Telescope Reviews: Sterling plossl in binoviewer

I don't own the Sterlings, and don't know that much about how different they are but it would seem that they can't be all that much different.

The Glass Path Correctors, OCSs, and other devices that boost magnification tend to all give higher or lower magnifications based on where the focal plane is with respect to the focal plane location for the stated magnification.

For "Shorty" barlow lenses, the magnification factor can vary considerably if the field stop of one eyepeice is in a much different location that the field stop of another eyepiece because as a percentage of the focal lenght of the Barlow, moving the field stop of the eyepiece makes a big change.

For example, if a Shorty Barlow is designed to give 2x when the field stop of the eyepiece is maybe 50mm away, then changing to an eyepeice with a field stop that is +/- 10mm from that distance, as a percenage of the focal lenght of the Barlow, that is 20%.

GPCs though are like the old time barlow lenses with the very very long barrels (which no one sells anymore). Because they were designed to work with binoviewers, the design is set up to work at the stated magnification when the field stop is way far away, like up in the eyepiece holder of the binoviewer.

This means that they are set up to give optimal magnification for a light path of about 120mm

Now doing the same math, if you had a field stop in this case that was +/- 10mm in difference, you would only be variying the distance by about 5% of the amount as when those same eyepcies were used in a Shorty barlow.

So, the power changes, but it does not change by much just because of the field stop location change.

And the magnification mught actually go down.

When you move the field stop away, the magnification goes up. When you move it closer, it goes down.

For eyepeices where teh field stop is in the eyepeice housing, the field stop lies outside of the eyepiece barrel holder (diopter, whatever). This would give slightly higher magnification factors.

Most long focal lenght plossls though have field stops that are in the barrel of the eyepeice. This actually puts them down inside the eyepeice holder.

For example, most 32mm Plossls either have field stops right down at the end of the barrel, or they use the end of the barrel itself for a field stop. This means that they focal plane is going to be just outside of the prism housing deep inside the eyepeice holder, which will give the lowest possible magnification factor.

So, find out where the field stop is for the eyepeice you are considering. if it is in the barrel, it likely will work at a bit less than for an eyepeice where it is in the housing.

My guess is the the GPCs are supposed to reach stated magnification when the field stop is even with the top of the eyepiece holder. Some eyepices will have field stops down inside the holder, and others will have field stops just above the end.

The variation in power will generally be very minimal because most eyepeices will have field stops that fall vary close to this range.

And even if it is 10mm down inside the barrel, givin that the GPC is likely giving stated power when the field stop is 120mm from the GPC, having a field stop 10mm closer will only give a small change, and in fact will lower the magnification.

But 10mm one way or the other is not going to make much of a difference in a refractor.

So find out where the field stop is and estimate if from that figure. A few millimeters will not make any differnce.