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# mattnedgus

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21

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• Rank
Nebula

## Profile Information

• Location
Sheffield, UK
1. ## Why does brightness reduce when magnification increases?

I think you could be right. From the internet searches I did there seemed to be many ways of saying the same thing which made finding something 'sound' (that I could understand!) feel like trying to run on treacle. From KIC 8462852 perhaps? That's an interesting link! (Unfortunately I can't open the other to Starizona yet - looks like they're down for maintenance!)
2. ## Why does brightness reduce when magnification increases?

In case it helps anybody in the future who finds this post, I found this link that made some real (intuitive vs academic) sense to me: https://www.astronomics.com/eyepiece-exit-pupils_t.aspx It links the exit pupil size with the brightness of extended objects, rather than using magnification as a reason only. So now as I understand it, for a given scope f ratio: if the magnification is doubled, the exit pupil diameter is halved and it's area is 1/4 of the size. This feels easier to understand than when I was trying to consider the field of view of the eyepiece! Hopefully it's the correct way of looking at it!
3. ## Why does brightness reduce when magnification increases?

I could really do with a good book to understand the theory I think! The effect of exit pupil is my new confusion! It does thank you - there's a diagram that shows it graphically here: https://en.wikipedia.org/wiki/Inverse-square_law which helps me visualise the rule. It's unbelievable how much I've forgotten over these last 15 years since I did an engineering degree! Thanks acey, I wanted to try and understand illumination and luminance better and read this Illumination vs Luminance Link where the author says that illuminance is measured as light striking a surface (the incident) and luminance is measured as the light coming off the surface that has light hitting it (the reflected). Then I got more confused and wasn't sure how that applied to your description or a telescope other than the light hitting/reflected from the objective and primary. acey, I think this is key (but then I get confused with Exit Pupil ) - this is what I was trying to show with those red and green circles: that when a higher power EP is used, it takes a smaller portion of the sky and makes it up to the same size. The reason I get confused with Exit Pupil is because the exit pupil also reduces with increased power and so fewer 'cones' in the eye are receiving input - which must also have an effect on the apparent brightness? But doesn't the image change in size with the exit pupil also? I think so too - when I drew that I was trying to understand the maximum field of view the scope could ever possibly see. I still think light from the wide angle I drew light would enter the scope and hit the parabolic primary, but because of the parabolic shape most of the light from extreme angles would be reflected at angles that would hit the sides of the scope rather than be focused onto the secondary.
4. ## Why does brightness reduce when magnification increases?

OK so just as I was writing my original and confused question I had a thought and think I might have worked it out. I've attached a drawing to try and show what I mean and help others if they're ever as confused as I was. I could only find sources that quoted brightness reduces four-fold for a two-fold increase in magnification, but I just couldn't wrap my head around and visualise it (too little mental exercise these days!!) - I knew it was something to do with the area of a circle but that's about it. Then I had a thought and made a little drawing: The blue circle represents what the telescope can see - for example it's maximum field of view (i.e. the most amount of sky it could ever possibly see: I worked out a theoretical 18.92 degrees for the SW200P - the second image shows how I came to this conclusion). The grey object (of no particular shape) in the middle of these blue circles is the same size because as far as the telescope is concerned the sky is the same scale (the scope still "see's" the same circle of sky). The red circle represents a lower power eyepiece's view and encircles a larger [field of view] area of the sky (the lower red circle show's what we'd see in the eyepiece: a smaller, brighter object) The green line represents a higher power eyepiece's view and encircles a narrower [field of view] area of the sky (the lower green circle show's what we'd see in the eyepiece: a larger, dimmer object) Am I right in thinking that the higher power eyepiece takes the light from a smaller [field of view] 'circle' in the sky than the lower power, but 'blows it up' to the same size in the eyepiece for us to see, as in the lower green and red circles? And if so is that what explains why the brightness goes down four-fold for a two-times magnification (because the light from a smaller [field of view] area is being shown at the same/similar size to the eye)? And essentially is this right: the scope always see's the same [field of view] 'circle' of the sky but the different eyepieces pick out different sized [field of view] 'circles' of this? Before I realised this I was under the delusional and confused impression that a smaller object in the sky might be brighter because the whole of the objective aperture could be used for that one object!!
5. ## SW200P First Observation - M13 and M31 different to "Left Turn at Orion"?

That's one thing I've discovered so far: the more I see the more I want to see, so I justify what I need on the thinking that I'm buying an 'experience'!
6. ## SW200P First Observation - M13 and M31 different to "Left Turn at Orion"?

