Pros: Longer eye relief than 5mm reticles, Inexpensive
Cons: None
The Bottom Line: The Orion 12.5mm plossl reticle is an effective way to get a mount aligned. It works for eyeglass wearers, and is economical, what more do you want?
Pirich's Full Review: Orion 12.5mm Illuminated Plossl Reticle (08450)
The Orion 12.5mm Illuminated Plossl Reticle is an accessory designed for precisely aligning astronomical telescopes on distant objects. This is allows an observer to very precisely align an equatorial mount, or for astrophotography, watch a specific star to correct the motion of the telescope on long exposures. The Orion 12.5mm Plossl illuminated reticule does this with a bit more eye relief than most. If you wear glasses, this is the reticule you are looking for.
Background
The reticule eyepiece is the least visible piece of important astronomy equipment there is. It is the part, which never seen, makes beautiful space images possible. Even if a large telescope is on a high-end computer guided mount, it was someone patiently aligning that mount to the Earth's axis with a reticule eyepiece who put that show on the road.
A reticule eyepiece is designed to project a set of double cross hairs in the same focus as the image. To make this possible, the eyepiece has a separate focus adjustment on top just for where the cross hairs will be in focus. So, after the image is in focus, you focus the cross hairs so they show up in the image. The cross hairs can be lit with an illuminator, which in the case of the Orion 12.5mm reticule is a variable brightness red LED. Thus far, I have found the dimmest setting when it comes on to be plenty bright.
Whether you are trying to do photography, or just want your mount to be able to track well for a while, the key to getting started is getting the mount's right ascension axis accurately aligned to be parallel with Earth's axis. Now, with that said, there are two major techniques for getting a polar alignment.
The first method is popular for German Equatorial Mounts, the ones with a counterweight arm on one side, is to mount a small telescope through the hub of the mount to be able to look and find the north star. Unfortunately, the North Star, Polaris, is actually over a degree away from true North, so while it is useful for getting your bearings on a hike, the fact this star moves around a circle 2 degrees across means a polar alignment on it will be approximate, at best. A version of this is performed by fork mounted telescopes such as the http://www99.epinions.com/content_107837886084, where the entire telescope will point to where it believes Polaris is after you align it, and you can adjust it to be a little better centered. And, since there is no Southern Hemisphere equivalent of the North Star, this method is even less useful to our friends in Australia, South Africa, Chile, and other locations on the other side of the Equator.
The second method is called drift alignment, and it uses a reticule eyepiece. The best instructions I have found for this process come from Starizona, one of the best astronomy shops there are. (yes, period): http://starizona.com/acb/basics/using_polar.aspx. The main concept with this approach is to use the mount's circle of rotation to get a precise alignment with the sky. So, for example, if you set up the tripod where the hub is pointed a little to the East of true north, then a star near the zenith (straight overhead), will seem to drop in the eyepiece of the mount since this error causes the scope to track so it will wind up North of where the star is by the time it gets to the horizon. If you have a reticule eyepiece, it puts a set of double cross hairs in the eyepiece so very small movements of a star seen in the image are obvious. The benefit of the Starizona instructions is they let you get the drift alignment in the minimum of time; about thirty minutes. This may sound like a long time, but it is actually faster than other versions of the technique. One pitfall of drift alignments done where the star is placed between the lines instead of on one is your perception may not be perfect due to a star showing a bit of glare due to poor seeing, and as a result you end up spending hours trying to figure out why the drift alignment keeps being slightly wrong (this is horribly frustrating).
The last part of this alignment is to get the latitude setting accurate, and you can either do this with a mount with an adjustment screw (the easiest), manually adjust a wedge (argh!), or adjust the leg on the tripod on that side (OK for light scopes).
Having done this, most telescope mounts will quite accurate, and will leave the scene centered in the field of view for long periods of time. The current crop of telescope mounts are surprisingly accurate, in general, as they have been upgraded with modern digital control for their drives. Once you have a mount accurately polar aligned, you will likely want to leave it that way as long as possible, so this can lead to a strong desire to put up your own little observatory.
