ollypenrice

I think that by 'stacking the stacks' Carole means combining a stack from one night with a stack or stacks from other nights, and you can certainly do this. You'd need to find the best way to weight them in the process. Under no circumstances would I put different sub lengths or settings into one stack. So, assuming consistent skies and the same settings, you'd just weight the stacks based on their total integration time.

The best way to do it, though, is to calibrate each individual sub in the entire shoot without combining them and so generate a new collection of individual subs duly calibrated. You then take these calibrated subs aside and stack them as a separate operation, obviously without any calibration files since that has been done to each sub already. This way you get the maximum value from the Sigma Clip algorithm and the best benefit from dither (even if you weren't dithering between subs.)  I suspect that this method would be of even greater benefit to DSLR imagers since it would attack the background 'colour mottle' problem most effectively.

Now, different sub lengths. Do it only if you know why you are doing it. Of the 100 or so images on my gallery site I've done it precisely twice, once on M42 (which everybody has to image in multiple sub lengths) and once on M31 (where I'm not even sure it did any good at all.) I never worry about white clipping stars. You can pull the colour into the cores in post processing and if you expose so as to avoid clipping stellar cores you won't go deep enough. The key thing is to look at your linear stack. If a galaxy core isn't burned out there then it doesn't have to be burned out in the final image because you already have the data in the stack.

Olly

38 minutes ago, Nigel G said:

Looks like the sensible thing to do is upgrade camera.

I have thought a lot about cameras and what to get.

I love the idea of a mono but would it be a good choice with so few clear nights to gather data, would a OSC be the wise choice.

I'm going to tackle this by reducing the exposure times and will try darks as it will do no harm. in fact I could add some darks to the data I have, I know its not ideal but I would see a difference to banding if it made a difference.

Nige.

My answer is always the same. Mono is fastest. On NB it is way faster and it opens up the moonlit nights for imaging.

Olly

12 minutes ago, tooth_dr said:

Well what’s the verdict

The fault, dear Brutus, lies not within our stars but within our wallets...

The arrival of monochrome CMOS cameras similar in format to DSLRs and at considerably sub-CCD prices inclines me to remind you of a phrase in your original post: Or am I in desperate need of a new camera?

I hesitate to answer this too assertively but if I were to do so the answer would begin with a 'y'.

lly

On 16/12/2017 at 14:14, Charic said:

Sorry Olly.

I read that the image  produced in my scope with DSLR attached would equal 42 magnification, rather than 42 times something? 
It said  "divide the telescopes focal length by the the diagonal of the sensor" and for my system, this results in 42x. 
 

The term 'magnification' is only meaningful if we know the dimensions of the thing being magnified by 42x. To know its dimensions we have to know what it is. When we look through a telescope giving 42x magnification we do know what it is we are increasing in size by 42x. It's the image perceived by the naked eye. But there is no such thing as a 'naked camera.' This might be a camera without a lens but in that case it wouldn't give an image at all. So my question' 42x what?' was perfectly serious. Of course we can certainly say that lens x will give 42x the magnification of lens y but this is only of interest to the owner of lens x. This magnification has no universal currency which is why we use the term resolution in arcseconds per pixel or, in professional circles, plate scale.

Besides, if I try two cameras in the same scope, one with small pixels and one with large, the plate scale will be unaltered but the resolution will increase, as will the size of the output image. It would be odd, though, to say, 'this chip magnifies more than that chip...' even though, in one sense, it does!

Olly

12 hours ago, Charic said:

 

The Skyliner will act as a 1200mm lens, and will provide a magnification in my DSLR of about 42x? This equates to the focal length of the scope divided by the diagonal of the camera's sensor?

 

 But 42 x what? (That's why the term isn't used in imaging.)

Olly

On 10/12/2017 at 12:38, GalileoCanon said:

 

If I'm not mistaken isn't a 50mm lens pretty close to what the naked eye sees? 

I think this is said because a 50mm lens's picture will present near and distant objects at the same relative sizes as they appear naked eye. In other words there is no 'foreshortening' effect as created by zoom lenses.

Since astronomical targets are all at infinity this effect does not exist for us.

Olly

Earlier on you used the term 'crop sensor.' This is a term best forgotten when you move into astrophotography and imaging with telescopes. Cameras, and particularly astronomical cameras, come in a wide range of chip sizes. In the image below I've modelled two chip sizes, large and small, in one of our telescopes. The object on the chart is M33 shown in green. The problem with the misleading term 'crop factor' is that it might lead you to believe that you are more 'zoomed in' (and capturing more detail) with the small chip than with the large. In fact you are not. If both chips have the same size pixels the image scale is literally identical as is the level of detail and the final screen size of M33 on your PC.  All that changes is that you have more sky around M33 wth the larger chip.

