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Stu

Peashooter & Light Thimble slug it out under suburban skies

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I hadn’t planned to observe tonight because of the cold wind, but it dropped during the evening and I couldn’t let the clear sky go completely to waste.

I also couldn’t face putting a large scope out or having too much kit to put away, so yet again these two have given me some observing when otherwise I would have had none. These two being the Zeiss Telementor II (63mm f13.3 achro frac) and the TAL Alkor (65mm F7.7 newt).

After comparing them on doubles the other night (where I would put the TAL as a winner on points), I thought I would try some DSOs tonight, just to see what is possible with a tiny aperture and skies that are about mag 5 NELM tonight, fairly decent transparency too.

Having checked out the Double Double, I went for M57 first. Finding it was fairly straightforward using the Rigel on the TAL then the RACI (🤪) on the Telementor. Starting at medium power (x88 in the TAL and x67 in the Telementor) there was an immediate difference in contrast, with the refractor on top. Upping to x133 in both scopes and whilst the Ring Nebula stayed very much an indistinct round blob in the Newt, the contrast in the refractor showed it as a clear ring with averted vision. Round one to the frac!

M13 next, and I must say a similar story. Straight in at high power, I was surprised how large and detailed the image was. Clearly not resolved, but in the Telementor it was definitely showing speckling and some sparkling of stars with averted vision. The TAL remained steadfastly a round ball, brightening towards the centre but with little definition to it. Round two to the frac again.

Could the little newt pull itself off the ropes or would it be a third round knock out?

So, could I spot a galaxy under these conditions with such small scopes? As good a chance as any I would have, given the position of M81 and M82 currently. Unusually for here, the little triangle of stars that forms the star hop for me (between h Ursa Majoris and the little line of stars starting at d UM) was just about visible with the naked eye. This made finding them fairly trivial and I quickly had M81 in the x67 view of the refractor looking surprisingly good. A fairly large oval, brightening in the centre. Finding M82 was a bit more of a challenge but I soon picked that up too, a thin line of light with some vague hints of definition/mottling in the centre. Moving to the TAL, it was clear that it was to be a knock out. M81 was much less distinct and M82 was virtually invisible. I picked up vague hints, but at any power the frac was a clear winner. I popped back inside to get my 24mm Panoptic and was delighted to have both galaxies framed beautifully in the field of view at x35 and just under 2 degrees. It was a remarkably competent and pleasing view of these two galaxies given the tiny aperture and far from ideal DSO LP levels. My night vision was not particularly well developed either, so under a dark sky and better dark adapted it would do even better.

Just as I was surprised that the TAL won out on doubles, I was equally surprised a how much difference there was in the Telementor’s favour on DSOs.

A quick but fun session, showing the benefits of refractor contrast in this battle of the titans, the Peashooter overcoming a challenging 2mm aperture deficit to deliver the win! 😉🤣🤣👍👍

 

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One could count the photons those two little guys take in, sounds like a load of fun, clash of the anti titans.

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13 hours ago, Stu said:

Moving to the TAL, it was clear that it was to be a knock out. M81 was much less distinct and M82 was virtually invisible.

Great report Stu!

It sounds like the TAL has well figured lenses and the Zeiss has excellent coatings and a fine polish that allows better views of the galaxies.

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2 hours ago, jetstream said:

Great report Stu!

It sounds like the TAL has well figured lenses and the Zeiss has excellent coatings and a fine polish that allows better views of the galaxies.

Thanks Gerry. It was quite a significant and surprising difference. I wonder how much impact the secondary obstruction has on contrast in such a small newt? I should calculate the percentage out of interest. The Zeiss certainly surprised me which how much it showed, yes the night was very transparent but still I often struggle to see these galaxies here in bigger scopes so was impressed!

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14 minutes ago, Stu said:

secondary obstruction

How big do you figure it is?  obstruction can do weird things IMHO

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Superb.  As an imager (if there is such a thing) I almost felt like I was there with you willing each OTA on.  Thanks, Stu.

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I would think that a couple of things might be at play here- the possibility of stray light reducing contrast- the u tube video shows much reflections with a light shone down the tube. Another thing is mirror reflectivity- even the best are quoted as "nominal" and also vary in reflectance across its surface. A typical 96% high quality mirror will produce in the low 90's -92%-93% ( a top coater told me this).

