AFOV and secondary shadow

[turboscrew]

How does AFOV affect the secondary mirror shadow?
I was thinking of getting another "finder eyepiece". I've been using a 40 mm Plössl with AFOV 52°, and the secondary shadow hasn't bothered me. I was thinking of getting a 40 mm Willam optics SWAN with AFOV 70°. Would that make the shadow worse?

[Lady Fraktor]

AFOV does not increase the secondary shadow it is caused by to low of magnification of the eyepiece.
This makes the exit pupil of the eyepiece larger than your dark adapted pupil.

What 40mm eyepiece do you have with a 52° AFOV? Normally they are between 40° and 45° AFOV.

With your telescope a 40mm would give a 10mm exit pupil... how are you seeing anything?

[notFritzArgelander]

The Orion Optics XV12 is f4 so a 40mm eyepiece gives a 10 mm exit pupil. You see a secondary shadow because of the exit pupil being larger than your dark adapted eye can handle. On an f4 scope when I was younger I would not use any eyepiece longer than 28 (4 times 7) mm focal length. Now that my pupil only dilates to 5-6 mm I’d put the limit at 20-24 mm. Then I can be fearless about AFOV.

[Ruud]

Yes, it is the exit pupil's size that causes the problem, and the exit pupil size depends on the focal length of the eyepiece. Example: if you have an f/5 telescope and you use a 30mm focal length eyepiece, you get an exit pupil of 30/5 = 6mm

The shadow of the secondary is in the centre of the exit pupil. If the secondary is 30% of the objective, the shadow of the exit pupil will be 30% of the exit pupil

When the exit pupil gets bigger than the observer's pupil two things happen:
     1. some of the light ends up next to the observer's pupil and cannot enter the eye
     2. the shadow of the secondary begins to fill up the observer's pupil

2. can cause the secondary's shadow to become noticeable.

Imagine watching the Moon using an 13mm wide exit pupil with a 3mm shadow at its centre. A 4mm observer's pupil hovering over this exit pupil will lose a lot of light when directly over its centre and taking in the full shadow of the secondary. When it drifts a few mm to the edge of the exit pupil, the shadow no longer obscures the pupil.

Another example: If you have an f/4.7 telescope and a 55mm eyepiece, you get an exit pupil of 55/4.7 = 11.7mm and you will definitely notice your secondary's shadow.

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I have a 500mm focal length f/5 refractor and used a 55mm Plössl as finder. Magnification was 9x and the exit pupil was 11mm. A refractor has no central obstruction so I saw no shadow, but I still did not like the eyepiece as a finder. A separate RACI has all the advantages: you can leave it in place when you don't need it, and you don't need to remove an eyepiece to do your finding. On top of this, the cross hairs of a RACI make it a more effective finder than a long focal length eyepiece will ever be.

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I discovered that I like a 6x30 RACI much better than an 8x50. The larger field of the 6x30 is definitely an advantage. My 6x30 RACI is a SkyWatcher and cost just under €50.

[kt4hx]

The longest f/l eyepiece I utilize now is (as mentioned by nFA) 24mm with my 17.5 inch f/4.5 dob. This yields an exit pupil of 5.4mm and I pretty much try to stay around 5mm as my maximum now. At 66 yrs of age, my eyes simply do not operate in the same manner as when I was 16 - then again, what does!

Where the AFOV comes into play is how much field the observer wants to deal with. The larger the AFOV the more eye movement it takes to scan the resulting TFOV. For a long time I always said I would go no larger than 82° AFOV as I was comfortable with that. However, within the past year I acquired a 13mm Ethos and its whopping 100° AFOV. Truth be told I don't find it a problem to utilize. However, not everyone feels that way. It gives me a nice roomy 0.7° TFOV at 152x in the 17.5 inch, which is nice for observing galaxy clusters.

[turboscrew]

AFOV does not increase the secondary shadow it is caused by to low of magnification of the eyepiece.
This makes the exit pupil of the eyepiece larger than your dark adapted pupil.

What 40mm eyepiece do you have with a 52° AFOV? Normally they are between 40° and 45° AFOV.

With your telescope a 40mm would give a 10mm exit pupil... how are you seeing anything?

