Focal plane and ep field stops

[mura_gadi (Steve)]

Hello,

I am trying to understand the ray trace from stellafane.org "newt for the web".

What happens when the eyepiece's focal plane is above the field stop of the eyepiece. I am after a F6 and set my max ep at 30mm which has a field stop of 43mm. My 75% band is 44.8mm atm, but if the field stop of the ep is below the focal plane, won't that narrow the band width of the 75% illumination to 43mm Xmm below the focal plane?

Assuming the focal plane of the ep is above the the field stop in the assembly. I am wondering if you actually can run closer to the field stop or even slightly less as the ray trace would be altered by the ep.


Thanks
Steve
Morpheus, ES and Tele vue eps as for general ep design.

[astro744]

I’m not quite sure what your asking but I’ll attempt to answer. Firstly the eyepiece field stop is at the focal plane of the eyepiece which when placed at the focal plane of the telescope will produce an image in focus and magnified by the ratio of focal lengths. A 31mm Nagler with an 82 deg apparent field has a 42mm field stop diameter. Your 30mm eyepiece with 43mm field stop diameter will have a slightly larger than 82 deg apparent field but it is also dependent on % distortion present in the eyepiece.

You also mention illumination of the field (also referred to as magnitude drop) and the goal here is to size your secondary mirror to give you about a 70% (Newt uses 75%). Mel Bartels used to quote 70% but now uses magnitude drop (0.4 as being acceptable).

I use the Mel Bartels diagonal sizing program and aim for 0.4 mag drop for my widest field stop eyepiece I use the most. A slightly greater than 0.4 mag is hardly perceived especially for a wide cone of light from a fast mirror. If you are a variable star observer you want the widest true field you can get (46mm FSD) and zero magnitude drop over that field. If you are a dedicated planetary observer you can drop a bit more provided your 100% diameter is not too small. (Old rule of thumb was aim for 12.5mm and this still works but typically these days with very wide field eyepieces it’s nice to have a larger fully illuminated field).

See https://www.bbastrodesigns.com/diagonal.htm

I personally prefer % illumination instead of magnitude drop and aim for a 70% drop at the edge of my widest FSD eyepiece but I have a couple of telescopes giving me 60% with my 41mm Panoptic and I can hardly tell the difference at the edge of field.

Note distance from secondary to focal plane can be adjusted at the design stage and sometimes after and this will affect % illumination.

[mura_gadi (Steve)]

Hi,

Thanks for the reply. I thought I had read that the field stop was not always at the focal plane of the eyepiece depending on the design of the eyepiece.

The scope will be predominantly used in a suburban area as a roll out scope, hopefully 6-8 dark sky nights a year. So, the majority of the time will be spent looking at bright objects. I was looking at between F5-6 as a general purpose scope, leaning towards F6 for:
a) F6 is easier to grind out and polish
b) Better visual optics
c) little more forgiving on eyepieces.
- at nearly 1.9 meters with shoes I can take a slightly taller scope as well

Currently I have either the Pan 35mm or the ES 30 82 as my wide field, the Pan aligns way better weight wise. The 17.5mm morpheus and either the 9mm Morpheus or the 10mm Delos for my high power.

The Pan 35mm is 1.03 degree and the ES 30mm is 1.11 degree, the Morpheus 17.5mm is .6 degree and the last two .31 degrees. I want to keep a full degree at low power and the morph was setup to take a full luna image.

My current fields for 100% and 75% illumination are .56 degree and 21.6mm and 1.18 degree and 45.2mm. The diagonal is at 18% obstruction with virtually no drop off 0.2 magnitude loss.

Given the scope will mostly be an urban backyard (bortle 5) and used for brighter objects such as planets, clusters and bright nebula like Orion, looking at your figures I could take on a lot smaller secondary than the 2.63" (18%) I currently have.


thanks
Steve

[astro744]

Just so I’m not guessing some important numbers would you mind listing the following:

Diameter of primary mirror.
I will assume desired focal ratio is f6 from previous comment.
Diameter of upper tube assembly and wall thickness of tube. (Latter not that crucial if thin).
Height of focused from outside of tube to fully racked in. I will add 25mm to desired focal plane position.
35mm Panoptic has a 38.7mm field stop diameter. Do you want a fully illuminated field over this diameter or are you ok with less, I.e 70% or 0.4 mag drop at the edge?

From the numbers you quoted I could quite tell if you had a 14”, 14.5” or 15” mirror. I would not go too small on the secondary but final size will depend on some of the numbers I’ve asked for.

[mura_gadi (Steve)]

Hello,

The specs are:
14.5" mirror,
16.5" On inside Tube diameter
.25" for the focuser mounting plate
Min. focuser height 1.88"
Focuser travel .94"
Focuser Diameter 2"

I was after very little drop off at the edge of field and I'm happy to take's Mel's charts for .4 mag reduction or less for illumination.