Now let's imagine, as might well be the case, that the larger chip has smaller pixels than the smaller chip. This time the larger chip will give a larger and more detailed image of M33 than the smaller one. This is because it will put more pixels under M33 itself and when it appears as a fullsize image on your screen (meaning one camera pixel is given one screen pixel) M33 will be given more screen pixels and, therefore, be bigger.

The size of the sensor has no bearing on the scale of the image and the person who coined the term 'crop factor' should be made to listen to an entire CD of yodellers in full voice!

lly

The OP is in India where I guess the nights are very warm. This might be a big problem for any uncooled camera doing longer exposures.

Olly

On 05/12/2017 at 17:37, Skipper Billy said:

Do 1/4  scale models count? 

https://sites.google.com/view/astro-imaging/other-hobbies/radio-control-model-aircraft

 

Most certainly!

Olly

1 hour ago, saac said:

Up until their exit from service Nimrod navigators were still trained in the art of obtaining a navigational fix from a star shot.  A little sextant type device was part of the equipment stored in the cabin just in case all else failed. Being based at Kinloss too would have been a delightful posting for any star gazer with the regular Northern light show during the winter months.  Maybe the connection is indeed real.

 

Jim 

Yes and, somewhat later than my Nimrod pilot, we had a retired RAF navigation officer staying who knew our pilot and had flown with him. He told me about his training in astro-navigation. Fascinating stuff.

I've just this minute finished David McCullough's Wright brothers biography and can honestly say that I've never read a more moving book.

https://www.amazon.com/Wright-Brothers-David-McCullough/dp/1476728755

Olly

2 hours ago, Martin Meredith said:

Per ardua ad astra could equally well be the motto of the amateur astronomer...

Personally, I've never flown anything. I do love reaching the tops of mountains though, so maybe the common factor is a subconscious desire to get above the clouds?

Martin

 

Or is it some kind of Icarus complex, or even Faustian? Indeed, in Marlowe's Dr Faustus, Faustus has mastered astronomy and, after his pact with the devil, has himself borne aloft on a flying chariot.

Olly

10 hours ago, Zakalwe said:

Astronomy is not a cheap hobby. Especially the ones that can afford to take astro-specific foreign holidays. Many of these may have retired from quite well paid careers and are still young enough to enjoy a variety of past-times.

It's highly probable that your sample size is very narrow and not at all representative of the hobby in general?

Another way of looking at it would be to sample a couple of flying clubs and see how many "do" astronomy. There may be a link, as amateur fliers will certainly have a far better grasp of atmospherics and basics like which way is North compared to the general public at large. For what it's worth, my local flying club doesn't seem to have many that peer at the sky, except to moan at the cloud ceiling (at least we have that in common!).

I'm not so sure. Firstly we are not astro-specific since many people come here for general rural holidays in the hills and, secondly, we are not at all expensive! (Our immediate region has two major tourist attractions, free flying and rock climbing. Astronomy is a distant third.)

In the first fifty years of my life I had never met anyone who had built his own aeroplane. Since coming here fourteen years ago I've met four. You're quite right, though, that sampling it the other way round would be instructive. For all that, I had the Flying Gnomuses last week and I have another PPL staying this week. And you yourself are keen on both. In my robotic shed three out of four scopes were owned by fliers until we lost Per. I'm going to take some convincing that this is coincidental.

BTW, I accidentally set up camp on a gliding-specific campsite the summer when I moved in here. I hadn't seen the sign saying it was for gliding club members. Nobody minded and I set up a scope that evening. The level of interest shown by the other campers was out of all proportion to that which I'd experienced on any of the many campsites on which I'd done the same thing, and the level of the questions I was being asked was very high indeed. It was a great evening, in fact, and I was invited to the annual dinner!

Olly

PS I've flown paragliders myself but never got to the stage of buying my own because I could see that it would be yet another opportunity to waste days, weeks and months waiting for the UK weather to sort itself out... I enjoyed it, though.

So there is indeed this strong mental connection between astronomy and flight.

2 hours ago, Jessun said:

Don't know what you're talking about ;-)

My ol' friend:

 

 Indeed, my very first guest at Les Granges was a retired Nimrod pilot. The connection is certainly there.

Olly

At a slight tangent to History of Astronomy, this, but I'm reading The Wright Brothers by David McCullough. It's a truly outstanding read with some entertaining astronomical connections.

Simon Newcome was one contemporary astronomer who spoke out on the absurdity of striving for powered flight, asserting that it would never happen. On the other hand another former astronomer, Samuel Langley, was the driving force behind several unseccessful powered flying machines and clearly believed it would be possible. Given that it took only 66 years to go from the first sand-skimmings at Kitty Hawk to the moon landing, Simon Newcome was wrong with a capital W!

By the way, I find as an astronomy provider that a passion for flight seems to go hand in hand with an interest in astronomy. Perhaps twenty of our guests in the last ten years have held PPLs, helicopter or gliding licenses and three have built their own aeroplanes. (One was on his fourth.)

