miguel87

4 minutes ago, Ags said:

At the focal plane (where the field stop is ideally) the light from the star is focussed to a point. If that point is inside the field stop (no matter how big the field stop is), 100% of the light of the star goes on to the eye lens. For example, given the same scope a star will be the same brightness in a 3.5 mm Ethos with a massive field stop or a 3.5 mm planetary eyepiece with a tiny field stop. The ethos let's through more light with it's bigger field stop, but it does so by showing more stars not by making individual stars brighter.

The percentage of the light lost around the pupil when the exit pupil is too big, is balanced out by the increased brightness of reduced magnification.

The only way to better a bigger exit pupil on a certain telescope is to decrease magnification. And as a result, brightness increases.

So yes, some light is lost around the eye, but I image is brighter anyway so nothing is dimmed.

9 minutes ago, Ags said:

I think what you are not getting is that there is exactly the same amount of light in the 10 mm and 1 mm exit pupil. That's because the 40 mm eyepiece is ten times further from the focal plane of the telescope so the same amount of light is simply ten times more spread out.

But the field stop on the shorter FL eyepiece would not intercept the whole cone of light, it selects a smaller area evidenced by the smaller TFOV. So it isnt the same amount of light.

Also remember that a star doesnt exactly behave like a true point source of light. It's best resolution in a telescope is the airy disc. The size of the disk depends the optics being used and resolution.

13 minutes ago, Ags said:

If I have an F4 scope and I use a 40mm plossl, the exit pupil will be 10 mm. This means most of the light from the star will not fit in your eye pupil. So the star will be fainter than if I had used a 4 mm eyepiece giving a 1 mm exit pupil. This only applies to point sources, extended object do get fainter at smaller exit pupils and reach a maximum brightness at exit pupil = eye pupil.

Even tho the star is a point source, it still reflects of every part of the primary mirror/lens, and because the exit pupil is an image of the primary, the larger the exit pupil, the brighter the star will be.

Even if the exit pupil is 10mm, you will still get 5mm+ into your pupil. Brighter than 1mm.

It might appear relatively brighter because the background sky will be much dimmer at 1mm. But if you measured the brightness of the star point it will be dimmer in a 1mm exit pupil than a 2, 3, 4 etc

27 minutes ago, Ags said:

I suggest you try this out with some eyepieces. Also refer back to the helpful diagrams posted earlier by @vlaiv

Have done, plus the maths behind it explains it anyway.

You would be right if a larger exit pupil spread the same brightness over a larger area (dimmer per unit) but it doesnt. We know that as magnification decreases, the image gets brighter. So increase exit pupil incrementally from 1mm up to your pupil size. As you go past your pupil size, the section 'stepped down' by your iris doesnt get any dimmer, you just miss out on the extra light of the larger pupil.

No dimming, just a stable maximum based on your pupil size.

Vlaiv was extremely generous and helped me to understand how an exit pupil functions. I never really got it before but he has a way of explaining things!

5 minutes ago, Ags said:

 

Nothing gets dimmer if the exit pupil is too big. It just fails to get brighter beyond that point.

Free online atlas, excellent quality, prints onto A4, laminate and into an A4 ringbinder. Quality waterproof star atlas for less than a tenner (or free if you have a laminater and spare folder).

www.charlotteastronomers.org › ...PDF
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The Mag-7 Star Atlas Project - Charlotte Amateur Astronomers Club

The thick cloud moved away from Swansea around midnight and despite the occasional gusts, the sky was looking very clear with sharp stars.

The 8inch newtonian came out for a nice night continuing my quest to sketch all 110 messier objects.

Last night I was able to tick off M13, M40, M97 and M108.

Afternoon folks

When the cloud finally cleared last night at around midnight it was CRYSTAL clear. Clearer than any of the nights we had in April (and there were lots of those).

I had a really nice night and continued my plan to sketch all the messier objects.

However, even with my red head torch (well reviewed black diamond) on, with parcel tape over the LED to dim further, to the point I can only just see my pencil marks well enough to sketch. And STILL this amount of light damages my night vision VERY noticeably. I tried with the head torch off but couldn't see the paper nearly well enough to sketch.

Instead of frequently going back and forth between eyepiece and sketch pad I have to try a longer study at the eyepiece then drawing from memory which is not ideal at all.

Does anybody have any tips or advice?

Thanks,

Mike

If you can see OK in daylight then no need for a moon filter. The moon is much dimmer than a sunny day.

The only benefit I see it trying to preserve some night vision if you are moving between different targets in the sky. The moon will ruin your night vision.

Exactly, so if I am using an eyepiece that somebody describes as wasting light.

The only ways to brighten the view are 1, new eyes, 2 new telescope.

If there is no eyepiece you can buy that will produce a brighter view, how can it be a light waster? It is surely a light maximiser!

I use an eyepiece with a 6.5mm exit pupil on a relatively fast newt. It is likely that my pupil is not often this large but the view is bright, beautiful and there is no sign of a secondary shadow even on the full moon. I certainly dont feel like I am wasting light.