So, if the TAL has 88% reflectance X2 for the secondary were down to 77% , add the eyepiece at 97% and were at about 75% system throughput. This does not take into account the reduction in aperture from the secondary.

The Zeiss will have top coatings and polish around 97% so were comparing a top 63mm (60mm effective with EP) to a scope operating around <48mm effective that has stray light issues (possibly).

The obstruction will make the TAL easier to split stars for sure IMHO- airy disk stuff. The CO might be a goodly number on this scope.

Glad you had fun Stu!

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2 hours ago, jetstream said:

I would think that a couple of things might be at play here- the possibility of stray light reducing contrast- the u tube video shows much reflections with a light shone down the tube. Another thing is mirror reflectivity- even the best are quoted as "nominal" and also vary in reflectance across its surface. A typical 96% high quality mirror will produce in the low 90's -92%-93% ( a top coater told me this).

So, if the TAL has 88% reflectance X2 for the secondary were down to 77% , add the eyepiece at 97% and were at about 75% system throughput. This does not take into account the reduction in aperture from the secondary.

The Zeiss will have top coatings and polish around 97% so were comparing a top 63mm (60mm effective with EP) to a scope operating around <48mm effective that has stray light issues (possibly).

The obstruction will make the TAL easier to split stars for sure IMHO- airy disk stuff. The CO might be a goodly number on this scope.

Glad you had fun Stu!

Thanks Gerry. All interesting stuff. I guess I hadn’t quite realised there would be such a difference but when you lay it out like that it is clear.

I just checked the TAL secondary and whilst it is hard to be totally accurate, I got the micrometer on it and reckon it is 18mm, which makes it 27.7%.

The mirrors could probably do with a clean, and I should probably also knock off a point for the dead spider suspended over one side of the primary! 😱😱😉

I wonder if it is worth, as a bit of a project and fun experiment, getting the mirrors HiLux coated and flocking the tube whilst it is apart? Might be fun, I could check performance and limiting magnitude before and after, wouldn’t cost much I don’t suppose.

I assume your comment about the Newt having an easier time of it on the doubles is because it puts more energy into the first diffraction ring so the primary airy disk is smaller, increasing the separation? Or am I way off with that?

This is proving far more fun and educational than I ever expected!

 

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7 minutes ago, Stu said:

This is proving far more fun

Fun is the name of the game!

Yes the obstruction does that and strangely enough increases contrast in the higher frequencies and keeps the frequency range intact for the aperture. I would guess, if the mirrors are 1/4 wave total that this scope would give the Telementor a run on the moon detail wise. I would definitely flock it, and paint the sec edge and holder with flat black paint with the smoke in it.  The drawtube and area would also get attention.

It would be nice to check reflectance before getting re coated- don't know how maybe someone here does? Roland Christen tested mass SCT's before the "Starbright" coatings era and some of those were extremely low in light transmission, as were some eyepieces. The SCT had the 88%x2 x  mirrors and about the same for the corrector...about 68% transmission and without counting the CO reduction in aperture.

With high reflectance mirrors, all flocked up etc I would be surprised if the TAL didn't offer similar DSO views. To its + side it is already APO chromatic.

With the most modern equipment available and from an optician that is VG -Terry O - my 97% nominal mirrors are about 93% true reflectance avg, which is top tier. There just might be a bit of hocus pocus with the mass produced reflectance numbers...but I think a lot are 90% or so.

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On 12/05/2020 at 18:32, Stu said:

I wonder if it is worth, as a bit of a project and fun experiment, getting the mirrors HiLux coated and flocking the tube whilst it is apart? Might be fun, I could check performance and limiting magnitude before and after, wouldn’t cost much I don’t suppose.

I'd ask Orion the question first Stu!😱🤣..

Could be a fun project though, as you say.

As regards the removal of dead spiders from the primary, I'm pretty sure you'll find helpful info on the web😁:hiding:

Dave

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Very interesting comparison. I wonder whether there is a difference in collimation in the two sessions, given that the TAL won on points on the doubles, which should have been a pushover for a refractor. Regarding photon efficiency of the optics, I do not think the Zeiss Telementor had top of the line coatings like the (Zeiss Oberkochen, West Germany) T* coating. I seem to remember many optics from that period in Eastern Germany having the distinct blue reflection of MgF2 single coatings (which were invented in Jena in around 1935). When comparing transmission, the losses in the optical glass itself should also be factored in, especially when considering older optics. With the thin lenses of a 63mm objectives, these are probably not huge, however

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