How low can you go?

[Josh Wilson]

I read somewhere that you need to be careful when choosing low power EP's for a mak, SCT, or newt telescope because too low magnification will show the secondary mirror obstructing the view. So what is the lowest magnification I could use for my 4.5" newt or (hopefully soon) an Orion XT8 dobsonian telescope before the secondary mirror shows up?

XT8: 203mm diameter, 1200mm FL, secondary mirror obstruction 47mm

4.5" newt: 114mm diameter, 910mm FL, 1" secondary mirror (i think)

[John]

It depends on the focal ratio of the scope because that, coupled with the eyepiece focal length, determines the size of whats called the exit pupil - the bundle of light coming out of the eyepiece. Too large an exit pupil and you will start to see the shadow of the secondary mirror. Your 4.5" newt is F/8 so a 40mm eyepiece (22.75 x) should work fine. With the XT8 you may need to drop to a 32mm (37.5 x) as it's focal ratio is faster - F/6.

I'm guessing your 4.5" only takes 1.25" eyepieces so you might as well stick with a 32mm in that as the 40mm will not show any more sky, despite having a lower magnification. The XT8 takes 2" eyepieces though so that opens up wider field possibilities at the 32mm focal length.

[E621Keith]

I think the secondary shadow issue only affects Newtonian. Most SCT and Mak are F10+, so even with a 56mm plossl, you will only get 5.6mm exit pupil, which is within the capability of most dark adapted eyes.

You can calculate an eyepiece's exit pupils by dividing the eyepiece focal length by the scope's focal ratio. You will be fine if you keep the exit pupil less than 5mm.

[Nevy]

I have a C11 (F10) and 12" dob (F5) ,and a 56mm plossl (2") works fine in both , I got it for the C11 but I was pleasantly surprised how good it works in the dob ( I can fit the Pleiades in the field of view). I can't see the secondary shadow , only when defocused but that happens with all eyepieces if you defocused enough.

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[Naemeth]

In addition to the above comments, if you would like to see what happens when the exit pupil is too large for your eye, use your lowest power eyepiece in the day, where you may see the whole secondary obstruction.

[faulksy]

I think the secondary shadow issue only affects Newtonian. Most SCT and Mak are F10+, so even with a 56mm plossl, you will only get 5.6mm exit pupil, which is within the capability of most dark adapted eyes.

You can calculate an eyepiece's exit pupils by dividing the eyepiece focal length by the scope's focal ratio. You will be fine if you keep the exit pupil less than 5mm.

always wondered how you work this out. thanks for that

[KaStern_Former_Member]

Hi Josh,

you have to consider several different things:

1) exit pupil diameter

2) eye pupil diameter

3) field stop diameter

The exit pupil diameter depends on eyepiece focal length devided by [telescope focal legnth devided by telescope optics diameter].

This if 7.9 for your 117/900mm scope and will be about 6 for your 200/1200mm scope.

So if you buy a 42mm fl eyepiece you get 5,3mm exit pupil with the 114mm scope and 7mm exeit pupil with the 200/1200mm Newt.

Your eye pupil diameter can get as big as 7mm when your eyes are fully adapted to darkness and your observe a dim object.

But eye pupil diameter can get as small as 1mm when you ebserve our Moon using a relative low magnification.

Exit pupil is entrace pupil devided by magnification.

If the 200mm apertured scope has 47mm Obstruction and you use a 42mm eyepiece maginfication will be 28,57x and

the central dark part in the exit pupil will mearure 1.64mm.

Observing a bright objekt as the full moon your eye pupil will be small and therefore you wwill get annoying black outs.

If you observe deep sky objects under dark skys there will be no problem at all.

Last thing to consider is the true field of view. To calculate it you will need to know the eyepiece field stop diameter

and the telescope focal legth.

In an 1,25" eyepiece field stop diameter is limited to 27mm diameter. Using the arctan formula you can calculate

the true field of view of your 900mm fl scope: 1.72°

With your 1200mm fl dob rezw field of view will be: 1.28°

There will be no change in true field of view no matter if

you use a 42mm, 32mm or 24mm 1,25" as long as

the field stop diameter is the same 27mm diameter.

With a 42mm 2" eyepiece you can get a 46mm field stop diameter.

It will not fit into the 1,25" focuser of your 114/900mm scope,

but it will give you 2.2° true field of view with your 200/1200mm Dob.

So I would recomend to buy a 32mm Ploessl or a 24mm widefield eyepiece for your 114/900mm scope

and a 2" widefield eyepiece for your dob.

If you observe the moon you better use higher magnifications than these.

Cheers, Karsten

[E621Keith]

always wondered how you work this out. thanks for that

The actual formula is the objective diameter (oD) divided by magnification.

The magnification is telescope focal length (tFL)divided by eyepiece focal length (eFL). (tFL/eFL)

As Karsten mentioned, this can be rearranged into eyepiece focal length divided by scope focal length divided by scope diameter.

Scope focal length divided by scope diameter is the focal ratio, so the formula can be simplified into eyepiece focal length divided by focal ratio.

As such a 56mm eyepiece in a F10 SCT will give 5.6mm exit pupil, while in a F5 Newt will give a 11.2mm exit pupil etc...

[Josh Wilson]

Thanks everyone for the replies. My 4.5 " newt is actually a .965" telescope. But fear not, I have an adapter. With the adapter on though, there's about 1cm of back focus so I'm considering getting a new focuser for it. Any way, thanks to Karsten too. I never would have thought that the field stop would have anything to do with lowest magnification possible.