If I replaced it with an eyepiece that matched my dilated pupil size perfectly, what would I be gaining? A bit more mag? I have that in spades in other pieces.

23 minutes ago, mikeDnight said:

effectively reducing the aperture of the scope.

I have an issue with this phrase.

How can the brightest possible image that YOU can see through the scope be considered reducing the aperture.

"Effectively reducing the aperture", people hear that and think wow, I'm not gonna do that, I dont want to dim my telescope. Truth is, Give me any example with the same telescope and same eyes of an image that is brighter? That is the brightest that your telescope can perform so how can it be considered reducing the aperture?

Just because a smaller telescope is capable of showing an image just as bright, doesnt mean that you are somehow degrading the bigger telescope.

"Reducing" implies that you could somehow not reduce. Brightness is maximised, not reduced.

Anyway, we all know what is happening I guess it's just a question of perspective/opinion 

14 minutes ago, vlaiv said:

Let's say that your pupil dilates to 6mm and that exit pupil of a given eyepiece for 7" scope is 7mm.

Now take an aperture mask that reduces scope from 7" to 6" - so its just a flat ring 0.5" wide that you put on your aperture.

What will happen to your view? Absolutely nothing - image will not become dimmer. It will remain the same. In that sense - using that eyepiece that gives you 7mm exit pupil on 7" scope is like using 6" scope with eyepiece that gives same magnification and 6mm exit pupil.

This is not because of eyepiece and exit pupil - it is because your eye cant get wide enough.

I cant tell if you are agreeing or disagreeing with me 😂

1 hour ago, bingevader said:

I've always understood this.

But why doesn't this then result in "wasted light" if the exit pupil is too big? That's where I need clarification.

Because you are maximising the brightness with ANY eyepiece over the max aperture of your pupil.

So why turn down an eyepiece if it's exit pupil is too big? You will still get the same FOV, magnification etc. And it will be as bright as any eyepiece can be for you.

Wasted light suggests that if you go smaller with the exit pupil you might not waste light and therefore get a brighter image. Which is wrong.

In essence a 'too big' exit pupil will not degrade your image in any way (unless we start talking about newtonian central obstructions).

Personally I dont see how you can be viewing the brightest possible image for your eye and say that there is wasted light?

That's my take anyway.

2 minutes ago, Skipper Billy said:

Not wishing to 'muddy the waters' but not all scopes will cover a full frame sensor.

I stand corrected. Not my area of expertise there. For sure somebody on this forum knows about your particular setup and I am 90% confident that your purpose built astrophotography scope could cover a full sensor.

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Just now, Cols Astro Journey said:

Thanks Miguel, I appreciate that its the same magnification from the telescope (750mm) but the sensor size in normal photography can effectively increase/decrease focal length by using the same lens on different cameras that have different sized sensors and I just wanted to confirm its the same using a telescope.

 

I guess I just need to know if its possible to use/focus this telescope (150pds) with all three sensors.

Anybody know if thats posible?

Yep definitely. The image created by the telescope I.e. the focal plane, never moves. It just hovers there in the focusser tube. So all camera chips have to reach the exact same position and boom, you are in focus. The only variable is how deep the chip is within the body of your camera.

Your focal length will always be 750mm.

8 minutes ago, Cols Astro Journey said:

Hi thanks to the responses but just checked on astronomy tools (as very kindly suggested) and came up with this (see attached) which is what I was expecting but it different from your responses so I am now even more confused.

 

 

You're just grabbing more of an already 'taken' image by the telescope.

So take your computer screen from your picture. The image of the nebula is at a certain magnification, regardless of what boxes your draw on the screen. All those images would be taken at the same magnification. Just a larger or smaller field of view.

I.e. I could not take a photo of the moon at 100x mag. Blow it up to twice the size on my laptop and post it online saying it was taken at 200x magnification.

12 minutes ago, Cols Astro Journey said:

 

Is it possible to cover different size sensors? as that would explain your response and if so what sensor size is suited to the S/W 150pds

I think... (somebody may correct me here)

That a full frame chip is better in any telescope. I think the image created by the telescope is bigger than normal camera chips anyway so I dont think people consider size matching sensors.

I am NOT an expert.

The telescope is creating a real image at the focal plane (where you place the camera chip). The image being sampled does not change, at all. Different chips will just pick up more or less of it in greater or lesser detail.

Magnification and focal length are fixed by the scope.

Imagine an image of the full moon, different sized chips will show more or less of the space around the moon. Or even not the entire moon if the chip is too small. So it might look bigger but only because it is taking up more of the sensor. It is not actually more magnified.

This is how I understand it anyway.

Just received my olympus DPS-1 10x50's in the post 😁

Obviously they have only been tested in daylight so far but wow! I know my last binoculars were poor and I cant compare to a 'proper' pair of Bino's because I have never had a proper pair before. BUT the image is sharp with lovely depth and the view seems enormous, very excited to try them out tonight.