You're right. It has AFOV 45°! I must have missed the note, The rest of the serise have AFOV 52°.
And I do see enough to get a target from finder into view. When I get the target about centered in the view, I switch to some useful eyepiece - often 25 mm. I'm really using the 40 mm eyepiece only for that.

[turboscrew]

Yes, it is the exit pupil's size that causes the problem, and the exit pupil size depends on the focal length of the eyepiece. Example: if you have an f/5 telescope and you use a 30mm focal length eyepiece, you get an exit pupil of 30/5 = 6mm

The shadow of the secondary is in the centre of the exit pupil. If the secondary is 30% of the objective, the shadow of the exit pupil will be 30% of the exit pupil

When the exit pupil gets bigger than the observer's pupil two things happen:
     1. some of the light ends up next to the observer's pupil and cannot enter the eye
     2. the shadow of the secondary begins to fill up the observer's pupil

2. can cause the secondary's shadow to become noticeable.

Imagine watching the Moon using an 13mm wide exit pupil with a 3mm shadow at its centre. A 4mm observer's pupil hovering over this exit pupil will lose a lot of light when directly over its centre and taking in the full shadow of the secondary. When it drifts a few mm to the edge of the exit pupil, the shadow no longer obscures the pupil.

Another example: If you have an f/4.7 telescope and a 55mm eyepiece, you get an exit pupil of 55/4.7 = 11.7mm and you will definitely notice your secondary's shadow.

---

I have a 500mm focal length f/5 refractor and used a 55mm Plössl as finder. Magnification was 9x and the exit pupil was 11mm. A refractor has no central obstruction so I saw no shadow, but I still did not like the eyepiece as a finder. A separate RACI has all the advantages: you can leave it in place when you don't need it, and you don't need to remove an eyepiece to do your finding. On top of this, the cross hairs of a RACI make it a more effective finder than a long focal length eyepiece will ever be.

---

I discovered that I like a 6x30 RACI much better than an 8x50. The larger field of the 6x30 is definitely an advantage. My 6x30 RACI is a SkyWatcher and cost just under €50.

I do notice the secondary shadow, but it hasn't bothered its use as a "finder eyepiece". I don't use it for observing at all.

[Lady Fraktor]

You are dimming the view down considerably though if wanting to search.
I would use a 25mm - 28mm and have a proper exposure.
Using the 10mm exit pupil is like hobbling a horse and trying to get it to run fast...

[turboscrew]

Yes, but I seem to need the TFOV to get, what I see in finder, centered enough to be seen with smaller TFOV eyepieces.
I have been planning to get a RACI instead of straight 6x30 finder.

[Lady Fraktor]

It sounds like your finder and telescope are not aligned together properly.
If they are, the centered object in the finder will be centered automatically in the telescope.

[notFritzArgelander]

It sounds like your finder and telescope are not aligned together properly.
If they are, the centered object in the finder will be centered automatically in the telescope.

Yes. I align the finder with every setup. Daylight first for ease then refined alignment with a star.

[turboscrew]

It sounds like your finder and telescope are not aligned together properly.
If they are, the centered object in the finder will be centered automatically in the telescope.

It is. It's just that it's hard to get the target centered so well in the finder that it is visible in the telescope view especially when one has to look in weird yoga positions.

[notFritzArgelander]

It sounds like your finder and telescope are not aligned together properly.
If they are, the centered object in the finder will be centered automatically in the telescope.

It is. It's just that it's hard to get the target centered so well in the finder that it is visible in the telescope view especially when one has to look in weird yoga positions.

Use a convenient daytime object that is somewhat distant and the problem will be solved.

[turboscrew]

It sounds like your finder and telescope are not aligned together properly.
If they are, the centered object in the finder will be centered automatically in the telescope.

It is. It's just that it's hard to get the target centered so well in the finder that it is visible in the telescope view especially when one has to look in weird yoga positions.

Use a convenient daytime object that is somewhat distant and the problem will be solved.

If you mean finder alignment? I did it yesterday daytime (while observing the spruce cones). The yoga positions are needed when I try to look up in a straight finder end that's 5 cm higher than my navel. The ground is too wet to kneel, and it still wouldn't help that much.
I'm only 57, but not that flexible any more.