Which eyepiece for the Quark?

[Stu]

Having used a few different eyepieces yesterday with the Quark, I thought I would post up a few brief findings. Anyone else please feel free to add your own experiences so as to build up a picture of what works best.

I've heard that the best contrast is obtained in the Quark with an eyepiece twice the focal length of the system in mm ie if you are using a quark in an f5 scope, you are at f21 overall including the Barlow effect so a 42mm eyepiece would give best results. I'm unsure of the reasoning behind this but it's what I've been told.

Given that my scopes range from f5.9 to f9, the implication is that a 40mm eyepiece or longer would be preferable.

Using the Quark information as guidance, I have stuck to simple eyepiece designs so have a 25mm Ortho and 25, 32 and 40mm TV Plossls. According to the TV website, the Plossls should have field stops of 21.2, 27 & 27mm respectively.

I have not measured the ortho yet but will add this later. I didn't have the 25mm Plossl with me either but this will be another comparison to make with the ortho.

First things first then. The Quark has a clear aperture of 21mm I believe (although I've seen mention of 25mm?). What I found was that with every eyepiece used, I hit the edge of the etalon aperture before I hit the field stop of the eyepiece ie the image disappeared behind a slightly defocussed circle inside the darker and sharper field stop of the eyepiece. This means that the additional field of view of the longer eyepieces is academic, the Quark is the limiting factor.

The reason for using longer focal length eyepieces then is purely for the magnification/image scale achieved and the associated sharpness and contrast. I found the 40mm slightly claustrophobic in that the fov was closed down the most out of the three, and only having a 43 degree afov in the first place it was a little like looking down a straw.

The 32mm felt more 'open', with less of an intrusion from the etalon, and combined with the wider afov was very nice to view through. Full disk was easy with this combination, and the small proms on show were nice and bright.

The 25mm Ortho was better still in terms of the fov. The etalon only intruded right towards the edge of the fov. Full disk was again possible but to properly see the proms required re-positioning of the sun in the fov.

In yesterday's conditions, with the 60mm scope the 32mm worked best I felt. There was nice detail present in all, and the higher power of the 25mm I can see will be useful in good conditions and with the sun higher in the sky, but the 32mm gave the best contrast, sharpness and field of view overall.

I will re-visit this again with different scopes, but I guess the fundamentals won't change ie the relationship between eyepiece field stop and etalon field stop.

Cheers,

Stu

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[Peter Drew]

Our experiments with a Quark on an Equinox 80mm and binoviewers resulted in 32mm eyepieces giving the best overall view. 

[Stu]

Thanks Peter, seems like we agree on that one then :-)

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[Moonshane]

hi Stu

I cannot really add much about the quark but it seems that full disk (or maybe half disk?) is what's likely to be achieved with a quark? I presume the larger the aperture, the more detail is visible even though the magnification is likely to be about the same?

Having just used my PST for the first time I was really impressed with the level of detail visible - actually quite surprised even at full disk with the 11mm TV plossl at 36x and 12.5mm BGO with barlow screwed on at 51x. Assuming that I get the same mags at 1000mm FL with the TAL mod (using 32mm plossl and 20mm plossl) then I am hopeful we'll be on a par and still getting full disk even with the 5mm original BF.

[Luke]

Great write-up, Stu.

My fave eyepiece overall with the Quark, using 60mm, 85mm, 100mm and 120mm scopes, by some margin, is...

... 32mm Plossl

[Stu]

hi Stu

I cannot really add much about the quark but it seems that full disk (or maybe half disk?) is what's likely to be achieved with a quark? I presume the larger the aperture, the more detail is visible even though the magnification is likely to be about the same?

Having just used my PST for the first time I was really impressed with the level of detail visible - actually quite surprised even at full disk with the 11mm TV plossl at 36x and 12.5mm BGO with barlow screwed on at 51x. Assuming that I get the same mags at 1000mm FL with the TAL mod (using 32mm plossl and 20mm plossl) then I am hopeful we'll be on a par and still getting full disk even with the 5mm original BF.

Glad the PST is living up to expectations Shane. I do think 'good ones' give excellent images, particularly given such a small aperture.

I used to find the 11m Plossl was ideal most of the time. The 12.5mm was nice but a little low (better in the wedge), I occasionally used 9 or 7mm BGO's if seeing was good.

