globular

I think my attempt to help probably hindered, so sorry about that. At least it was in tune with Edmund not in opposition. Good luck with your finder project... look forward to seeing it.

I equated the two to try and make understanding it easier, not because it is the "right" way of thinking about it. The length A is really the clear aperture... slightly larger typically to give a crisper margin. If you trimmed arbitrarily then, yes, you are changing A but you are not changing the clear aperture (unless you trim too much). The 1.7071 only holds if your A is "optimally" trimmed. If you trim differently then you might get 1.6, say. But 1.6A(arbitrary)=1.7071A(optimal)=A' Absolutely. If it didn't then you would get some rather distorted images! The reason you get an extra 0.7071A is not because of the roof, but because of the extra glass needed to achieve a roof over the whole clear aperture. Maybe this way of thinking about it is easier.... The light path through the red bit of glass is A and the light though the two bits of blue glass is 0.7071A, giving 1.7071A in total.

I think A in the roof prism is... And if you equate this prism to a "normal" one then it's A' would be...

Their answer is consistent with my response above and does seems to work with their quoted figures for my non amici version - but I agree the figures don't add up for the amici. Surely for an amici prism the glass path has to be approximately twice the clear aperture - as the light starting on the far left has to travel to the far right (= clear aperture distance) as well as through the prism (another clear aperture distance (ish)). So if the 31mm clear aperture is right the glass path should be around 60.... or if the glass path is 50 then the clear aperture must surely be less than 31. The figures they quote for the 2" version of the amici are; glass path 80, clear aperture 44..... this seems just about ok. I suspect one or more of the quoted figures for the T-2 Amici Prism diagonal is wrong. My guess is the clear aperture more like 27.

Puzzling. Please post back when you get an answer.

My understanding is that glass path of 50 means there is 50mm/1.56=32.1mm of glass - with the light travelling 50mm due to refraction. And optical length of 47.5mm means there is 47.5mm-32.1mm=15.4mm of additional light travel from the glass surface(s) to the connection flange surface(s). (Probably 7.7mm each side). Hence the movement in focal plane of using the prism (compared with straight through) is 50mm+15.4mm = 65.4mm. I don't have this particular prism to measure it for you - but I do have a baader prism with reported glass path of 50 and reported optical length of 100 and I can confirm my measured optical path in use (~117mm) agrees with this same formulaic approach, i.e. 50 + (100-50/1.56) =117.9mm (I'm also happy that the reported clear aperture of my prism (47.5mm) is pretty accurate. But again can't comment on the one you're interested in.) Hope that helps.

Someone drove a land rover over one.... not sure about a tank though...

The guy in this link is a bit of a **** but he does point out the right and wrong way to calibrate. (The 'figure of 8' method may not be optimal)

settings...compass true north and location permissions are both there.

Do you have "Use True North" enabled? And allow compass access to your location? (These two options are used to correct magnetic north to true north based on your current location). And is your phone in a case with a magnetic clasp? If so try it out of the case.

It sounds like your phone needs the compass calibrating... rather than leaving it uncalibrated and offsetting the error in other apps that use the compass.... You don't say what phone you have.... but I suggest you google it and you'll find instructions for android and ios.... HTH YMMV

I added a long cable tie to my focuser. Run your finger up or down the tie for largely shake free fine focus adjustment. Slips on and off easily for repositioning. Not sure if it will work with your focuser? Worth trying though.

Great offer. I accept. I'll bring my rods too.

Indeed. But by the same argument there are now fewer that look like the background and more that look slightly different to the background. So me might take more notice of the slight difference.

However many there are, there are still 4x as many when you double the aperture... a change bigger than the 7% JND.... so just might be noticed.

a few receptors slightly activated..... brain says "nah it's probably noise, ignore it" lots more receptors with same very low activation... brains says "too many to ignore this time, see what you make of it"

Congratulations on the scope. Nice sketches. (Especially like the snowball nebula θ)

I'll just run with keeping it horizontal. Pretty easy to do in an alt-az. Trial and error will see me through.

Thanks Mark. I suspect, once I'm outside and trying it, all will be well. I went with a single knob control version to adjust prism-prism angle and rotation to make it as easy to operate as possible. But, as you might have noticed, I like to understand the theory as well as trying it in practice. I get that the rotation has to be perpendicular to the parallactic - because the dispersion happens along the parallactic - but I thought this angle changes as distance from meridian increases and not as altitude changes. (e.g. at all altitudes long the meridian the parallactic angle is always vertical - so the ADC is always horizontal. But away from the meridian it's not vertical so the ADC is not horizontal). Thanks Andrew. I found that same formula for the parallactic angle and have written an app for my phone that allows me to read out loud the inputs (that I get from SkySafari) and it will speak back the angle..... but I can't seem to get it to produce sensible results. Either the formula is wrong or my inputs are not in the correct format. 🤔 It's probably a degrees vs radians thing in the sin, cos, tan functions....? Could you perhaps give an example location, time and object and provide the inputs in the correct formats to slot into the formula?

I'm using an alt-az mount and have my shiny new ADC ready to try on my next clear night. Hopefully most of the time I'll see red and blue colour fringing.... and then I align the ADC 90° to a line through the centre of the colours..... ... but if there is no obvious colour fringing, but still sharpness issues that the ADC can help with, I'd like to know how to correctly align the ADC. If I'm observing something at the meridian then it will have a 0° parallactic angle and the ADC should be at 0° from horizontal.... yes? (hence the bubble level on some ADCs) But if my object has a 30° parallactic angle, say, then the ADC should be at 30° from horizontal??? And how do I calculate the parallactic angle? Does skysafari give it out for me? And if I have my diagonal rotated, say 10°, should the ADC angle relative to the horizontal still be 30° or now be 40°? My head hurts 🙃

You also have different coordinates on PC verses Phone… (54.55,-1.30) verses (54.57,-1.33)…. and this will lead to some differences too.

The formula for the magnification of the common barlow design is M = 1 - X / Fl where M = magnification, X = distance of focal plane to barlow lens group, Fl = focal length of barlow lens group. Trouble is you probably don't know the Fl of the barlow (they are seldom published) nor the position of the barlow focal plane. However if you do a few measurements with your particular barlows / gpcs then you can produce good estimates for these. (e.g. see how M changes with changes in X (e.g. with and without a 20mm spacer, say) and you can estimate Fl.... and then can estimate X with published M). And you can then recalculate M with binoviewer by adding the lightpath of the binoviewer to your estimated X and recomputing. (Note a powermate has a different design (it's not a barlow) and this formula does not apply. But the 2x powermate is pretty close to 2x no matter the distance. So only your GPC will be varying.)

I've had a play with that area but never thought golf ball.... so no I've not seen it.

I love swooping the veil area. Even with my more limited 8" and 1.3° fov I get some fantastic views and often spend an hour or more soaking it up. My eyes agree with you that Pickering's Triangle / Fleming's Wisp is often brighter than the Western Veil. I've found a lot of nebulous structure here.... keeping the scope moving over the area (rather than lingering on one spot) seems to help. Is the 'half golf ball' this?....