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Posts Made By: Brendan Sheehan

May 27, 2013 05:16 PM Forum: Equipment Talk

2" diagonal internal restriction issue

Posted By Brendan Sheehan

Hi, It's come to my attention that some 2" diagonals have internal restrictions that are smaller than the maximum 2" eyepiece field stop of 46mm (the inside diameter of the eyepiece barrel itself). I have seen one with an internal safety stop in the chimney section (to prevent eyepiece from hitting mirror) that had an internal diameter of only 41mm.

In particular, if one has a large sct and intends to remove the stock sct thread adapter to avoid it's 38mm limitation, it seems a restrictive diagonal would defeat some of the intended purpose.

Just curious is others are aware of this and/or what your thoughts are... also if you are aware of any 2" diagonals that maintain a full 46mm+ inside diameter all the way through. I suspect Lumicons do, but I don't have their 2" version.

June 3, 2013 02:44 AM Forum: Equipment Talk

SCT illumination curves

Posted By Brendan Sheehan

I made the attached plot of the illumination percent across the FOV, from center to edge, for the case of the maximum field stop of 46mm. There is a separate line on the plot for each of 6 potential restrictions, which are listed in the plot label. The plot was produced with a short MATLAB code I wrote, and is based on the approx. geometry of my 10" Meade. The results depend on the exact dimensions and positions of things. I measured where possible, but a few had to be estimated, so here are the numbers I used.

secondary mirror: 75mm diam. 625mm from field stop

baffle: 57mm inside diameter, ends 425 from field stop

38mm adaptor/43mm nosepiece/46mm nosepiece: Inside diameter 38/43/46mm respectively. In all cases 100mm from field stop.

41mm eyepiece stop/46mm eyepiece stop: Inside diameter 41/46mm respectively. In both cases 50mm from field stop.

The results are based on how much of the secondary mirror is blocked by the various obstructions. The calculation is done for each of 100 points equally spaced along an "equator" across the field stop from center to edge. The resulting 100 values are connected to produce the lines in the plot. The calculation of the percent of the secondary visible is very tedious since ray tracing or intuition shows a crescent shaped section is what gets blocked. The area of the crescent is computed with a difference between two "area of a circle cut off by a chord" calculations. The two relevant circles are the secondary itself, and the circular area that is visible through the restriction from the relevant point along the field stop equator. Basically, where the two circles overlap, the secondary is visible and where they don't overlap, the restriction is eclipsing a crescent shaped portion of the secondary. The visible area (total - crescent) as a percentage of the total area of the secondary is what is given as the "percent illumination"

Note that the line for the baffle drops below 100% illumination first, meaning the baffle is what limits the fully illuminated portion of the field. However the other obstructions block light more aggressively nearer to the edge.

Disclaimer: I'm not an optics expert so I can't say with 100% certainty that my method is foolproof and that my plot is accurate. It's just my best effort at a pretty tedious calculation and for what it's worth I don't know of any reason why it wouldn't be accurate...

May 29, 2013 02:51 AM Forum: Equipment Talk

2" diagonal internal restriction issue

Posted By Brendan Sheehan

Thanks for the replies. After some measuring, sketching, and thinking, I can see that the sct baffle tube necessarily causes under illumination at the field edge of widefield eyepieces, but that a narrow sct thread adaptor, diagonal nosepiece, or eyepiece stop, does have the potential to block additional light beyond that blocked by the baffle tube. I agree it'd be difficult to detect with the eye, but technically I think it does happen.

I sent the question to Williams Optics and Lumicon. Williams gave a terse response saying there were no performance issues with their diagonals and did not invite further conversation. In contrast, Lumicon responded with exact inside diameter measurements for their diagonals and invited any other questions I might have.

The Williams I have has a 43mm I.D. nosepiece and a 41mm I.D. eyepiece stop. Lumicon says their "enhanced" and "lumibright" diagonals have a 47mm nosepiece and another 47mm opening where the body meets the chimney which acts as the eyepiece stop.

May 30, 2013 06:24 PM Forum: Equipment Talk

2" diagonal internal restriction issue

Posted By Brendan Sheehan

Consider the following thought experiment:

Take a refractor, it doesn't matter what focal ratio but say the aperture is 100mm (just so the aperture is bigger than a 46mm field stop). For simplicity eliminate the diagonal, so that the eyepiece is inserted directly into the focuser, "pirate scope" style. Let the eyepiece be a 2" model with the max field stop, so 46mm inside diameter for the field stop/barrel.

Now imagine shrinking yourself down so you are a small fraction of a millimeter tall, and can walk around inside the OTA.

Stand at the left edge of the field stop and look towards the objective. How much of the objective can you see? Not all of it, cause the eyepiece barrel, which extends in front of you creates a "horizon". Roughly speaking, since the horizon is 23mm to the left of the optical axis, you can't see any part of the objective that lies more than 23mm to the left of the optical axis. So, there is a crescent shaped portion of the objective that cannot contribute light to the edge of the field at your position.

Now put a 41mm diameter eyepiece stop, or a 38mm adapter somewhere out in front of you. These thing necessarily "raise" the horizon and block an additional crescent shaped section of the objective, increasing the degree of under illumination at the field edge.

If the restriction is severe enough, you won't be able to see the objective at all, meaning the degree of under illumination at the edge has reached 100%.

Now begin floating to your right, towards the optical axis. Slowly more and more of the objective becomes visible over the horizon, however you may find that before the entire objective is visible, the end of the focus tube appears, and is now the thing creating a horizon and blocking a smaller section of the objective. You are now in an annulus of field that is unaffected by the barrel/other local restrictions, but is still under illuminated due to the focus tube. Moving further to the right, at some point the entire objective becomes visible. You are now at the edge of the fully illuminated portion of the field.

In this example the focus tube (or baffle in an sct) is what limits the size of the FULLY illuminated field, because it forms the last horizon, when moving towards the optical axis, before the entire objective becomes visible. However, the 38mm - 46mm restrictions closer to the field stop cause additional under illumination at the edge beyond what the focus tube would cause by itself because they form the relevant horizon when your standing at the field edge. So, removing anything smaller than 46mm gives you some additional light in an outer annulus of field. May not be detectable to the eye, but I think it's interesting.

Using the geometry of my 10" meade, the barrel of a 2" eyepiece allows over 75% of the secondary mirror to be visible from the edge of a 46mm field stop. With a 38mm adapter in place, less than half the secondary is visible from the same position.

I first did this by drawing the rays, but thought the shrinking idea would be easier to explain than the drawings. Let me know if this makes sense...



June 13, 2013 01:28 AM Forum: Astro-Physics

Globulars of NGC5907 and a Quasar

Posted By Brendan Sheehan

I don't know anything about imaging, so I have a question. How do you figure out the magnitude, redshift, and nature (glob, quasar, etc.) of the identified points in the image?