The advantage you will have with your PST mod is full disk views at 100 (or 90mm with the D-ERF you have) aperture giving the benefits of the resolution even on the lower mags. You will then be able to zoom in to higher power for the detail.

With the Quark you are pretty limited in the eyepiece choice, so the field of view is dictated by the scope focal length. It's almost like you need to change scope with a Quark rather than the eyepiece!

Full disk is possible in the 60mm, but given the x4.2 Barlow, the focal lengths of the system are always longer than your TAL. The FS60 is at 1491mm and the SW120ED at 3780mm so the latter is all about high power views of a section of the disk. 32mm gives x47 in the 60 and x118 in the 120.

It's the same old story really, no scope is ideal. The Quark suits me though with my scopes and lack of time/facilities to put together a PST mod. It is also great for portable/travel use as I posted elsewhere. The mod will do a great job for you though I'm sure.

Stu

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[Moonshane]

cheers Stu

I keep forgetting about the extra long focal lengths created with the barlow.

[Davey-T]

Got TV 32mm plossl yesterday, was going to get 40mm but they didn't have one, haven't tried it in 100mm f/6  yet as it hasn't stopped raining since

Dave

[Peter Drew]

Shane. I'm afraid you won't be able to get a full disc solar image with 100mm FL and a 5mm BF, you will need a 10mm BF to do this. It is the size of the BF that determines the field, not the eyepiece. The 5mm BF vignets the field but not the aperture fortunately provided that the donor scope is not less than F10. This is hardly any different than viewing the Moon at enough magnification such that the full lunar disc is not available.    

[Moonshane]

sorry for veering off topic Stu but hopefully as it is kind of related you don't mind. Peter I was assuming that with the same magnification and the same afov the true field visible would be the same. It is not really a problem.

[John]

Although I've no immediate plans to get into solar h-alpha viewing it's a subject that I like to evesdrop on

........ It's almost like you need to change scope with a Quark rather than the eyepiece!

 

With the Quark costing around £800 and achromatic refractors between 70mm and 120mm being available for £70 - £150 apiece on the used market I wonder if Stu has hit the nail on the head with this comment. Maybe the Quark requires a re-think on the eyepiece / scope relationship ?

[Stu]

sorry for veering off topic Stu but hopefully as it is kind of related you don't mind. Peter I was assuming that with the same magnification and the same afov the true field visible would be the same. It is not really a problem.

No probs Shane, it's all interesting stuff :-)

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[spaceboy]

I have no doubt getting the combination of eyepiece and scope right with the quark would offer some unbeatable views but having recently read that ED or APO are best suited to the job I doubt a cheap achro would ever show the true potential of the quark so anyone considering buying a quark would have to be friendly with their bank manager. Not to mention the recommended eyepieces for use in the quark of Televue plossl which again aren't the cheapest even used.

[Peter Drew]

There have been some excellent Quark images posted on Solarchat using standard short focus achros. As far as eyepieces are concerned, as the FOV of the Quark is dependent more on the blocking filter size then Plossls are adequate, doesn't have to have green writing on them but nice if you can afford them. 

[spaceboy]

I agree Peter but I was thinking more visual requiring ED or Apo as imaging is often CCD and processing to get good images where the MK1 eyeball is you get what you get. Although a totally different set up I have noticed going from my better corrected and slower 80mm f/6 and lunt wedge to a 102 f/5 that while the views appear more steady the edge performance is notably softer. This I would also assume to be even more obvious if using a quark as the solar limb will be right on the edge of view. Would it not stand that better optics and colour correction would mean less of a notable impact of aberrations to the view ? I know from having several achromatics that there is a slight loss of contrast on the planets due to colour fringing.

[John]

I didn't think CA was an issue with H-Alpha viewing ?

A decently well figured objective would be better though.

[Stu]

I didn't think CA was an issue with H-Alpha viewing ?

A decently well figured objective would be better though.

That's my understanding too John. The benefit of an apo should only be if it is generally better figured than a cheaper achro.

The other point which is I mentioned somewhere else is that Ha is obviously red end of the spectrum and achro scopes are often optimised for the green so their performance in HA is not as good.

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[spaceboy]

The other point which is I mentioned somewhere else is that Ha is obviously red end of the spectrum and achro scopes are often optimised for the green so their performance in HA is not as good.

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Sorry this is what I meant. Basically what Stu said. Forgive me I'm not totally clued up on the whole aberration terms of things.