Wide 2” eyepiece - Help please!

[PhotoGav]

I am investigating the options available for a wide field 2” eyepiece. It is for use on a 10” refractor. The current widest eyepiece I have is a Meade 5000 series 40mm 2” and it is great. What would be appropriate that is longer than that and would offer a wider view? The only thing I have seen so far is the Masuyama 2” eyepieces at 50mm and 60mm listed on FLO - would they be good?

Many thanks in advance.

[rockystar]

I think you're going to find the exit pupil way too big. 50mm will yield a 10+mm Exit Pupil.

[Stu]

Do you mean 10” refractor Gav? What is the focal length?

There is the 55mm Televue Plossl which is up to their normal standards. Bear in mind you won’t get much if any additional field of view because they only have 50 degree afov compared with say a 68 degree on the 40mm eyepieces (or the 41mm Panoptic). The main benefit is the extra exit pupil but that probably isn’t an issue depending upon your focal length. 

[Stu]

Just now, rockystar said:

I think you're going to find the exit pupil way too big. 50mm will yield a 10+mm Exit Pupil.

Depends what the scope is and what the focal length/ratio is

[rockystar]

37 minutes ago, Stu said:

Depends what the scope is and what the focal length/ratio is

Indeed it does - I've assumed they meant reflector  and 250/1200.

[Stu]

9 minutes ago, rockystar said:

Indeed it does - I've assumed they meant reflector  and 250/1200.

Yes, that's most likely unless it really is a monster f10 10" refractor!!!

I'm sure @PhotoGav will confirm soon.

[PhotoGav]

Thank you for your replies @Stu & @rockystar . As you can tell I am not very well versed in the mathematics of eyepieces. However, the scope in question is indeed a monster 10” refractor. It’s an absolute beauty - a Thomas Cooke 10” made in 1860 on a monster equatorial motorised and computerised mount. It’s the oldest GoTo telescope... in the world!! It has a focal length of 3.8m and a focal ratio of f15. I am very privileged to have access to it. The views that I have had through it of the Sun, the Moon, the planets, double stars, clusters and planetary nebulae are simply breath taking.

Please can you explain the subtleties of how eyepiece focal length, apparent field of view, exit pupil and eye relief all work! In terms that an imager can understand, please!!

[Stu]

Thanks Gav, that clears it up!

Firstly can we assume it takes standard 2" eyepieces?

So, basic equations....

Magnification is found by focal length of scope divided by focal length of eyepiece.

Actual field of view is found by dividing the apparent field of view of the eyepiece by the magnification

Exit pupil is found by dividing the aperture by the magnification, or the focal length of the eyepiece by the focal ratio of the scope.

[Stu]

So for the beast and say the 55mm TV Plossl with a 50 degree afov you get:

Mag = 3800/55 = x69

True Field of View = 50/69 = 0.72 degrees

Exit Pupil = 55/15 = 3.67mm

 

[Stu]

In terms of focal length it is pretty close to the 8" f20 Mak I used to have which was 4m. Pretty extreme and you are limited to around 0.7 degrees field at best. Fab for planetary and Lunar, and globs I should think.

[PhotoGav]

@Stu - thank you for your very clear explanations. Yes, it takes standard 2” eyepieces. I will do some maths and see what numbers the Masuyama eyepieces generate. 

Can you recommend any other top quality long focal length 2” eyepieces that I should consider?

[rockystar]

1 hour ago, PhotoGav said:

However, the scope in question is indeed a monster 10” refractor. It’s an absolute beauty - a Thomas Cooke 10” made in 1860 on a monster equatorial motorised and computerised mount. It’s the oldest GoTo telescope... in the world!! It has a focal length of 3.8m and a focal ratio of f15.

Wow, very nice.  Not much to add now stu has given you the formulas. Can't help with longer length Esp, sorry.

[Ags]

Shouldn't you be looking at 3" eyepieces for an observatory scope like that?

http://apm-telescopes-englisch.shopgate.com/category/3439303532

I have heard somewhere of people using reversed camera lenses...

[Stu]

2 hours ago, Ags said:

Shouldn't you be looking at 3" eyepieces for an observatory scope like that?

http://apm-telescopes-englisch.shopgate.com/category/3439303532

I have heard somewhere of people using reversed camera lenses...

That’s certainly the way to get a wide field of view, I wonder if the scope could take them, in theory you would think so. The larger field stop will allow the extra field.

That 80mm would give x47, a 1.1 degree field and a 5.3mm exit pupil, very nice! Almost as spectacular as the price!!

I’m afraid I don’t have any experience beyond the 55mm TV Plossl, I’ve only used 41mm Panoptic, 42mm Vixen LVW and a 40mm TMB Paragon, nothing longer. I’m sure someone else will be able to assist

[PhotoGav]

@Ags that is a very interesting idea, thank you. Not sure what the observatory director will make of the budgetary requirements, but that would certainly give the significantly wider field of view we are after. I’m intrigued by the camera lens idea - do you have any further information please?