I hadn't considered that! I can see 1 degree and Andromeda looks to be over 3. I'll be happy just to see more of the disc though in a dark sky than the core I have seen - for now! I'll give those a miss for now then and put the money towards everything else [on this growing list of things!] I want to buy.
7. ## SW200P First Observation - M13 and M31 different to "Left Turn at Orion"?

beka; I think that's what I was seeing: just the bright central part! Cheers Alan, I managed to get out a little last night between clouds for another look at Andromeda - When the sky was clear by-eye there were still moments where the visibility through the scope varied. I was amazed by how much! rockystar; yeah I feel quite privileged having seen what I have so far considering people might go their entire lives and never see them - but I'm craving more. On the first clear night after obtaining a RACI/telrad I'll be off out to a dark skies site! :-) Have you ever used a light pollution filter? I was wondering whether these could help with the ambient for home-observing?
8. ## SW200P First Observation - M13 and M31 different to "Left Turn at Orion"?

Thank you for such a quick response Timebandit. I thought that [the ambient light etc] might be a large factor. In the wider lense there's a definite orange hue to the sky. I live only about 35 minutes away from a Peak District Dark Skies site so I reckon the next time there's a clear sky I'll make a trip out there!
9. ## SW200P First Observation - M13 and M31 different to "Left Turn at Orion"?

Hello again, So I finally got out last night (30th July) for my first ever observation with the Skywatcher 200P I've had collecting dust for about 2 weeks now! The sky was cloud-free but with urban light pollution. I really struggled to use the standard straight finderscope - I don't know how people cope with those: I needed yoga positions Back Bend, Table Top and the Camel just to get my eye even close! I'll try it some more but I imagine I'll be purchasing a RACI finder asap!! By scanning with the main scope though I'm fairly sure I managed to find M13 at first. It didn't look quite like it does in "Left Turn at Orion" though - to get a similar-sized view of the Cluster I had the magnification set at 133x which is nearly double the view in the book for a Dobsonian under medium power (75x or 40'). I wondered whether this was more likely because the ambient light was drowning much of the size of the cluster out? It was really impressive to see nonetheless! Then I tried to find Andromeda... Which I thought I had, but it wasn't nearly as large or impressive as the book suggested. I was far more impressed with the Hercules Cluster through both viewfinders, in terms of size and clarity. Is there a chance that I hadn't in fact found Andromeda but some other Galaxy, or could it be a problem with the urban lighting/ difference in direction I was looking that made the difference?
10. ## Should I return Celestron TS70 in favour of Skywatcher Mercury 705 with EQ3?

Ohh! I never considered this... The ball head I've been using can be tilted over to 90deg. It doesn't have anything for altitude control except the ball in that position (unlike the in that link happy-kat posted), but it would be easier to adjust that and use the swivel for azimuth than trying to adjust the entire ball like I have! EDIT: Except, I just realised, without having some dovetail adapter there's a chance the weight of the scope (being off-balanced as it is) would try to unscrew the thread - I'll have to check which way it wants to fall!) Thank you! I've made a note (old-school type!) of all that you've said and will bear it in mind when I start looking at EP's! (It won't be long now I think I've settled on a scope!) On another note I spoke to Ian at Rother Valley Optics and I think I'll hang onto the Travel Scope for now and just use it as an actual travel scope (since it would be easier to carry-on or pack than the Mercury 705!) and as a secondary to the Dobsonian I'm itching to buy! The TS70 with it's current EP's and mount set-up is not much good for aiming and focussing on planetary targets but it is fairly good at seeing a patch of sky and pulling out the stars or looking at the moon! And at least I'll have something to compare with the Dob so that should improve my leaning and understanding (like comparing diffraction spikes or helping me understand whether something is mis-collimated etc etc).
11. ## Should I return Celestron TS70 in favour of Skywatcher Mercury 705 with EQ3?

Cheers faulksy - I live literally 5 minutes away from them so I'll go up in the morning!
12. ## SKY-WATCHER SKYLINER-200P or BRESSER MESSIER DOBSONIAN 8-INCH

Oh, and Charic: I wear glasses too but I've been finding I put them on when someone else is looking through the scope just so I can see the stars by eye! I think I understand what you mean though: that to move from scope to finder on an optical finder means you don't need to put them on. I hadn't considered this when I'd thought about using a red dot finder!
13. ## SKY-WATCHER SKYLINER-200P or BRESSER MESSIER DOBSONIAN 8-INCH

I think for now I'll just be happy to have a decent telescope I can use, even if there are more perfect (focuser) options out there - I've been coping with a Celestron Travel Scope 70 and it's true what I've read: the best telescope is the one you have to hand. I think I'll go to Rother Valley Optics in the morning and maybe pick up the Skyliner. I am really tempted with the Bresser because of the Focuser but I have some reservations about their design and I don't want to be drilling anything just yet.