Captain Scarlet

Nice by proxy! The earthshine was very noticeable last night for me too, but it was all that was ... there was a small band of cloud-free sky just about where the Moon was, but that was it.

M

I'll try to address the precise distance-to-pole thing tomorrow.

The FL of the scope as used for the photo, at 2799mm  +/- 17mm, with this combination of camera and adapters, I am highly confident of. In fact I've done quite a bit of measurement of this, ending up with the following chart. I had an extra approx 40mm of back-focus from the "B" point in the chart below. A longish thread (Im the OP on it )about this can be seen here.

OK raw image attached here...

_S7A5565.CR2

I find that a rocket blower gets rid of perhaps half the dust, the rest stays firmly in place.

I've read of 3 extremely respected people in the industry recommending and using the fingertip method on their optics, and I do, or rather did, buy the arguments about it, before I read that there's significant quartz in house-dust.

The story is that if a surface is quartz-coated, quartz being really quite hard, and you gently, I mean gently, rub your soft fingertips over the mirror or lens surface in the presence of water + detergent, you'll end up with a good result. My own direct conversation was with a senior person at Leica who recommended the technique for their spotting scopes and binoculars. He even went on to say to do it under a slowly running warm-water tap, but their optics are nitrogen-purged waterproof! This is actually how I clean my bins and spotting scopes.

Sapphire in the jewellery world is indeed coloured, but synthetic "pure" sapphire is clear, and incredibly hard. Almost no commonly-occuring substances are harder, only diamond dust really. Sapphire's colours arise from other contaminants in the Carborundum crystal structure. In fact there's an in-joke in the gemstone world that says there's only one colour in which Sapphire gemstones don't occur: Red. That's because they have a different name: Ruby! Ruby is in fact red sapphire.

M

I stepped away from this thread for a while and returned to find it covering 10 pages!

This is going to be incredibly useful. I already have an FFT algorithm which I wrote a few years ago as a function for Excel for different purposes (analysis of accelerometer data for a rowing boat - see my sig/blog where there are some more details, not of the signal-processing which was inconclusive, but the acceleration data itself). I I may well be bale to bring this tool to bear on this.

I do have a couple of questions though:

- The straight edge? Should it be Black/White, or is any darker/lighter interface useable?

- Does it matter if the "dark" is simply out-of-focus background with the bright edge being in focus?

- I've attached a photo: will the edge indicated be useable for this? It's about 125m away, taken at prime focus with my Skymax180 at 2799mm FL and EOS 7Dmk2. And against a greenish background (fields). I've only attached a jpeg for now as I don't intend this to be "the" photo, it was getting dark and shutter speed was slow. I'll re-do it on a bright day.

Cheers and I love this thread, Magnus

It's my understanding that most mirrors and lenses these days are, in addition to anti-reflection coatings, coated with a scratch-resistant layer on at least the surfaces that are exposed to the elements and likely to get contaminated and therefore cleaned and touched. I've tried to search the literature for what those coatings are actually made from, and it's surprisingly difficult to get any concrete information.

The search results are mostly of opticians trying to sell you scratch-resistant coatings for your new specs. Even the remaining 5% of search returns say "we employ CaptainMagenta-Lux,  the latest proprietary anti-scratch technology" or somesuch without giving any useful details. Schott's site says something like "give us a call and we'll give you more info via personal conversation". It all seems very hush-hush. Commercial secrets, I guess.

I believe that most such coatings are SiO2, i.e. Quartz, which is pretty hard stuff and should resist a reasonable amount of abuse. My only qualm came when further looking for information on household dust, to find that a reasonable proportion of household dust comprises, yes indeed, small particles of quartz! Which makes me reluctant to use the "soft fingertip" method of cleaning lenses and mirrors.

Another far harder material that seems to be used for some, especially IR,  optics and watch-glasses is Sapphire or Carborundum, which is much harder than Quartz. So my question: is Sapphire ever used for scratch-resistant coatings? If so, I'd be much happier to use a fingertip cleaning-method on it. Indeed if I were confident my scope had a Sapphire coating, I could happlily scrub away with wire wool! Well perhaps not, but you get the point.

Comments or descriptions of actual industry experience welcome...

Cheers, Magnus

Valentine’s Day present ... 100% increase in my filter count, adding to the Oiii I already have.

Cheers, Magnus

Carlo Rovelli’s Reality is Not What it Seems (or perhaps The Order of Time) is a superb mainly qualitative discussion of just this stuff. How a photon halfway from Andromeda is in Andromeda’s past but our future. How it’s possible indeed commonplace for any of us to time-travel forwards but not backwards. By far the best explanations of these mind-exploding things I’ve read, anyway.

Magnus

It’s a balancing act, or optimization if you like. Lower magnification gives you wider field. More aperture gives you more light to detect those more stars, but also comes with increased exit pupil, which is fine until you hit your actual pupil’s size after which extra aperture makes no further difference. To keep the pupil size down as you increase aperture, you need to increase magnification.

More independent of other effects is “darker skies” which of course also show more stars so for any sort of scope and any sort of observing darker skies are better.

I hanker after a big dob, and the RFT concept is front and centre of my thinking for it...

Cheers, Magnus

Can just about make out the E in the penultimate one? Impressive for a smartphone in Bortle 7/8.