You mention "I will add 25mm to desired focal plane position.", I thought at 1st it was for 1.25" ep use, but that seems too large an addition. What is the 25mm addon for?

The Pan 27mm was a fore runner as that would make the weights of the 3 ep's 450/415/408 grams. The real requirement is to keep the wide field above 1 degree, then the .6 degree at x100+ mag or better (x126 atm), but I didn't want to jump to the very wide fields due to weight variations, price, ER etc.

Weight is very easily dealt with, but I need to find the focal length of each of the 3 ep's with long ER. Preferably 15-20mm of ER (actual) or better without hitting premium class ($400+) for the mid and high powered ep's. Hence the Morpheus's (that and they have great reviews across nearly all the focal lengths).

With the focuser mounting plate, can it be metal plate? Or is metal a no-no due to flexing from temperatures?


Thanks
Steve

[astro744]

Originally Posted by mura_gadi

Hello,

The specs are:
14.5" mirror,
16.5" On inside Tube diameter
.25" for the focuser mounting plate
Min. focuser height 1.88"
Focuser travel .94"
Focuser Diameter 2"

I was after very little drop off at the edge of field and I'm happy to take's Mel's charts for .4 mag reduction or less for illumination.

You mention "I will add 25mm to desired focal plane position.", I thought at 1st it was for 1.25" ep use, but that seems too large an addition. What is the 25mm addon for?

The Pan 27mm was a fore runner as that would make the weights of the 3 ep's 450/415/408 grams. The real requirement is to keep the wide field above 1 degree, then the .6 degree at x100+ mag or better (x126 atm), but I didn't want to jump to the very wide fields due to weight variations, price, ER etc.

Weight is very easily dealt with, but I need to find the focal length of each of the 3 ep's with long ER. Preferably 15-20mm of ER (actual) or better without hitting premium class ($400+) for the mid and high powered ep's. Hence the Morpheus's (that and they have great reviews across nearly all the focal lengths).

With the focuser mounting plate, can it be metal plate? Or is metal a no-no due to flexing from temperatures?


Thanks
Steve

Ok. The 25 mm is added to position the focal plane 25mm above the fully racked in position. This is to provide ample inward travel and accommodate most eyepieces and possibly even the Paracorr if you ever want to use one at f6. (I don’t have the P2 and am not sure of the required inward travel). Most will say you don’t need a Paracorr at f6 but it depends on how clean a view you want. The Paracorr increases the size of the sweet spot for a coma free view but typically at f6 it is for the purist. (I notice a difference on my 10.1”/f6.4 but typically don’t bother using one, P1 not P2).

In any case Paracorr or not you need to allow some inward travel and typically 25mm is enough. Now in your case you seem to have a low profile focuser with a very small amount of travel. i.e. less than 25mm. If you position your focal plane only 0.5” above the fully racked in focuser you only have 0.44” of outward travel remaining. You can always use extension tubes but if you run out of inward travel then there’s not much you can do other than let the collimation screws on the primary all the way out but why not give yourself more travel to start with.

I plugged in the following numbers into Mel Bartels site (note a trick is to set magnitude drop to greater than desired, e.g. 0.6 as this will give you the next sized secondary mirror and show you its performance graph.

14.5
87 (=14.5*6)
11.32 (=16.5/2+0.25+1.88+0.94). 0.94 should be 1 with extra travel to spare
1.52 (=38.7/25.4) this is FSD of 35mm Panoptic giving 1 deg field.
1, 1.3, 1.52, 1.83, 2.14, 2.6, 3.1, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 12
0.6
Click on update chart

If you go with 2.6” secondary and change to a taller focuser with more travel you can play around with the focal plane position (of available focusers) to drag the magnitude drop from 0.25 to 0.4 with the 2.6” diagonal. A 2.6” diagonal will obstruct 18% by diameter and give a 0.8” of 100% illuminated field. A 3.1” will obstruct 21% and give a wider 100% illuminated field.

If you put a taller focuser on the 2.6” you will reduce both 100% and 70% (0.4mag) fields. Note too the diagonal holder will have a lip around the edge of the diagonal further reducing effective sec. diameter. I would probably go with a 3.1” secondary and a taller focuser with more travel unless I specifically wanted a planetary ‘scope with smaller 100% field and lower % obstruction. In practice I have found the 21% obstruction on my own 10.1” f6.4 gives me excellent contrast on planets and even though I also have a 1.83” secondary I have seen no reason to swap them. I think the importance of a smaller sized secondary is often over stated and anything under 25% but closer to 20% is an excellent choice.

As to the material used to hold the focuser, I’ll let someone else chime in here as it’s not something I’ve ever needed to consider. My custom built ‘scope is a tube not truss and the only reason I’m familiar with secondary sizing is that I got into it in some depth perhaps more than I needed to when I was sizing my diagonal and even then I bought two sizes because it was borderline but have kept using the larger diagonal.