Should I use 2" eyepieces?

[mal-j]

Hi,

I've just acquired a 12" f4 reflector. I only have 1 1/4" eyepieces now but would I better be served with 2" ones?

Thanks

[Andy_E]

It all depends on what you want. 2" ep designs are going to offer you a wider field of view, but at f4 I imagine the majority of your observing would be fine with 1.25s... unless you want to branch in to the 100deg FOV market.

[Stu]

There is some discussion along the same lines here too, for different scopes but the principles are the same.

http://stargazerslounge.com/index.php?/topic/200608-2%22-or-1%2E25%22-eyepieces%3F

It really depends upon your targets and required field of view.

With 1.25", you are pretty much limited to just under 1.4 degrees fov with say a 32mm Plossl or 24mm Panoptic.

With 2", you can get Widefield eyepieces in longer focal lengths eg 28mm UWAN which would give you 1.9 degrees or 31 Nagler giving 2.1 degrees approx, these are just examples, plenty of other choices out there.

It's is a case of weighing the benefits eg field of view, with the cost, and weight of the larger eyepieces. F4 would also be pretty demanding so you would need to buy a decent quality ep to avoid the image being poor in the outer parts of the field

Cheers,

Stu

[John]

If you don't already use one, you might want to think hard about getting a coma corrector if you move to 2" eyepieces for their wider fields of view. As you see more of the off axis field, coma will be a real issue with an F/4 newtonian.

[Reeny]

I think it depends on field of view and eyepiece focal length.

There are too many if's or but's depending on the eypiece specification.

For example:

The 82 degree Explore Scientific eyepieces need 2" at 18mm

The 70 degree ES eyepieces need 2" above 20mm

The 68 degree ES eyepieces need 2" above 24mm

Once up to 2" - the FoV is restrictive at the loweset focal lengths where an even wider barrel is needed.

As shown here for the same design of 2" eyepiece

http://www.telescopehouse.com/acatalog/info_BC008.html

30mm - 70 Degree field
42mm - 65 Degree field
50mm - 55 Degree field
 

[RoysSkies]

It's is a case of weighing the benefits eg field of view, with the cost, and weight of the larger eyepieces. F4 would also be pretty demanding so you would need to buy a decent quality ep to avoid the image being poor in the outer parts of the field

Cheers,

Stu

I have a 12" F4.9 SW Dob and I use all 2" EP's, I got the SW 38mm panaview and I lose only a little bit in the outer FOV but the images in the center are crystal clear.

I would recommend that you ask your EP supplier if he would let "Try before you Buy".. I ahve never use an F4 so I can't give you concrete advice, but I reckon

a 32mm or 26mm would be Okay with very little coma.

Cheers Roy

[John]

In a given barrel size the apparent field of view (AFoV) you have is defined by the diameter of the field stop which in turn is limited by the internal diameter of the eyepiece barrel.

For a 1.25" eyepiece the largest field stop diameter that can be accommodated is around 27mm-28mm and with the 2" size it's 45mm-46mm.

This means that 1.25" eyepieces can have max AFoV's along these lines:

40mm - 44 degrees

32mm - 52 degrees

24mm - 70 degrees

18mm - 82 degrees

13mm - 100 degrees

In the 2" fitting you can have:

40mm - 70 degrees

31mm - 82 degrees

25mm - 100 degrees

These are approximates.

You also need optical quality and design capable of providing distortion free views across these large fields of view and, for use in a fast scope, that is not easy to achieve and does not come cheap !

[mal-j]

Thanks for all of your advice.

I have decided to purchase a secondhand Explore Scientific 20mm 100-degree 2" eyepiece.

We'll see if it's good for my setup and may invest in a coma corrector.

[John]

Thanks for all of your advice.

I have decided to purchase a secondhand Explore Scientific 20mm 100-degree 2" eyepiece.

We'll see if it's good for my setup and may invest in a coma corrector.

It's a great eyepiece but you may need to re-balance your scope when you use it - it's very heavy at around 1 kg.

I see slight coma at the edges of the field of view when I use mine in my 12" F/5.3 dobsonian.

