Wide field of view 2" eyepiece upgrade, 8" dob.

[LaurenceT]

Now there is slightly more darkness I've dug out my stock 2" eyepiece that came with the 200p Dob and I've been truly amazed by the view offered. Up till now I've been using my Baader zoom but the 28mm 2" eyepiece is a different experience. I am however conscious of it's limitations, star sharpness falls off very quickly moving away from the centre so I'm looking for an effective upgrade.

As ever there are financial constraints so I'd be looking to buy used with a budget of around £70, any comments gratefully received.

[Zermelo]

This discussion here has some relevant comments:

 

[Louis D]

9 hours ago, LaurenceT said:

As ever there are financial constraints so I'd be looking to buy used with a budget of around £70, any comments gratefully received.

There's not much in that price range, new or used, that will vastly improve the view in the outer reaches of the FOV at f/6.

If you doubled your budget, I'd recommend the 40mm Lacerta ED as a decent step up that gets you pretty close to 40mm Pentax XW level correction.  At 30mm, the APM UFF and it's other brandings is exceptional while not being TV priced.

You'll probably want to invest in a coma corrector at some point to clean up the edges and flatten the field a bit once you get eyepiece astigmatism under control (but not before).  The GSO CC is probably the best bang for the buck out there.  It's sold under many different brand names worldwide.

[LaurenceT]

21 minutes ago, Louis D said:

There's not much in that price range, new or used, that will vastly improve the view in the outer reaches of the FOV at f/6.

If you doubled your budget, I'd recommend the 40mm Lacerta ED as a decent step up that gets you pretty close to 40mm Pentax XW level correction.  At 30mm, the APM UFF and it's other brandings is exceptional while not being TV priced.

You'll probably want to invest in a coma corrector at some point to clean up the edges and flatten the field a bit once you get eyepiece astigmatism under control (but not before).  The GSO CC is probably the best bang for the buck out there.  It's sold under many different brand names worldwide.

Thanks, increasing the budget sufficiently is unlikely given the current economic situation.

Interesting about the coma corrector, I hadn't thought about that, I have a Baader coma corrector lurking in a drawer. I'll have a play with that.

[bomberbaz]

Hi Lawrence, have you looked at the Kitakaru eyepiece.

StellaLyra 30mm 2" KITAKARU RPL Eyepiece | First Light Optics

Now it might not have a fantastic FOV at 62 degree but I have not heard anything bad about these.

The reviews I found refer to extremely flat and colour free results at the eyepiece. And your DOB at F6 will be a perfect match giving 1.5 degrees at mag 40. Slightly over budget but you might struggle getting one 2nd hand. 

There is also the superview eyepiece also by stella lyra but I believe this is a copy of the GSO although should still work well in an F6 and better still, under budget. However I am not able to offer any more info about it but might be worth pursuing further.

StellaLyra 30mm 2" SuperView Eyepiece | First Light Optics

I would like and may yet get a Kitakaru 18mm for the DOB for use on smaller targets. I love my 102 eyepiece but sometimes smaller objects look better framed in a smaller fov. 

[LaurenceT]

11 hours ago, bomberbaz said:

Hi Lawrence, have you looked at the Kitakaru eyepiece.

StellaLyra 30mm 2" KITAKARU RPL Eyepiece | First Light Optics

Now it might not have a fantastic FOV at 62 degree but I have not heard anything bad about these.

The reviews I found refer to extremely flat and colour free results at the eyepiece. And your DOB at F6 will be a perfect match giving 1.5 degrees at mag 40. Slightly over budget but you might struggle getting one 2nd hand. 

There is also the superview eyepiece also by stella lyra but I believe this is a copy of the GSO although should still work well in an F6 and better still, under budget. However I am not able to offer any more info about it but might be worth pursuing further.

StellaLyra 30mm 2" SuperView Eyepiece | First Light Optics

I would like and may yet get a Kitakaru 18mm for the DOB for use on smaller targets. I love my 102 eyepiece but sometimes smaller objects look better framed in a smaller fov. 

Thanks for that Steve, I'll have a look at them. It seems that the real issue here is will I get enough improvement over the stock Skywatcher 28mm 2" EP by buying a sub £100 EP to justify the expenditure.

[Louis D]

6 hours ago, LaurenceT said:

Thanks for that Steve, I'll have a look at them. It seems that the real issue here is will I get enough improvement over the stock Skywatcher 28mm 2" EP by buying a sub £100 EP to justify the expenditure.

Probably any 4 element or more eyepiece would be an improvement over the SW 28mm three element eyepiece.  It's probably a Kellner or Konig variant, so not very good at f/6 or at 56° (28° half angle) AFOV.  Notice how spread out the dot plots get at just 10° off axis for simple designs at f/5:

However, notice that the Erfle type, even with 5 elements, is far from perfect.

In my experience, at f/6, most of the superwide eyepieces under $100 are probably sharp to about 50% to the edge and then start to degrade slowly until at around 75% out where sharpness rapidly declines to the edge.  If you only look on axis, they'll probably perform satisfactorily.  If you tend to let objects drift from edge to edge and watch them the entire way, you won't be pleased.