Olly

This is one of the very few targets which cannot be imaged well in a single exposure time. Personally, I blended sets of exposures of 11 seconds, 50 seconds and 10 minutes (15 would have been better.) Each setup will vary so you do need to experiment.

Once it comes to combining them, some thought is needed. I prefer to do this in Photoshop using this tutorial. http://www.astropix.com/html/j_digit/laymask.html

Jerry has updated his website since I first learned the layer masking technique from him but the basics remain unchanged. In the latest version he blends two exposure lengths but I would still shoot three, I think, with my setups.

Olly

Doh. I'll just go back to my original interpretation which was the same as Steve's! However, Optcorp did subtract 1mm when calculating my adapter. I didn't argue because if it didn't work it would be their fault, but it did work (and it's quite sensitive for a full frame chip with the TEC, I gather.)

I'll let someone else draw the right set of graphics!! I'm on toothache antbiotics so no sangria for me... Well, maybe just one!

Olly

51 minutes ago, Filroden said:

You’re trying to move the light to reach the chip by changing the spacing but the light path cannot change (not without adding lenses). You have to build more space to move the chip to where the light now reaches focus. 

OK but at least I can now understand the existence of the 'subtract' argument which I think I first heard of from QSI and which was adopted by Optcorp when I asked them for the correct spacer for my TEC140 flattener to full frame chip. (This works perfectly, for what it's worth.)

So what's the error in my graph? In the second version the overlay at the bottom brings the focal point to that of the chip distance. The angle of convergence had been respected (because it was a cut and paste job from the original light path with filter.) On the other hand my overlay shows the filter intersecting the light path further up the light path (within the pink 'spacer' section) so that's not geometrically correct and could be the sorce of the error.

What bugs me in this whole question is that the 'subtract' camp are not idiots but I've known myself for long enough to know that I might well be an idiot!

Olly

13 minutes ago, RayD said:

Me neither. I think it is confusion where people work out the space, then subtract the 1mm,  then need to add that to the spacer. 

I took ages going through this as I really struggled with spacing initially. 

But if we take as gospel the sentence 'angle of convergence remains the same' then we get this:

????

Olly

Nightmare! Yes, I now think you're right! I've always said 'add' myself but now I can't see where the 'subtract' argument comes from.

Olly

2 hours ago, RayD said:

Simply put if you need a 16 mm spacer without factoring filters, and you have Baader filters @2 mm (just an example as I don't know how thick the Astronomik ones are), then you will need a 16.66 (or 16.7) mm spacer.  As noted by @wxsatuser you need to add 1/3 of your filter thickness.

Personally I would go with your 16 mm and make the difference up with delrin rings as there is no guarantee that your sensor is spot on 17.5 mm.  There may be a bit of trial and error to get it spot on.

Edit:

Here is a simple diagram which I believe explains what I see as the theory of the filter changing the light path and pushing back the focal point, which is why you need to add a bit (this is not my image).

I always struggle with this so I took Ray's diagram and added the hardware around it. The image below is how it came out. It shows why Davey T is right. The lengthening of the light path means you need a shorter spacer. Even as I write this it feels wrong, but see what you think of the diagram. For me, at least, it has brought a bit of clarity since, in the past, I though adding length to the light path meant adding it to the spacer. It seems not... but maybe I'm up a gum tree again!

Olly

What about using it wide open and making a front aperture mask to stop it down? I did this to avoid diff spikes which are pretty intrusive on starry fields of view. 

Thanks Mark, one to have a play with!

Olly

21 minutes ago, Astro Buer said:

Yes, well I had a thought earlier today that I can put my cameras in APS-C mode which when using the WO ZS61 would give me a focal length of 540mm,

The idea that focal length and chip size are related is an error imported from the daytime camera world via something called 'crop factor.' I would ignore all this completely. It is utterly meaningless. Keep it nice and simple in astrophotography and you won't go wrong. You have a focal length, determined by your scope's optics. This is a simple and non-negotiable fact. You have a chip size of x by y, also non negotiable. Then you have your pixel size, which (allied to you focal length) specifies your resolution in arcseconds per pixel. Your chip size simply speciifies how much sky you cover. It has absolutely no effect whatever on your resolution. (That derives from focal length and pixel size. The number of pixels you have is also irrelevant.)

Olly

Edit: Crossed with Adam J but we are saying the same things.

I would look at it this way: the mount has a maximum payload and a maximum tracking accuracy. Both, not just one, have to be respected. Payload is obviously easy - you just weigh what you are putting on the mount. Accuracy is more difficult. The unit that matters is resolution measured in arcseconds per pixel. This value is derived from focal length and pixel size, so focal length alone is not a useful term. (Long focal length with big pixels equals short focal length with small pixels in terms of resolution, stating it simply.)

I think that the best information will come from published images in which the user states what camera, at what focal length, was being used. To turn this into arcseconds per pixel you can use a number of online calculators such as this one: http://www.12dstring.me.uk/fovcalc.php

Olly