Let us know how you get on with it  

[AlexB67]

Hi, mal. Look forward to a little report , It is not often you hear any brands able to stand up to f/4 and maintaining good edge to edge performance.  TV state it explicitly, but interestingly, even many of  the ES eyepieces do not state a recommended focal ratio cutoff.

There are a couple of Maxvision eyepieces at 82 degrees like this one,

http://www.optical-systems.co.uk/maxvision-82deg-okular-24mm-p-25559.html

without splitting you wallet in half  and claim good performance at f/4. Never seen eyepieces at that price anywhere else claiming f/4 that I am aware of, at that FOV anyway, but no idea how good it is. Seems almost a bit too good to be true at that price.

MV also do a 18mm at that FOV claiming f/4.  I've not heard much on the 82 degree maxvision eyepieces here, though the 68 degree ones are very popular and claim f/5. Could the MVs be the same with just a different casing and coatings or something else like the ES, who knows, but nothing stated about recommended focal ratio cutoff on the ES.

The information mystery that surrounds eyepieces.

[Andy_E]

Despite the claims, I doubt there are any truly 100% coma free UWAs at f4... someone may correct me though.

[John]

Despite the claims, I doubt there are any truly 100% coma free UWAs at f4... someone may correct me though.

Coma is a scope aberration. I guess you mean astigmatism ?

[AlexB67]

Despite the claims, I doubt there are any truly 100% coma free UWAs at f4... someone may correct me though.

I guess, you'd need to CC to find out anyway, the eyepiece is not going to do anything for that in this case, at least I do not know of any eyepieces with a build in CC AFAIK, but there are varying degrees of coma correction, some better and more successful at the task than others I gather. 

edit:  beat me to it.

[AlexB67]

I meant to add.

Coma is not as big a deal sometimes and astigmatism perhaps overlooked, but here will come a point where it will be annoying  in wide FOV eyepiece and faster scope, a cheap eyepiece will suffer form both.

The only eyepieces I can talk about from experience is the MV I own and a 25 degree BST, they both show coma, BUT, the MV is more pleasing to look through on the outer edges, because it is mainly coma that makes the outer edges become less well defined, whereas in the BST astigmatism is the more noticeable aberration in comparison. I do not find it an issue a lot of the time at that FOV, but the sort of targets in view like wide clusters that fill the FOV, I can see the MV is a better eyepiece in that regard.

[Andy_E]

Coma is a scope aberration. I guess you mean astigmatism ?

 yes...doh!. Just been shopping for a coma corrector so have it on the brain

[astronymonkey]

Watch for exit pupil too. At f4 a 5mm exit pupil is at 20mm eyepiece and 6mm at 24mm...so I wouldnt go above 24mm max or youll just loose light...one of the compromises of a fast scope im afraid.

cheers

[Andy_E]

I meant to add.

Coma is not as big a deal sometimes and astigmatism perhaps overlooked, but here will come a point where it will be annoying  in wide FOV eyepiece and faster scope, a cheap eyepiece will suffer form both.

The only eyepieces I can talk about from experience is the MV I own and a 25 degree BST, they both show coma, BUT, the MV is more pleasing to look through on the outer edges, because it is mainly coma that makes the outer edges become less well defined, whereas in the BST astigmatism is the more noticeable aberration in comparison. I do not find it an issue a lot of the time at that FOV, but the sort of targets in view like wide clusters that fill the FOV, I can see the MV is a better eyepiece in that regard.

Sorry %*(^$$ up so I did (see above post).

[John]

I meant to add.

Coma is not as big a deal sometimes and astigmatism perhaps overlooked, but here will come a point where it will be annoying  in wide FOV eyepiece and faster scope, a cheap eyepiece will suffer form both.

The only eyepieces I can talk about from experience is the MV I own and a 25 degree BST, they both show coma, BUT, the MV is more pleasing to look through on the outer edges, because it is mainly coma that makes the outer edges become less well defined, whereas in the BST astigmatism is the more noticeable aberration in comparison. I do not find it an issue a lot of the time at that FOV, but the sort of targets in view like wide clusters that fill the FOV, I can see the MV is a better eyepiece in that regard.

You will notice coma and astigmatism more if you use eyepieces with even wider fields of view as these aberrations increase in severity as you move away from the optical axis of the scope. 

[mal-j]

Thanks guys.

