2" EPs for 8" Celstron SCT?

[DavidK]

I have a range of EPs and diagonals that were used for refractors.
Mostly 1.25", a couple of very nice 2" EPs.

I've just bought a 8" Celestron SCT (via this site).

I'm wondering if I should be wanting to use the 2" EPs with this scope.

Will 1.25" EPs use all the light the scope collects?

David Kinston
Vic 3162
0414 637999

[mental4astro]

Hi David,

Easy! Use the very same eyepieces you are using with your refractor with your C8! :D

A C8 has no different requirements from a refractor. If the new scope was a Newt, then there's a few things to consider, but not with an SCT.

And yes, 1.25" EPs use all the light collected, just like it does with your refractor. No different. Bigger aperture does not mean needing to use 2" eyepieces. Otherwise I'd need to use 3" eyepieces with my 17.5" dob...

The difference between using 2" or 1.25" eyepieces has nothing to do with aperture. The reasons have to do with other considerations, but not having a larger aperture. You can also use 2" eyepieces with a 2" scope...

Good to know the why so you can make the best choices for you.

Take two straws, same length, but one larger in diameter than the other. When you look through each, what do you see? The larger diameter shows more of the background than the narrower straw. This is more significant when it comes to long focal length eyepieces. You will find that there is a distinct maximum length of focal length in both barrel size and Apparent Field of View (AFOV).

In 1.25" EPs, with and AFOV of 52° the longest focal length you will find is 35mm. Yes, you get a 40mm 1.25" eyepiece, but the AFOV will be smaller, and it will show exactly the same True Field of View (TFOV), that is the exact same amount of sky. The longest with a 68° AFOV is 24mm, with 82° it is approx 15mm, and 100° it is 13mm. You just cannot get a 1.25" EP of 30mm focal length with an AFOV of 100° - the barrel size just won't allow it.

With 2" eyepieces, the focal lengths of course get longer for the corresonding AFOV. 52° approx 50mm, 68° it is 42mm, 82° it is 31mm, and 100° is 21mm. And again, you just cannot get an EP of 50mm with a 100° AFOV as the barrel just won't allow it.

Alex.

[ab1963]

35mm panoptic

[mental4astro]

Good to know what David has first before just blind dropping a "get this", and with no reason.

[ab1963]

Hi David

I read it wrong my apologies thought you wanted advice on an eyepiece not ones you already have ,My first scope was a Meade LX90 8inch sct and i had a 35 Panoptic with eyeguard extender that gave 57x mag ,1.19 degrees and was fantastic in the LX90 and you could just about get the whole of the pleiades in the fov and was my favourite low power ep through this scope

[Wavytone]

David, your refractor eyepieces should work fine.

You’ll get as many opinions as there are users here most of whom expressing an opinion based on a sample size of 1 (eyepiece and scope).

However in an SCT at f/10, virtually any modern eyepiece will work reasonably well..

[mental4astro]

David?

We can't help you unless you let us know.

You already have experience with SCT's as you have a C5. How did you find the eyepieces you have with it and your refractor? You never replied to the posts made in the thread you started asking about your C5.

[Tropo-Bob]

Hi David,

If you have some nice 2" EPs, there is no good reason not to use them. If the scope currently only takes 1.25" EPs, the scope can easily upgraded to take the larger size. Give Bintel or any other trusted dealer a call re this.

With regards to the EPs using all the light that the scope collects, this normally relates to the Exit Pupil of the EP. The 2" EPs may give a larger exit pupil if they are low powered.

2" EPs can also have the ability to provide wider fields at lower powers, which are great for scanning the Milky Way etc; depending on their design.

My best advice is just try the EPs and see what U like using.


Bob.

[astro744]

The rear opening of the C8 is 37-38mm. If the field stop diameter of your 2" eyepiece is greater than this then there will be some vignetting although it will unlikely be noticed because of the position of the rear opening in the light stream. Eyepieces with field stop diameters greater than 38mm include 31mm Nagler (42mm FSD), 41mm Panoptic & 55mm Plossl (both 46mm FSD).

A 35mm Panoptic has always been the recommended eyepiece for a C8 since its FSD is 38.7mm.

See https://www.celestron.com/blogs/knowledgebase/i-want-to-use-2-in-eyepieces-with-my-sct-and-or-mak-will-there-be-vignetting-from-the-ota-itself

Note I always saw the C8 rear opening reported as 38mm yet the Celestron post above says 37mm.

Whilst 2" eyepieces are one way to get low power on your C8, you do need a 2" visual back and 2" diagonal. Note too the effective focal length of your telescope becomes greater with a 2" rear assembly than with a 1.25" assembly. A better way to get low power on a C8 is to use a 0.63x focal reducer/field flattener screwed to the rear opening followed by a 1.25" visual back and 1.25" diagonal. Then use say a 24mm Panoptic which has a 27mm FSD and you will get the widest field possible and at the design focal length of the telescope (x0.63 because of the FR/FF). So in effect your 27mm FSD becomes 27/0.63=42.8mm without the vignetting.

Note three 1.25" eyepieces can give you the widest true field; Tele Vue 24mm Panoptic, 32mm Plossl & 40mm Plossl. Each has 27mm FSD but gives different powers and exit pupils.

[Loupy31]

I have a pre starbright coatings, C8, 2 Inch adapter on scope, 2 Inch diag. , Best view , for me, is a Explore scientific, 30mm 2 Inch, its big, its heavy and it has amazing views through the C8

[DavidK]

Thanks everyone for the replies. Apologies for the duplicate post - for some reason I didn't see the replies.

No longer have the C5. My main scope is the 8" EdgeHD Celstron SCT with a 2" Bintel diagonal, 40mm WO 2" EP, 2" Hyperion 8-24mm zoom EP and Celestron 2" focal reducer. It will be set up when we move in a week or 2.

Also a 6" Meade LX80 OTA on a Skywatcher Star Discovery mount as my portable scope. Several WO and Meade 1.25" EPs from 4mm to 26mm.

Looks like I'll be sticking with the 2" accessories on the 8" SCT, which will only rarely be moved around.

[Camelopardalis]

David, word of advice with the Edge HD...you can see the difference between cheap eyepieces and better quality ones ;) this may not be true of lesser SCTs, but you certainly get a better experience out towards the edge with better quality ones.

[Ukastronomer]

YES

How many times have I said, unless you ask a complete question what you have, what you want to achieve, what you want to spend how can someone reply.

Just saying the make off an eyepiece and size is no help