[ED Splitter]

14 minutes ago, Louis D said:

Probably any 4 element or more eyepiece would be an improvement over the SW 28mm three element eyepiece.  It's probably a Kellner or Konig variant, so not very good at f/6 or at 56° (28° half angle) AFOV.  Notice how spread out the dot plots get at just 10° off axis for simple designs at f/5:

However, notice that the Erfle type, even with 5 elements, is far from perfect.

In my experience, at f/6, most of the superwide eyepieces under $100 are probably sharp to about 50% to the edge and then start to degrade slowly until at around 75% out where sharpness rapidly declines to the edge.  If you only look on axis, they'll probably perform satisfactorily.  If you tend to let objects drift from edge to edge and watch them the entire way, you won't be pleased.

I have the same eyepiece as the OP. Is this lack of field sharpness the reason why very bright stars develop a single a spike unless looked at perfectly on axis? 

[Louis D]

45 minutes ago, ED Splitter said:

I have the same eyepiece as the OP. Is this lack of field sharpness the reason why very bright stars develop a single a spike unless looked at perfectly on axis? 

 To my knowledge, the only cause of a single spike in a Dob/Newt would be if the secondary is held by a stalk rather than a spider.

If you were using a correct image diagonal in a refractor or cat, the Amici prism can also introduce a single spike on bright stars.

You're not referring to a lobe pointing away from the center of the FOV are you?  That could be coma, astigmatism, or lateral chromatic aberration.

Edited August 10, 2022 by Louis D

[ED Splitter]

1 hour ago, Louis D said:

 To my knowledge, the only cause of a single spike in a Dob/Newt would be if the secondary is held by a stalk rather than a spider.

If you were using a correct image diagonal in a refractor or cat, the Amici prism can also introduce a single spike on bright stars.

You're not referring to a lobe pointing away from the center of the FOV are you?  That could be coma, astigmatism, or lateral chromatic aberration.

I should have clarified. I am using a refractor. Depending on head/eye position the spike protrudes from the centre of the star and can move around the centre of the star much like a second hand around a watch face. So if I move my my eye off axis of a bright star the spike moves around the star if that makes sense. This only happens on bright stars. Perhaps I need to make some more observations and start a new thread. 

[Don Pensack]

It sounds a lot like astigmatism.

Cause #1, if it rotates in angle with your head, is you own eye.

If it goes away at high powers, but is worst at low power, that is a good sign it is in your eye.

Cause #2, if it doesn't rotate with the head , is in the optics of the scope.

That could be the star diagonal or the objective.  You can eliminate the star diagonal by using/borrowing another and seeing whether the problem goes away.

If it doesn't, it is likely in the objective.

I have seen tons of small refractors (50-90mm) with serious astigmatism in the lenses, so that would not be surprising.

[bomberbaz]

11 hours ago, LaurenceT said:

Thanks for that Steve, I'll have a look at them. It seems that the real issue here is will I get enough improvement over the stock Skywatcher 28mm 2" EP by buying a sub £100 EP to justify the expenditure.

Depends how you define improvement Lawrence, for some it is greater fov, better eye relief, better experience all round etc.

From what i have read the Kitakaru RPL (Really perfect lens) will give you a colour free, flat view across the full fov of the eyepiece if reports are true (and I have no reason to disbelieve them). It may be 62 degrees, but I would rather have a near perfect 62 degrees than a shabby 82 if you get me. 

[Louis D]

4 hours ago, ED Splitter said:

I should have clarified. I am using a refractor. Depending on head/eye position the spike protrudes from the centre of the star and can move around the centre of the star much like a second hand around a watch face. So if I move my my eye off axis of a bright star the spike moves around the star if that makes sense. This only happens on bright stars. Perhaps I need to make some more observations and start a new thread. 

So, the spike spins around the star on the side opposite to wherever your moved your eye?  It sounds a bit like a reflection somewhere.  I've had bright planets have ghost images in some eyepieces that I can chase around the view in this same manner.  It's just weird that it extends as a spike rather than as a dimmer point of light.  Look down your eyepiece, diagonal, and focuser tube in daylight to look for shiny surfaces that may need flocking or at least blackening.

[ED Splitter]

1 hour ago, Louis D said:

So, the spike spins around the star on the side opposite to wherever your moved your eye?  It sounds a bit like a reflection somewhere.  I've had bright planets have ghost images in some eyepieces that I can chase around the view in this same manner.  It's just weird that it extends as a spike rather than as a dimmer point of light.  Look down your eyepiece, diagonal, and focuser tube in daylight to look for shiny surfaces that may need flocking or at least blackening.

Many thanks for the tip. I thought it was the cheap eyepiece but it could be a number of things. I will investigate further. Mthanks again. 

[Stu]

10 hours ago, ED Splitter said:

Many thanks for the tip. I thought it was the cheap eyepiece but it could be a number of things. I will investigate further. Mthanks again. 

Did you confirm whether you are using an erecting prism (amici) or a mirror diagonal? The Amici prisms can cause a spike due to the meeting of the prisms in the centre of the field of view. Fine for daylight viewing but can cause problems for astro.

[ED Splitter]

4 hours ago, Stu said:

Did you confirm whether you are using an erecting prism (amici) or a mirror diagonal? The Amici prisms can cause a spike due to the meeting of the prisms in the centre of the field of view. Fine for daylight viewing but can cause problems for astro.

It’s the standard dielectric mirror diagonal that comes with the skywatcher 100ed. Didn’t get chance to view last night as it was a bit murky and the moon was so bright.