I have just bought a paracorr coma corrector from telescope house as they have them at a good price.

Hopefully this will help in most of the issues raised if not the balancing one!

[AlexB67]

You will notice coma and astigmatism more if you use eyepieces with even wider fields of view as these aberrations increase in severity as you move away from the optical axis of the scope. 

Yes, for that reason I am reluctant to go even to anything bigger than 60 - 70 degrees eyepiece in my scope, without using a CC I think I would get annoyed with it. If I did buy something like an 82 degrees myself I'll be saving for that, so I can  invest in a decent CC at the same time, to stop the astigmatism at that sort of angle. I can only suspect it is going to be costly .  It will need some thought when the time comes, but in no rush for the those ultra wide views right now and hopefully get to try a few before  I buy anyway.

[John]

Yes, for that reason I am reluctant to go even to anything bigger than 60 - 70 degrees eyepiece in my scope, without using a CC I think I would get annoyed with it. If I did buy something like an 82 degrees myself I'll be saving for that, so I can  invest in a decent CC at the same time, to stop the astigmatism at that sort of angle. I can only suspect it is going to be costly .  It will need some thought when the time comes, but in no rush for the those ultra wide views right now and hopefully get to try a few before  I buy anyway.

I went the other way and found a newt with a F/5.3 focal ratio. Coma is much reduced at that ratio and only shows itself right out by the field stop of the 100 degree eyepieces. Added bonuses are that accurate collimation is easier to get and maintain and the proportionately smaller secondary obstruction helps with contrast. I wish the chinese 10" dobs were available at F/5.6 or similar - F/4.7 does pose a number of challenges.

[ollypenrice]

In our 20 inch F4 of strictly average optical quality the default EP is a 2 inch 26 Nagler. The TV quality cleans up the edge of field. We don't bother with a CC. The wide FOV is so relaxing, though. It's a nice view and no mistake.

Olly

[AlexB67]

I went the other way and found a newt with a F/5.3 focal ratio. Coma is much reduced at that ratio and only shows itself right out by the field stop of the 100 degree eyepieces. Added bonuses are that accurate collimation is easier to get and maintain and the proportionately smaller secondary obstruction helps with contrast. I wish the chinese 10" dobs were available at F/5.6 or similar - F/4.7 does pose a number of challenges.

I did always ask for that magic f/6 or so 10 inch Dob  when I was considering the ideal scope for me, I remember pleading on here and asking why not make an SW in that f ratio. In an ideal world I would have bought a VX10L , but it was just not in my price range, and I needed the compactness of the flexi in any case.  For the kind of FOVs I am using I am  happy with that for the foreseeable future.

In a few years who knows, I may well rethink the DOB as well and go for one of the OO optics with a bit more civilised f ratio, and then the very big TV wide angle Whoppers alongside, but seeing the latter are so costly, if you can afford that you can probably afford the OO Dob    I am not at the stage yet where I feel I need to be thinking Big TVs wide angles, and OO dobs to satisfy my appetite, only observing for 7 months or so, instead of 30 years or whatever number years of under the belt. 

Of related interest, to gain an advantage in another way,  It has been more in the back of my mind ( and the missus too) to buy a house more out of the city and save for better skies, perhaps in the next 5 years or so is our plan, a little bit more country and garden we both like. 

It is amazing the difference in skies you can get where I am with a 15 - 20 drive east where we live now.  Only last Saturday was another good reminder of what my little 5 inch can show versus my 10 inch at home, in what weren't even very good conditions.  

[John]

......., only observing for 7 months or so, instead of 30 years or whatever number years of under the belt. 

......It is amazing the difference in skies you can get where I am with a 15 - 20 drive east where we live now.  Only last Saturday was another good reminder of what my little 5 inch can show versus my 10 inch at home, in what weren't even very good conditions.  

Both good points 

You have far better equipment than I had when I'd been in the hobby less than a year. It took me around 5 years to save enough for a 6" newtonian - things are so much less expensive now, which is great. 

On viewing under dark skies, I very much agree. I took a 6" to the SGL star party a few years ago and that showed DSO's somewhat better than the 10" would from my back yard.

You are right - there is no rush. The Universe isn't going anywhere soon