@Stu - thank you very much for your help - I will have a Jaffa cake or two for you at the SGL SP!

[John]

Once you have got to a ~40mm with a 70 degree AFoV you have reached the widest true field that the 2" format eyepieces can deliver. You would need to consider a larger format eyepiece to see more sky. You can get longer focal length 2" eyepieces - up to 80mm in fact, but the true field that they show will not be any larger and the reducing apparent field, as the focal length gets longer, can seem claustrophobic. The main reason for accepting this and going for a longer focal length than 40mm would be to get a more effective exit pupil size rather than to show more sky.

 

[PhotoGav]

1 minute ago, John said:

Once you have got to a ~40mm with a 70 degree AFoV you have reached the widest true field that the 2" format eyepieces can deliver. You would need to consider a larger format eyepiece to see more sky.

Thanks John.

I have to say, SGL has delivered yet again - earlier today I was fumbling in the dark with regards to eyepieces. Within a few hours I have been educated and informed by you generous souls and have a great deal more understanding of the whole eyepiece world. Thank you all. You have also found me a wonderful way of spending 2000 plus Euros - the other great skill of SGL, finding ways of spending shed loads of cash!!!

[John]

Hopefully SGL helps you get shed loads of value out of the investment anyway !

[PhotoGav]

1 minute ago, John said:

Hopefully SGL helps you get shed loads of value out of the investment anyway !

Absolutely! And in terms of the value of the telescope in question, even a 3” eyepiece system would be peanuts!

[Paz]

3 hours ago, PhotoGav said:

Thank you for your replies @Stu & @rockystar . As you can tell I am not very well versed in the mathematics of eyepieces. However, the scope in question is indeed a monster 10” refractor. It’s an absolute beauty - a Thomas Cooke 10” made in 1860 on a monster equatorial motorised and computerised mount. It’s the oldest GoTo telescope... in the world!! It has a focal length of 3.8m and a focal ratio of f15. I am very privileged to have access to it. The views that I have had through it of the Sun, the Moon, the planets, double stars, clusters and planetary nebulae are simply breath taking.

Please can you explain the subtleties of how eyepiece focal length, apparent field of view, exit pupil and eye relief all work! In terms that an imager can understand, please!!

Oooooh - we would like to see some pictures of that sometime! 

[vlaiv]

Depending on focuser used on telescope (is it 2", 2.5" or 3" focuser, and whether it is "upgradable") - you might consider using some sort of FL reducer (usually used for AP) and regular 2" eyepieces.

https://www.teleskop-express.de/shop/product_info.php/info/p8118_Astro-Physics-27TVPH-2-7--photo-visual-Telecompressor-0-75x.html

Or something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p8704_Explore-Scientific-3--0-7x-Reducer-and-Korrektor-for-Apos--ACF-----.html

Even this:

https://www.teleskop-express.de/shop/product_info.php/info/p5125_APM-Riccardi-APO-Reducer-and-Corrector-0-75x---M82-connection.html

Any of these will boost your true field of view, but might be sensitive to correct distance.

[PhotoGav]

51 minutes ago, Paz said:

Oooooh - we would like to see some pictures of that sometime! 

I’ll post something when next on my laptop.

@vlaiv - that’s another interesting idea, will investigate the possibility when next at the telescope. I know that the focuser is a rickety old brass thing with an odd screw fit end and various 2” push fit adapters. It certainly isn’t ‘upgradable’ only ‘addable to’!

[Ags]

If you look on CN you will see some threads about using gaussian lenses like a 50/1.8, but I doubt camera lenses are going to beat the 70mm field stop of the 80mm 3" Masayuma. You may also want to have a look here:

http://www.siebertoptics.com/SiebertOptics-eyepieces-observatory.html

...although I must say the Siebert website makes my eyes bleed ?

[Louis D]

On 07/10/2018 at 06:40, Ags said:

If you look on CN you will see some threads about using gaussian lenses like a 50/1.8, but I doubt camera lenses are going to beat the 70mm field stop of the 80mm 3" Masayuma. You may also want to have a look here:

http://www.siebertoptics.com/SiebertOptics-eyepieces-observatory.html

...although I must say the Siebert website makes my eyes bleed ?

Siebert has made 4" inch eyepieces for some large observatory telescopes for visual use.  You should investigate if the scope's focuser can be adapted to 3" or 4" eyepieces.  Many observatory scopes simply have diameter reducers in place to allow for 2" eyepieces, but actually have much larger focuser openings for photography on large sheets of film.  It would just be a matter of removing the 2" adapter and having a 3" or 4" adapter machined to replace it.