# Nothing to see, but loads to see....

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I re-read this, I didn't mean to sound 'off'. You have prompted me to go and measure it now, on the next sunny day!.

There's no glue on this scope to melt so that potential problem hadn't crossed my mind.

Trying to do some physics I think it goes like this :

The primary is 150mm and the secondary is 72mm in diameter, but reflects 85%s so I make that about 2.75W from solar collected by the scope ( primary shaded by secondary ) and 2.75  from direct solar heating to the surface of the secondary facing the sun, the lack of difference being due to the shading effect of the primary vs the lower reflectance of the front of the secondary.

The equilibrium temperature will be the temperature at which the object ( the secondary mirror unit) will radiate the same amount of energy it receives.

ie using Stefan-Boltzmann, P = σAT^4 where sigma = Stefan-Boltzmann constant σ = 5.670374419...×10−8 W⋅m−2⋅K−4.[5]

So T = ( P/sigma.A)^(-4) which is ... 330K   or 57 Celsius

Using W = 1350 W/m^2 as the solar irradiance, 150mm as the diameter of the primary, 72mm as the diameter of the secondary, 85% as the reflectivity of the aluminised surface and 50% as the reflectivity of the surface facing the sun.

If I repeat this for an OG of 100m diameter I get

input power into the glass = 1350* PI* 50mm^2*0.045 = 0.5W

where the transmissivity of BK7 glass is 95.5% so the absorptivity is 4.5%

Which means the equilibrium temperature turns out to be .. 153K . or roughly -120C

Well, nice to see its a chilly sunny day for the refractor users!

Anyone see where I went wrong ?

No problem, I’ve been known to be a bit grumpy on occasion too

I’ll let someone else with a bit more knowledge try to solve the riddle!

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I didn't realise just how big that is. Very nice.

Thanks Mike. Nice couple of 4” scopes which work really nicely side by side. Views today were excellent, particularly in Ha, but I shouldn’t really say that on a thread ‘promoting’ the joys of white light solar observing .

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7 hours ago, Stu said:

Thanks Mike. Nice couple of 4” scopes which work really nicely side by side. Views today were excellent, particularly in Ha, but I shouldn’t really say that on a thread ‘promoting’ the joys of white light solar observing .

Nice views in Calcium II K too, lol

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• 2 months later...
On 10/04/2020 at 09:03, Stu said:

Observing the sun in white light yesterday, it struck me that whilst the disk was basically blank, there was a huge amount of detail to see still, enough to keep me observing for quite long periods each time when the seeing was good.

Many people think white light observing is very boring unless there is an active region visible. I suspect that is because either the setup they have is not optimised or just that they have not trained their eye to see the detail that is there.

Yesterday for example, there was a lovely patch of faculae which has appeared over night around the limb, and at least one other small area elsewhere on the limb. Outside that, the granulation visible when the seeing calmed was quite stunning. At high power, and by high power I mean x200, the tiny granulation cells started to open up, being around 1 to 2 arc seconds in size.

Quite often, when I first look, I struggle to see anything as my eye has not picked up the focus point properly, and it can take a few minutes to get there. I normally focus on the limb, then on any feature such as faculae, then start looking in the centre of the disk. Sometimes I can see the granulation is there but my eye is still not catching it properly, so I find that panning the scope then gives enough movement for it to suddenly snap into focus.

Focus is another key point. Accurate focussing is essential; I always refocus every time I come back to the scope and frequently during each session just to make sure I am spot on. A fraction of a turn of the fine focuser is rough to make a difference and by fraction I mean 1/10 of a turn or so. It really does affect what you see.

I find that the granulation is most vivid in the centre areas of the disk, presumably because that is where you are looking directly into the cells and the contrast is best. Towards the limb the granulation tails off but limb darkening means that this area is where you will see faculae at its best. I find that I perceive the detail in the central area of my vision, beyond that it tails off so wide fields of view aren’t necessary.

With the excellent seeing yesterday, I was seeing amazing detail. Everywhere I looked, I saw different patterns, and in various places there were denser, more complex areas with tight knots, almost approaching small spots but not as dark, along with lighter lanes and in one case an oval of light with a darker centre. At one point I spent fifteen minutes looking at the same spot. It took effort but I could definitely see changes in the patterns during that time as most cells only last about ten to twenty minutes.

My kit has developed over the years, and I’ve tried many different combinations. What I have now is very convenient to use, portable but also gives a quality of image that keeps me happy. I use a Tak FC100DC with FeatherTouch focuser. I then use a Baader CoolWedge with Continuum filter, Baader Zeiss Mark IV binoviewers with a x1.7 GPC and Zeiss 25mm Orthos (converted microscope eyepieces which are very sharp, have excellent transmission, low scatter and good eye relief). To get to high power, I add an AP Barcon and then add extension tubes via Quickchanger fittings when I want higher power. It is an unusual setup which I don’t suggest replicating but it avoids changing eyepieces and does work for me. I certainly find that barlowing long focal length eyepieces works better than trying native short focal lengths, the images merge more easily and the eyepieces generally have bigger exit lenses and better eye relief.