M

18 hours ago, John said:

... Markus was able to supply me with the copies of the original paperwork and tests for my scope - he keeps them all on file apparently. The previous owner of my scope had lost the copies that had been supplied with the scope so I was glad to be able to get another set of them.

Funnily enough when I bought my APM-LZOS 105/650 from a fellow SGL-er I emailed Markus to ask about some handwritten numbers on the inside of the lens cell, and the veracity of the scope (yes all was good) and to ask if he had a better copy of the test report for the scope, as the report I had was a bit tatty and water-stained. He replied to say that no he didn’t have a copy of that particular report.

M

23 minutes ago, John said:

If I did not already have a TMB/LZOS 130mm I would have been onto that one 😀

I have got away with two 4 inch refractors but I suspect my other half will not see why two 5.1 inchers are "essential"

It's quite simple. It needs to be presented as an optimization problem. The correct number of scopes, obviously, = n+1, where n = no. scopes you own at present.

But also, the correct number of scopes = s-1, where s = no. scopes that would cause your partner to leave you.

Combining these, you get s = n+2. Which means your partner will only leave you if you get 2 more scopes. So there's room for just one more.

Ad infinitum...

There is a TMB 130 in the for sale section at the moment...

Seriously though I’d be tempted by the 8” CC but would be worried by diffraction stripes off any bright planets. Such planets through my 12” newt, which of course has spider vanes, are in my view slightly spoiled by the diffraction artifacts because they stream off the whole width of the disc in all 4 directions. Whereas I don’t mind the spikes on bright stars.

Magnus

... or even a Questar 7” perhaps? Has to be considered in the mix and a snip at only $13k 😳

M

Ah I should have added I’m not an astro-imager (yet ... it’s only a matter of time), just visual for photography. For terrestrial use and bird-photography in particular it’s been amazing for me. Though I did stick it on a skytee2 and photograph a full moon once and stacked a few tens of frames with good results. 

6 minutes ago, aderyn said:

... I'm guessing you're in eastern England at the moment, with the howling wind ...

Farthest SW Ireland actually, lovely and dark when eventually the skies clear!

“The price of the Canon lens is significantly more £12,000 new compared to the RASA at £2000.”

While that’s true for the very latest model of the Canon 400/2.8, you should be able to pick up a top-condition 400/2.8 L IS USM for around £3k I reckon, perhaps cheaper. All the versions of this lens-spec have had outstanding image-quality I understand. What’s changed has been that it’s been made lighter and lighter.

I know because I own one, and it produces amazing images. But mine is gen 2 or 3 I think, the first IS version, and it’s heavy! Around 4.5kg. I totally love it.

Magnus

Really enjoyable read, thank you. As I read it I heard HOWLING wind outside, it made me very jealous! And “naked eye Beehive” is a sign of lovely dark sky, so lucky you! When you get a chance, look also for the Coma Berenices Cluster naked eye, a similarly fuzzy patch roughly at the centre of the arc made out by the “pan-handle” of Ursa Major.

Magnus

How about the az-eq5? Very good in az mode, and can transform into an eq mount whenever it suits. I have an az-eq6, which is basically the same but scaled up, and it’s brilliant.

Cheers Magnus

I had a go a couple of nights ago. I got about halfway through your tour when Cass clouded over. I moved to Ursa Major and was gratified to quite clearly see M81 and M82 in the 12x50s.

M

Nice read, fascinating, thanks. As you know it's one of my standard go-to targets as well, a lovely sight.

M

@Peter Drew I found this post of yours from a few years ago:

”Dall used an erector transfer system which picked up the primary light cone further from the primary than usual and then transferred it to the eyepiece. This typically reduced the secondary obstruction to about 15%, gave an erect image and was completely free from sky flooding even with a single spine tube. The inside of his aluminium tubed Maksutovs were unpainted!. I made a number of Maksutovs to this design and can vouch for their high contrast.”

What was the system? A roof prism inside the baffle tube?

On 05/02/2021 at 16:28, Avocette said:

I have just completed a very quick (but careful) dismantling and reassembly of my SkyMax150 Pro, taken some measurements, taken some photos and blown away a very few dust motes from the optics. The primary mirror diameter   (measurement K in Magnus’s nomenclature) was 163mm overall and around 160mm if I discount the ground edge which is chamfered at about 45°. The Primary Mirror Retaining Ring (J) is 47mm. The Secondary Baffle Skirt is 52mm (H) at the wide end, and has a depth of 43mm (C&I). 

The metal tube is 2.2mm thickness and 178mm internal diameter. In my scope, this means that the eight screws, which attach the Corrector Plate Rim and the Rear Cell, distort the circular profiles when tightened appropriately.

My scope at least ten years old and possibly somewhat older. It has the ‘Black Diamond’ paint job and the 2” visual back but with a 66mm thread rather than the SCT compatible rear thread of more recent manufacture.

Here are a few photos for the curious!

 

Brilliant thanks for this! Allows me to fill in a few blanks in my specs for my 150.

M

5 minutes ago, jetstream said:

 @Captain MagentaExcellent Magnus, missed this thread unfortunately. What did you end up figuring the central obstruction was in the end? for the 180mm

Thanks.

It seems to be in the region of 33-35%, as predicted from dimensions and confirmed with images. A shame, as I reckon SW could have reduced it quite significantly by using a thin retaining-ring for the primary rather than the big thick thing they have used. Oh well.