Is it normal.... for the field of view to change as you rotate an eyepiece....

[rw5373]

Just back from my first night out with my first ever telescope - Celestron Astromaster EQ 130. Super chuffed to have glimpsed Saturn.... but I have a question about eyepieces and what is 'normal'!

The telescope came with 2 eyepieces - a 20mm 'Erecting' Eyepiece, and a 10mm (non-erecting) eyepiece.

 

When I use the 20mm erecting eye piece - is it normal/correct that, when I look through it:

- the image is not quite perfectly circular.... about one third of the circumference is slightly shadowed - as if the circumference of the image disc is being squashed slightly from one side

- also  - if I rotate the eyepiece within the eyepiece holder - I was expecting the image I saw just to circle around perfectly on its centre point.... like watching a disc spin...  but it doesn't quite do this - the centre point seems slightly offset - so the as I rotate the eyepiece, the field of view of the image changes slightly - bringing in different parts of peripheral  image  that I can only see with the eyepiece in certain positions.   

 

When I use the 10mm eyepiece I don't see these same issues - the image is perfectly circular, and when I rotate the eyepiece the image spins around a central point like a disc.

 

Is this something intentional about the design of the 20mm lens, or have I got a fault optic?  As best I can tell the telescope mirrors are all aligned OK - but could the eyepiece be out of kilter?

 

 

Thanks in advance fir any adivce

 

 

Robert

 

 

[John]

Hello and welcome to the forum. This thread from 2018 covers the same issue that someone else came across with the 20mm erecting eyepiece and the same (or very similar) scope:

 

 

[johninderby]

The 20mm errecting eyepiece is for daytime terretrial observing use and  very poor at night time observing. What you are seeing is “ normal” for such a poor eyepiece.

You would be better to pick up an inexpensive £10.00 20mm eyepiece on eBay or Amazon..

[Demonperformer]

Hi, Robert, and welcome to SGL.

Good advice given above.

Enjoy the journey.

[banjaxed]

I have never tried to rotate the eyepiece as once fitted and the fixing bolts are clamped on the EP it can't rotate. Am I missing something ?

[bingevader]

2 hours ago, banjaxed said:

I have never tried to rotate the eyepiece as once fitted and the fixing bolts are clamped on the EP it can't rotate. Am I missing something ?

Nope, release the bolts slightly and your EP should rotate.

Not that it should make any difference, unless you have the same set up and EP!

14 hours ago, johninderby said:

You would be better to pick up an inexpensive £10.00 20mm eyepiece on eBay or Amazon..

Now, EP suggestions are a completely different matter all together!

As a replacement for the 20mm, I don't think you could go wrong with a Vixen plossl.

Wouldn't break the bank and would happily transfer to another 'scope should you get hooked and need an upgrade!

[johninderby]

The Vixens are good eyepieces but if you don’t want to spend as much the Ascension plossls are not half bad and are certainly usable.

https://www.ebay.co.uk/itm/Ascension-Super-Plossl-Eyepiece-20mm-FMC-UK/322899831103?epid=9010229777&hash=item4b2e54693f:g:zIMAAOSwhpZaFs45

Edited July 30, 2019 by johninderby

[rw5373]

@John Thank you for that link.... apologies - I did search this forum before posting but clearly didn't do a through enough job as I didn't find that post!

Everyone else and John  - thank you for such quick replies with advice.

 

nice to find a friendly Internet forum!  🙂

 

Bobby 

[Louis D]

If you're curious about the 20mm eyepiece's internals, a contributor on CN did a tear-down, clean-up, and rebuild recently.  It's actually a rather complex little beast.