I use binoviewers to mitigate  the floaters which inevitably happen with eyes my age (50) and high powers with a 4” scope and a bright object.

A good setup will cost money of course, but doesn’t have to cost a fortune. I do find the Baader CoolWedge to be a notch above the Lunt wedge, particularly at high powers, but that said the Lunt is still excellent. I don’t have experience of the other options out there. I think the scope is likely to make the biggest difference though and here I would recommend something with a well figured objective, with excellent correction for spherical aberration as poor performance here can really kill the detail. Somewhere around 100 to 120mm is probably optimum to make the best of our normal seeing conditions. Larger will definitely show more due to the additional resolution under good conditions but it is likely that they will be affected by poor seeing much more often. Fast achros should be avoided I think as they often suffer with this. That said I suspect the 152mm f5.9 scopes perform pretty well as they are well corrected, but in the 4” category something like a good TAL 100 or Lyra Optics 102mm f11 (or clone) would be a very good choice. The ubiquitous 120ED is also an excellent option for solar, being at the upper end of the optimum aperture and with very fine optics which will deliver excellent results. Inwards focus if using binoviewers is something to be mindful of, but is likely to be less of an issue because you often use a Barlow to get to the powers necessary. A dual speed focuser is highly recommended as mentioned earlier.

In terms of binoviewers, I have found the TS Optics units or similar spec clones are very good quality, with self centring eyepiece holders and individual focusing on each eye. Careful balancing of the focus for each eye is another area that makes a difference to the detail seen. My Mark IVs do not have this feature but the eyepieces do so I can still easily achieve excellent balanced focus between the two. The Mark IVs do have very tight tolerance on the eyepiece holders so merging of the images is never a problem. Careful setting of the interpupilliary distance is also very important to make sure both eyes are recieving full illumination. Repeating wink of alternate eyes when in position shows whether this as correct; both views should look equally well and evenly illuminated.

In terms of eyepieces, there is plenty of choice out there but I would think that some of the 12.5 or 18mm orthos out there would be a good option; simple lens designs with sharp optics and low scatter are what you want to look for.

Finally, a good solid tracking mount is very handy to give a stable view and allow you to concentrate on pulling out the detail. I normally use my Vixen GP with dual axis drives. Accurately polar aligned it with track the sun all day without problem and is quick and simple to use. I also use an adjustable observing chair so I can get comfy for fifteen or twenty minute sessions and to be able to relax.

Gosh, this has turned into a bit of a monster post, I kept thinking of more to add. Well done if you have got this far, and I hope it is of some use. There are many ways to skin the solar cat though so if you have a setup which works well already, all the better. As the Sun slowly wakes up I hope we all get some amazing views of lovely Active Regions over the coming years. If your setup shows granulation well, then the detail which can be perceived in the ARs is amazing, often rivalling many images I find.

Solar White Light observing; it’s the future I tell you .

Stu

Well Stu, only two years late, but I just came across this post. .  Excellently written and very informative, I've bookmarked it for future reference!

I've been thinking of buying the Baader ceramic wedge for a while, and your article has probably edged me a bit nearer.

Just a  ouple of things.  Presumably your continuum filter goes onto your  camera adapter and then into your wedge?  Also, is it worthwhile buying the photographic versions - are the extra ND filters actually very useful?

Thanks again for the post Stu, even if it does mean some extra  expenditure .

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3 hours ago, paulastro said:

Well Stu, only two years late, but I just came across this post. .  Excellently written and very informative, I've bookmarked it for future reference!

I've been thinking of buying the Baader ceramic wedge for a while, and your article has probably edged me a bit nearer.

Just a  ouple of things.  Presumably your continuum filter goes onto your  camera adapter and then into your wedge?  Also, is it worthwhile buying the photographic versions - are the extra ND filters actually very useful?

Thanks again for the post Stu, even if it does mean some extra  expenditure .

Glad you found my crazed ramblings finally Paul

I just have the visual version of the CoolWedge so don’t have the additional filters. The Continuum and ND3.0 filter are fitted internally and I don’t remove them. This makes it very safe as I can never forget to add the filters.

I’m not an expert on the imaging side at all, I’m assuming from the web description of the imaging setup that you use the Continuum, ND3.0 and one out of the ND0.6, 0.9 or 1.8 filters but someone else may be better to confirm on this. If I’m correct, then you would still have the two base filters fitted internally then add one of the additional filters to the camera as needed.

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Many thanks Stu, that's very helpful, thank you.

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