afov to tfov

[DWood]

My wife got my daughter a Gskyer 70mm 400mm for Xmas. I've been doing reading and trying to learn. We will give early for the saturn jupiter conjunction.

 

Searched site but unsure b of the venaqcular I need.

 

What is the lowest return on investment for low power eyepiece?

At what point does low magnification start to deteriorate the actual volume the scope can handle? Eyepiece in regards too.

[jambouk]

Have a read of this:

https://britastro.org/node/10878

James

 

[Stu]

2 hours ago, DWood said:

My wife got my daughter a Gskyer 70mm 400mm for Xmas. I've been doing reading and trying to learn. We will give early for the saturn jupiter conjunction.

 

Searched site but unsure b of the venaqcular I need.

 

What is the lowest return on investment for low power eyepiece?

At what point does low magnification start to deteriorate the actual volume the scope can handle? Eyepiece in regards too.

Without wishing to over complicate things, there is a concept called ‘exit pupil’ which is the diameter of the bundle of light exiting the eyepiece and entering your eye. This varies with telescope aperture and the magnification being used.

It is calculated by dividing the aperture of the scope by the magnification being used, OR, dividing the focal length of the eyepiece by the focal ratio of your scope.

Why is this important? If the exit pupil is too small (say below 1mm, definitely below 0.5mm), then you see any floaters which you might have in the view. Above about 6 or 7mm you start to waste light because your pupil can only dilate to about this size (and this size reduces with age). The other thing with large exit pupils is that the sky background can appear very washed out if you live in a light polluted area.

So, with your scope, a 40mm eyepiece would give a magnification of x10, an exit pupil of 7mm and probably 5 degrees field of view (assuming a simple Plossl design), plenty to fit Jupiter and Saturn into even now as they are about 1 degree apart.

You would likely do better with a 20mm or even a 15mm which would give x20/3.5mm/2.5 degrees or x26/2.6mm/1.9 degrees respectively. Both would be plenty to fit the planets in during the conjunction. You could of course go even higher power than this.

[bomberbaz]

following on from @Stu regarding exit pupils and magnification, you should also bear in mind that the higher you go in magnification, the more shaky the view will appear and eye placement becomes more difficult.

I would certainly recommend getting the scope out as soon as possible and trying it out during the day on distant terrestrial targets to get you all used to this. 

A 8mm eyepiece would give you x50 magnification which has an exit pupil as referred to by Stu of 1.4mm and a true field of view (TFOV) of the sky of around 1 degree. This assumed the (AFOV) apparent field of view of the eyepiece is 50 degrees. 

At x50 you will be able to see the moons of Jupiter, some cloud detail and you will also be able to make out the rings of Saturn, seeing conditions allowing. 

If you are considering getting a different eyepiece then something in the 8-10mm range would be a decent mid range eyepiece to work with until you get used to using it.

Celestron X-Cel LX eyepiece | First Light Optics 9mm is in stock and hopefully with you for the conjunction. 

 

[bomberbaz]

5 hours ago, DWood said:

My wife got my daughter a Gskyer 70mm 400mm for Xmas. I've been doing reading and trying to learn. We will give early for the saturn jupiter conjunction.

I notice that this scope actually has a 3x barlow and a 25mm eyepiece. I am not sure of the quality of either of these but used together in the scope you have an eyepiece which is effectively 8.33mm, this will give you the x50 I mentioned above and so you should maybe hold off buying anything until you have played around with the eyepieces it comes with and the 3x barlow in combinations. 

Edited December 12, 2020 by bomberbaz

[DWood]

Exit pupil is what I was needing.

Apeture÷magnification=exit pupil

Focul Length(scope)÷fl(eye piece)=mag

So I'm using

70mm aperture 400mm fl

So 32mm eye piece 400÷32=12.5

70mm÷12.5=5.6mm exit pupil.

 

Is that right? So that in reality would be the lowest we would want to go.

[Stu]

11 minutes ago, DWood said:

Exit pupil is what I was needing.

Apeture÷magnification=exit pupil

Focul Length(scope)÷fl(eye piece)=mag

So I'm using

70mm aperture 400mm fl

So 32mm eye piece 400÷32=12.5

70mm÷12.5=5.6mm exit pupil.

 

Is that right? So that in reality would be the lowest we would want to go.

I believe this is all explained in my post Dave.

You only really reach a fully dilated pupil under decently dark skies, with light pollution or glare from lights your pupil will be smaller.
 

I would suggest the 32mm would probably be the lowest, for two reasons. Firstly the exit pupil, but secondly increasing to a 40mm eyepiece doesn’t but you any more field of view because the apparent field of view will reduce from 50 degrees with a 32mm to around 42 degrees for a 40mm, so you get lower Mag and no more sky.

[DWood]

Thanks