems like a compelling direction with some promise, if subtle.

Pete

#5 WRAK

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Posted 13 May 2013 - 01:43 PM

First light with the Tak Mewlon 210 was a hard landing - I had ordered it with a "perfect" collimation by an optical professional but transport seems to have its own hidden agenda. So what I got were no crisp spurious disks but badly distorted spots of light due to lost collimation.
I will certainly try to do my own best with Polaris on next opportunity even when I have hard feelings about wasting precious moments of good seeing with the task of collimation. Besindes: Reversing focus not only produces focus shift but also huge image shift by some arc minutes - did not expect this poor craftmanship from Tak.
Maybe I come on terms with the C925 so I can skip the Mewlon collimination task.
Good news: The 225mm diameter iris is meanwhile replaced and works now properly.
Can only hope for better weather - is rotten again now.
Wilfried

#6 Ed Wiley

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Posted 13 May 2013 - 08:11 PM

Hi Wilfried:

I would expect any DK (or SCT or Newtonian) to loose collimation when shipped. Mine looses collimation when I transport it by car in a padded case. Mine (Royce 8" DK) is easy to collimate, takes me just a few minutes, but I am not fighting a moving mirror. Never collimated a Mewlon, does it have adjustments on the primary as well as the secondary?

#7 WRAK

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Posted 14 May 2013 - 03:47 AM

Ed, adjustments only for the secondary. Tak claims a very stable collimation so I thought it worth a try to order a pre collimated scope.
Regarding Royce - the claim is here refractor like performance. How would you compare your 8" DK with a refractor?
Wilfried

#8 Ed Wiley

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Posted 14 May 2013 - 11:00 AM

Hi Wilfried: I have no 8" refractor to compare to the Royce. In fact, I did not take the Royce to the TSP to check it against the SCURT (resolution target) so I cannot even give you a quantitative evaluation. Worse, I don't do visual work, all mine is based on video high resolution imaging and autocorrelation reductions, so I cannot even give you a qualitative evaluation. However, the Royce has been shown capable of high resolution imaging of doubles at about 1" separation under indifferent conditions (3/5) in eastern Kansas. (Results within error of interferometry measures by professionals with big scopes.)

As an aside, my C11 edge did very well with the SCURT, averaging below the Dawes Limit on three runs at the target. Not bad for a mass-produced scope. I would expect the same or better for a Mewlon or a Royce given excellent collimation. I do not think there is any way around collimating each session. I totally buy into Legault's analysis:

http://legault.perso....fr/collim.html

Ed

#9 WRAK

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Posted 15 May 2013 - 01:56 AM

Could yesterday try my C925 but seeing was only about Pickering 5 so I could only assume that the distorted diffraction pattern also showed signs of a slight mis collimation. Makes things a bit complicated if you depend on good seeing for good collimation.
Wilfried

#10 Ed Wiley

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Posted 15 May 2013 - 09:22 PM

I find this resource useful under less than ideal conditions:

http://www.astromart...?article_id=718

For the hopeless case when collimation is really screwed up:

http://www.robincasa...ges/collim.html

Naturally bad seeing makes collimation difficult, thus an artificial star is useful. The TSP one was very convenient, I am going to try one in the backyard

Ed

Ed

#11 WRAK

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Posted 16 May 2013 - 03:16 PM

Thanks Ed, using a full moon night with good seeing for collimation with Polaris seems a good idea indeed - even if it is not true that a full moon night is of no use for double star observing. The use of a stick ro replace somebodys hand to find the direction of mis collimation seems also a good idea. Use of a Duncan mask seems even more efficient. An artificial star may replace Polaris very well but at the price of an unusual angle of the scope and an unrealistic distance to the object.
I have to go through the collimation challenge to get the best out of my SCT for resolving unequal doubles and with more experience it will become easy I hope.
Wilfried






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