For your info, my previous pair were cheap Bushnell bino's with 25mm primary lenses and 7-15x zoom. Not a great image at all.

8 minutes ago, bomberbaz said:

I used to have a chart for exit pupils for various filters but generally the tighter the bandpass of a filter, the larger the exit pupil needs to be. This is because you are dramatically cutting the levels of light passing into your eye and therefore a larger exit pupil is needed to make the image being looked at bright enough.

As a UHC has a larger bandwidth passthrough than say a HB, you can get away with a smaller exit pupil.  However this also depends on the target option. EG planetary nebula such as the ring nebula are quite bright and so will stand a lower exit pupil, even down to 1mm and be viewed from a light polluted back garden.

The horshead nebula on the other hand is very dull object, really needs a HB filter and a 5mm exit pupil is optimum. (in ink black skies)

Your 20mm plossl will give an exit pupil 3.3mm which is great for very many nebula.

Steve

Interesting thought Steve, didnt consider that. My 200p is the EQ so an even bigger pupil at 4mm should be ok with a UHC. Would be my first filter.

1 minute ago, John said:

Skywatcher 1.25 inch eyepiece adapters are not usually threaded for filters, unfortunately.

 

 

Thanks John, will check mine later when I am setting up. Fairly sure it is not threaded tho, shame as it would save me money!

Although I have been thinking about maybe going for the 1.25 inch filter anyway. I only have 1 plossl in my collection (20mm NPL) and it is my lightest (least glass) eyepiece by a good margin. I sometimes moan about it for not having a view that is as contrasty as other eyepieces.

Now I am thinking maybe this is because it allows the most light through and shows more sky glow. If this is the case, maybe it is the ideal EP for a filter to have a noticable effect. It is also my second longest focal length (after my 2inch eyepiece).

26 minutes ago, rwilkey said:

Hi Miguel, always go for 2" as you can use these with either 2" or 1.25" ep's by attaching the filter to the 2" end of your adapter, you don't need to attach filters to ep's at all and it saves faffing about when you switch ep's.  A filter can be placed in any part of the focal chain.

Thanks for the tip. I'm not sure about attaching to the end of the adaptor tho?

My 1.25 to 2 inch adaptor is just flanged at the end and tightened with two finger screws as if it was an eyepiece.

Maybe I am missing something?

Also would there not then be a problem with reaching focus with a 1.25inch eyepiece?

34 minutes ago, Don Pensack said:

The exit pupil is an image of the primary mirror.  It is not the image of the sky.  It is the distance we hold our eye from the eyepiece to see

the entire field of the eyepiece.  To show an eyepiece is not focusing the light, back away from the eyepiece.  The image stays in focus, but you are progressively seeing less and less field.

Exit pupil and field go hand in hand, but focus is in the eye.

I think for novices like me, when people say the exit pupil is the image of the primary it doesn't help. It may even make things work, because I imagine a reflection of the night sky in a mirror. I understand now why this isnt the case, but...

I point a mirror at the sky, what do I think the image of the primary mirror is? Probably the night sky.

20 minutes ago, Don Pensack said:

To show an eyepiece is not focusing the light, back away from the eyepiece.  The image stays in focus, but you are progressively seeing less and less field.

You could say the eyepiece is refocusing the image. From the 'real' image on the telescope focal plane to the unfocused light travelling out of the exit pupil.

Thanks 👍 it's always nice to raise one's level of understanding.

4 hours ago, andrew s said:

Mike I am glad @vlaiv managed to explain things better than I could. I thought the following might be helpful.

a) show vlaiv's diagram above and in b) I have blanked out all but the bit shown in you Blue diagarm posted above (but missing the eye).

In c) I have shown how the eyepiece forms an image of the objective at the exit pupil (note the rays are an inverted mirror image but at a steeper angle due to the angular magnification) with d) showing the same thing for a double headed arrow.

Regards Andrew

Thanks, I do have a better understanding but not perfect. I am content for the moment 😂 In fact, as I write this and look at diagram c) I do have a bit of a eureka moment of understanding. Its cuts to the core of my misunderstanding. I will try to explain.

Here is what I was writing before:

I look at the different parallel Ray's coming out of the exit pupil (let's imagine a 5mm exit pupil) and think, if i shrink that pupil, cut it off at the top and bottom so that it is now 2.5mm. Then the parallel photons coming along the optical axis and passing through the exit will reduce by 50%. The parallel photons coming from the edge of field see the exit at an angle (it therefore appears narrower, like a doorway seen from a sharp angle) and therefore a smaller amount of photons will pass through....

Then I realised I was wrong because the angled photons also saw the original 5mm exit from an angle. The relative losses from any particular angle will still be 50%, they just may not have started at the same brightness.

So I was wrong all along, sorry guys.

If your pupil shrinks and cuts off the edge of the exit pupil, the whole image will dim by an equal amount.

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