ES 20 100 deg field stop position

[Scooot]

I've been trying to work out the correct spacing for my GSO coma corrector for when this eyepiece arrives and need to know the position of the field stop in relation to the shoulder/reference point of the eyepiece.

I contacted explore scientific and they told me it was 15.2 mm below the shoulder. I've read on another forum that the paracorr setting for this eyepiece is the same as the 21mm ethos, which implies to me that the field stop positions should be similar. The position of the ethos field stop is about 9mm above the shoulder ie a 24mm difference. So I'm wondering whether I've been given the wrong info by explore scientific and they meant 15.2mm above the shoulder.

can anyone offer any guidance on this.

[crashtestdummy]

Hoping to try the same eyepiece in my paracorr tonight so would be able to tell you difference between a 30mm es 82 and the 20mm 100° if that helps?

[Scooot]

Yes that would be great. I've managed to get through to them on their support chat line. I think they have made a mistake. They're now telling me its -15.2mm but a minus number means above the shoulder on televues site. Should know in a minute. There servers been down all evening, that's why I thought I'd ask here.

[John]

This is not exact but I've just compared the ES 20 / 100 to the Nagler 31 in my ED120 refractor and the ES is very close to being parfocal with the Nagler. I'd say the exact focal plane of the ES requires just a mm or so of outward focus travel over the Nagler - just a quarter turn of the 1/8 speed knob.

If you used the Paracorr settings for the 31mm Nagler you would be pretty close I'd think.

Hope that helps a bit.

[Scooot]

Thanks John, the nagler field stop is in exactly the same position as the 21mm ethos so that makes perfect sense. They're going to send me an email with a diagram but I'm fairly sure it's above the shoulder now.

incidentally, I saw on the other thread you balance your OTA with a 2pound weight. What do u attach it with and where if u don't mind me asking.

[John]

incidentally, I saw on the other thread you balance your OTA with a 2pound weight. What do u attach it with and where if u don't mind me asking....

I bought a cheap magnetic knife rack on e.bay for around £6 and fastened it to the tube with super strong double sided pads. The magnetic strips on the knife rack are not quite strong enough to stop a 1lb or 2lb iron kitchen weight from sliding when the scope is pointing above around 40 degrees so I stuck a couple of thin 20mm bolts through the strip at the bottom and 2/3rds of the way up - these stop the weights from sliding and they stick quite firmly in place in those two positions. The combinations of those two points and 1lb and 2lb weights give me enough options to cover all my eyepieces. The lighter eyepieces need no counter weight at all. It's quick and easy to keep the counterweights by the scope and add / subtract them as needed.

I ought to give Damo the credit for this idea - I nicked it from him

Here is a hastily snapped pic with the 1lb weight in the upper position:

[Scooot]

I ought to give Damo the credit for this idea - I nicked it from him

ingenius, thanks to you both

[Scooot]

Well this is the diagram explore scientific helpline sent me. (They have been trying to be very helpful I must say).

So according to this diagram the field stop is bang on the eyepiece shoulder or reference surface, not above or below, and there has indeed been some confusion. Hmm I'm still not convinced they've told me correctly but if its near there it shouldn't be too much of a problem.

pity they don't publish the info like televue!