Description
The Orion 12.5mm Illuminated Plossl Reticle is about the size of most Plossl eyepieces with the illuminator, which is about the size of a cap from a fountain pen, attached to the side. The construction of the eyepiece has this as a screw-in accessory, and there are non-battery illuminators which can run off of a separate power supply out there. The two parts come separate in the box, and the illuminator must be attached separately.
The whole top of the eyepiece is the focuser for the reticule, and when I had it focused for myself, has a standoff from the main eyepiece body of about 1/8 inch. The eyepiece cap will go on with the illuminator attached, but not all the way to snug, since the illuminator is very close to the top. In Arizona, I expect removing the illuminator on a regular basis will result in a lot of dust getting into the eyepiece, I leave it attached and have been living with the partial fit of the top cap.
The cross hairs appear black when the telescope using the eyepiece is pointed at a light background with the illuminator off. At night, they are nearly invisible in this mode. The illuminator switch clicks to a low brightness position (it is visible at the lowest brightness) and gets brighter as the knob is advanced. The reticule lines themselves appear thicker when lit, and if slightly out of focus they have a bright red flare on either side of them.
Usage
The 12.5mm prescription is a bit different from most illuminated reticules, which are 5 mm or 6 mm focal length, so they have approximately double the magnification of this one. Since I have started wearing glasses, though, eyepieces below 10 mm in focal length have begun to lose most of their usefulness for me since I have to use them without glasses, and with the full effects of astigmatism. So, the longer eye relief of the 12.5mm eyepiece, where the image is projected well away from the eyepiece, makes this a lot more useful to eyeglass wearers. It is true more tracking accuracy is visible at higher magnification; I suggest a barlow lens. If you are doing photography with a digital SLR, the camera will have a focal length of 35 mm, so the accuracy of the alignment with the reticule will, if you do a 5 minute track as recommended in the Starizona instructions, the alignment without a barlow will be good enough for an exposure up to about 2 minutes.
This comes to the second use for this eyepiece, tracking for astronomical photos with long exposures. After a good polar alignment is achieved, the performance of a telescope mount will not be perfect for very long exposures, so guiding is needed. The concept here is to have either a second telescope called a guide scope (best), or an off axis guider (not always possible) to put up a reference image. The photographer then watches this and gives the mount's drive controller manual commands to keep a reference star on an intersection of the cross hairs (hey, if you're doing this, my hat is off to you). With the advent of digital photography, the need to guide very long exposures has been reduced by the ability to combine exposures taken with digital cameras.
The image in the 12.5 mm plossl part of this assembly is of OK quality- it is only 40 degrees apparent field of view, and the sharpness is good enough for guiding, but if this were an eyepiece without the reticule, it would probably stay in the parts box. With that said, the image is flat, and moving a star from side by moving the telescope will keep it on a reticule line, so it meets the criteria needed to be useful as a guiding eyepiece. The eye relief is just enough to be able to use the eyepiece with glasses on, and this is a definite improvement over versions with shorter focal lengths.
Conclusions
The Orion 12.5mm Illuminated Plossl Reticle is an effective way to deal with the dirty job of getting a telescope mount aligned. Given the cut-and-dried nature of the task, it is hard to justify spending $270 for a Takahashi long eye relief reticule when this one will do the job. In that light, the Orion 12.5 mm plossl is a solid value since it does get the job done as surely as any other model would. Do I love doing polar alignments? No, nothing is going to change that, but the Orion 12.5mm Illuminated Plossl Reticle makes it a straightforward task you only have to do once in the evening.
Great for accurate polar alignment, the 4-element Plossl design is also well suited for off-axis guiders, where light gathering is minimal. Fully mult...More at Orion Telescopes & Binocu
This 4-element Plossl is well suited for off-axis guiders where light gathering is minimal Great for accurate polar alignment Fully multi-coated lense...More at Amazon Marketplace
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