AndresEsteban

Right, Mike! 👍🤜🤛🔭 Your description of how to proceed is excellent! I just wanted to give some numbers and the general formula so our colleagues would understand why it's essential to put a reasonable distance between the laser and the wall, I'd like to use 10 to 20 meters. I forgot to mention narrowing the laser beam even more! 1st - Regulate the laser lens to have the point focused at the calibration distance. 2nd - Open the collimator and under the 45º diagonal, just in front of the laser exit (internal) glue an aluminum foil, press your finger on it so the central hole appears, and with a needle, punch a small hole at its center. This will reduce considerably the laser dot diameter, which is not a dot because the solid-state laser beam is generated at a rectilinear junction. The laser point should have an internal diaphragm, which has not. So we make one at the exit. This mod really improves the laser collimator performance making the dot smaller! Regards and clear skies for us all!🙏🔭 Andy

I agree with you, bosun21! A very important point you mention by the way! Indeed this is a problem with many drawtube's eyepiece holders...😢 Laser collimators and Cheshires will show this off-centering when we tighten the retention screw... A partial solution is to make the collimator body fit more tightly (a scotch tape around usually will do), being the recommendation not to tighten the screw when we achieve a tight fit using this method.

A well-collimated laser collimator when rotated on itself, will generate a 10 mm circle on a wall when at a distance of 30 feet (9.144 meters). Adopting this criterion it's clear that the collimator has a deviation angle from the centerline (circle's center) equal to alfa, where alfa = 1.8 arc-minutes. So: A well-collimated collimator must have a deviation < 1.8 arc-minutes. Let's simplify and assume a maximum tolerance of 2 arc-minutes of deviation for any laser collimator. In that case, the deviation r [mm] at a distance L [mm] will be: r [mm] = L [mm] x tan ( 2 arc-minute) = L [mm] x tan (1/30) = L [mm] / 1719 r [mm] = L [mm] / 1719 Therefore, on a collimator with alfa < 2 arc-minutes at 30 ft (9144 mm) from a wall, we are considering it generates a circle with a diameter < 10.6 mm. A 10 or 11-mm circle is quite small so we need to increase the distance by at least 20 meters to have a circle of 23 mm, which has enough room for the laser dot and will let us draw the circle easily on paper and center it to make the proper corrections! Regards Andy

Yup! Thank God! 👍🔭🙏 Knowledge knows no time constraints! For many people, these threads are quite important and elucidative! Let's keep them alive!😊👍

Thanks for the observation, Don! You're right! 👍👍 A prism diagonal provides more backfocus than a mirror diagonal. The focal plane is farther from the objective than in a mirror diagonal! Therefore, more in-travel distance is available at the drawtube! 🤜🤛🔭 Thanks again, Don! Text above corrected! 👍 Regards and clear skies for us all! Andy

Exactly! The prism "eats" less light path from the objective than a mirror, therefore the drawtube doesn't need to be racked in so much and there's still a portion of it outside the focuser. In other words, prismatic diagonals provide more backfocus than mirror diagonals. With a prism, the focal plane is farther from the objective than with a mirror, so more in-travel is available! (Thanks to Don Pensack for the feedback!👍🤜🤛🔭) Regards and clear skies for us all! Andy

Hi Louis! The schematic is correct and it shows what really happens! You may use the virtual image instead of the real one for practical purposes. Both mirror and prism diagonals advance the focal plane toward the objective. Mirror diagonals move it more forward than prisms. As OPM -the optical path of a mirror- is greater than OPP -the one of a prism- the new focal plane with a diagonal mirror will be closer to the objective than the prism. The drawtube goes further inside with a mirror diagonal than with a prism one. That's why more drawtube length is available when a prismatic diagonal is used! Remember that both diagonal types shorten the path length, but prisms shorten it less than mirrors. That's the reason to use prismatic diagonal when our drawtube is almost all racked in and has no room for further in-travel. Regards and clear skies for us all! Andy

Hi folks! IMHO, my understanding is that a prism diagonal has a shorter optical path OP than a the mirror diagonal. Let's call the optical path of the prismatic diagonal as OPP ~65 mm for a 1.25" one. Let's call the optical path of the mirror diagonal as OPM ~75 mm for a 1.25" one. Now let's consider our refractor with focal length FL used without a diagonal, pointed to an object far away with any eyepiece you'd like. The drawtube will be almost all racked out. Pick up a CD marker and make a line at the joint between the drawtube and the focuser body. Now measure with a ruler the distance between this line and the drawtube end (we will assume here lies the focal plane... OK?). Let's call this distance LDO = Length of Drawtube Out. This is how much the draw tube goes out to achieve focus. 1) Now put your prism diagonal with the same eyepiece and focus again. The drawtube needs to be racked in, in order to have the object focused again, right? 🤦‍♂️ Make a new mark on the drawtube and write "P" on it. Now measure the distance between the drawtube end and this line and let's call it DPD = Distance for Prismatic Diagonal. This is how much the drawtube goes out to achieve focus with a prismatic diagonal.😄 2) Now replace the prismatic diagonal and put the mirror one. Put again the same eyepiece on it, focus. We'll realize that now the drawtube needs to be RACKED IN a few millimeters more! Make the same line with the CD marker at the junction between the focuser body and the drawtube. Measure the distance between the drawtube end and this line. Let's call it DMD = Distance for Mirror Diagonal. This is how much the drawtube goes out to achiEve focus with a mirror diagonal.😄 3) Now we have three measures: LDO, DPD and DMD. And we've measured that DPD > DMD.👍 Ok, let's see now whats going on here 😎. Before using any diagonal, the focal plane was at LDO from the focuser body. Then, when we put the prism diagonal, the focal plane went near the objective, thus, we saw that the drawtube WENT IN and now is at a distance DPD from the focuser body. Therefore, the focal plane "travelled" LDO - DPD, this is the Optical Path for the prism: OPP = LDO - DPD😎 Again, but now with the mirror diagonal, we have the optical path for the mirror diagonal OPM = LPO - DMD.😎 Now let's crunch some numbers in the real world. I've used a vintage 60 mm FL=910 mm achro DAN BEAM (Circle K), of amazing qualty and optics, from the '60. Also 25 mm Vixen NPL plössl (36.3x). Values measured: LDO= 107 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter without star diagonal!!!) DPD = 41 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter with prism diagonal) DMD = 30 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter with mirror diagonal) So: OPP = LDO - DPD = 107 mm - 41 = 66 mm OPP = 66 mm OPM = LDO - DMD = 107 mm - 30 mm = 77 mm OPM = 77 mm OPD = 66 mm OPM = 77 mm In other words: the mirror has a greater optical length than the prismatic one, therefore the focal plane needs to travel 77 mm towards the objective, whereas the prism "eats" only 66 mm of the FL, so the drawtube is not so much inside the focuser WHEN A PRISM IS USED, in fact, we have more drawtube out with the prism than with the mirror, thus, leaving some room to focus other types of eyepieces! Just my 2 cents... Regards and clear skies for us all! Andy