light gathering comparisons

[Kevdan]

Hi all,

Been working out light gathering differences between scopes and this is what i've found, this is based on the skywatcher reflectors from 76mm (3") up to 350mm (14")

I think this may be helpfull to a few because when you read up spec of a scope it only gives the light gathering difference between the scope in question and the next lower sized scope. for example like myself i will be in the market for a 250mm - 300mm scope in the next few months but with this table I can compare these scopes with any other size scope. I stopped at 14" due to the cost of these scopes being a little higher than most of us are prepared to go to.

Hope it helps,

Kev.

Light gathering differences.pdf

[patomlin76]

Very useful thanks, especially as I am wanting to move up from a 70mm refractor - I've been looking for this earlier today!

Paul

[Kevdan]

Im not entirely sure wether the result will be the same for a refractor as it is for a reflector.

[Rapallokid]

Great table, nice work. Surprised at the gain form my 6" sct and my 12" DOB.

[Moonshane]

Im not entirely sure wether the result will be the same for a refractor as it is for a reflector.

it certainly will. the only effect generally from the secondary is a slight loss of contrast and possibly sharpness but this is negligible if your secondary is 20% or less by diameter cf. your primary.

[John]

I think refractor systems have a slightly higher % of light throughput than reflective systems but the difference is only around 5% I believe, maybe less.

[Moonshane]

especially less if using a reflective diagonal :0)

as always it depends on the optical and coating quality of the optics and in reality so many other factors affect things to the extent that the only reliable thing is aperture as this is constant.

[John]

especially less if using a reflective diagonal :0)

....

Good point !

I think the decent ones have 99% reflectivity though, thankfully.

I've never understood why that could be achieved for the mirrors in diagonals and not, as yet, for primary and secondary mirrors

[Moonshane]

very true mate. I never really worry about this sort of thing as I am not 100% convinced we can see even a 10% drop in reflectivity or light passage at the eyepiece - although I do of course flock and paint matt black all bits on my newts to increase every bit of contrast (not the same as light gathering though I s'pose).

[umadog]

You may see a 10% difference if you can set up two scopes with such a difference in the field and go back and forth between them. Otherwise, you won't notice it.

[TractionMan]

Very nice table! Good to have and read when the time comes to upgrade the scope (probably in a year or so). Out of curiosity, does anyone know how much light the various scopes gather compared to the human eye?

[sologuitarist61]

Good table and well done for working it all out - you can tell it is raining can't you?

[Kevdan]

Very nice table! Good to have and read when the time comes to upgrade the scope (probably in a year or so). Out of curiosity, does anyone know how much light the various scopes gather compared to the human eye?

well now thats a question , the human eye is about 5mm pupil so pi r squared = pupil radius of 2.5mm = 19.63mm squared so therefore the 114mm dia mirror collects surface is 18145mm2 and the eye is 19mm2 so 18145 / 19 = 955% more light gathering than the 114.

My head hurts now.

Kev

[mdstuart]

I would suggest adding an approximate limiting magnitude for each size of scope in say a sky where you can see stars of mag 5.5 overhead

So from my notes

Eyes 5.5

50mm binoculars 9.0

70mm refractor 11.0

110mm reflector 11.9

150mm reflector 12.4

250mm reflector 14.5

These are approximate! ..remember mag 12 is ten times fainter than Mag 11 etc..

I am sure you could put some figures in for the other sizes that would be a best fit..

Mark

[Kevdan]

I am sure you could put some figures in for the other sizes that would be a best fit..

Please feel free to ammend the table or send me the info and I'll add it to the table.

Kev

[John]

I would suggest adding an approximate limiting magnitude for each size of scope in say a sky where you can see stars of mag 5.5 overhead

So from my notes

Eyes 5.5

50mm binoculars 9.0

70mm refractor 11.0

110mm reflector 11.9

150mm reflector 12.4

250mm reflector 14.5

These are approximate! ..remember mag 12 is ten times fainter than Mag 11 etc..

I am sure you could put some figures in for the other sizes that would be a best fit..

Mark

These sound about right to me Mark. We are talking about point sources here rather than extended objects I assume ?

[mdstuart]

Yep point sources..Do not worry about putting them in your table Kevdan I just thought my mags might add to the thread...

Another idea might be to list how many galaxies you can see in each size of scope to demonstrate that the 250mm can see 750 wheras the 110m can pick up about 120 etc...

Mark

[CFC]

And while it's raining Mark, could you list the actual galaxies that can be seen by each size, this thread would become a perfect newbie resource then!

[JamesF]

it certainly will. the only effect generally from the secondary is a slight loss of contrast and possibly sharpness but this is negligible if your secondary is 20% or less by diameter cf. your primary.

When you actually work the numbers out it's surprising how small a percentage of the total area of the primary is blocked by the secondary if you stick to that limit.

James

[JamesF]

In fact, for the record (and assuming I have my maths right a secondary that is 20% of the primary diameter obstruction yields a 4% reduction in the area of the aperture which is the same as not having the secondary obstruction and having a 2.2% smaller diameter primary. Or with a 200mm scope and a 20% (40mm) central obstruction, you have the same light-gathering area as an unobstructed 195.6mm scope. Not exactly much to panic about, is it?

James

[aGreyarea]

I read somewhere that the high reflectivity coatings (dielectric 99% etc) are prone to moving, contracting in the cold I think. Apparently thats fine on a diagonal, because the percentage of area coated compared to substrate volume is very low, so only the edges deform and you can't see them anyway. But apparently on a large, comparetively thin mirror that you want to see the edges of, it can warp the whole thing.

[faulksy]

I read somewhere that the high reflectivity coatings (dielectric 99% etc) are prone to moving, contracting in the cold I think. Apparently thats fine on a diagonal, because the percentage of area coated compared to substrate volume is very low, so only the edges deform and you can't see them anyway. But apparently on a large, comparetively thin mirror that you want to see the edges of, it can warp the whole thing.

sort of true but thats why they use quartz glass or pirex which doesnt alter with thermal input