andrew s

Looks like "the face" is blowing Ciaran a kiss in gratitude. Regards Andrew

Don't forget the Door mouse is two dimensional and the house mouse confined. Regards Andrew

Fantastic images. I could hang it on my wall. Regards Andrew

Yes it was the inner baffle tube I was thinking about. But, as I say pure speculation on my part! It's a fine balance to stop sky flooding and let the whole light cone pass. Regards Andrew

I have no additional data but badly designed baffles could act as an aperture stop. Regards Andrew

I thought it was just the next size up in Takahashi refractors! Simple. 🤔 I know eyepiece, tripods mounts, etc. It is often the best bit, which is why I have enjoyed building telescopes and spectrographs as there are more bits to choose and fiddle with. Regards Andrew

Yes, but I am shifting towards AZ EQ5 for the mount as I might try my hand at sketching and a driven mount would seem to be the thing from the advice I have been given. I do have some leanings towards a TS 6" or 8" Classical Cas or a 6 or 8" Mak but the optical quality of the Mewlon pulls me back even though the off axis spot size might be bigger, if only it were f15 like the Intes-Micro Alter 715 . Then again there is the compact omc 140...... Anyway, half the fun is agonising over the choices. Don't hold your breath, it may be a while. Regards Andrew

I read something that maybe of relevance in Telescope Optics by Rutten & van Venrooij. They were commenting on field curvature of eyepieces and how difficult it is to remove without introducing astigmatism. The also commented on the fact that the f number the eyepiece has to work at is the same as that of the telescope and that they had to work at much higher off axis angles. All in all this makes aberration control difficult especially field curvature. It also gets more challenging at shorter focal lengths. Thay also pointed out that the eye has to accommodate to account for the field curvature of the eyepiece and that this gets harder as you get older. Thus all factors point to a longer focal ratio, longer focal length telescope paired with a longer focal length eyepiece being easier on the eye. Regards Andrew

Sorry if this is the wrong place to ask but how important is having a drive mount for sketching? How many manage without? Regards Andrew

@Stu, when words fail try a diagram. Not very good but I drew this to scale just using geometric optics rules. I hope it show what I tried to explain in words above. Regards Andrew PS It occured to me that the fundamental difference between the linear and angular magnification is that when you scale a triangle all the lengths increase in proportion but all the angles stay the same.

I do remember it but more in the context of aberrations. Aberrations cause there to be no clear focus point while aberration free optics snap in. Regards Andrew

That is the what you get a prime focus if you take an image. I said the same in an earlier post. It is not what you get if you view it visually with an eyepiece at the same magnification. The difference is with the image you are looking at linear size at prime focus on the chip, while visually you are looking at the angular magnification as the eyepiece produces an image at infinity which your eye then focuses on. Regards Andrew

Your so managerial @JeremyS , who cares as long as we are having fun! Regards Andrew

Note I made a mistake before now corrected. Regards Andrew

Ok here is my logic having checked the equations more carefully!,,,, The angular size of the airy disk "a" is a = l/D for wave length "l" objective diameter "D" The apparant angular size of an image "b" in an eyepiece is such that b/a = M = F/f where M is the magnification F and f the focal length of the objective and eyepiece respectively. Thus the angular size in an eyepiece is b = l*M/D so at constant magnification the image has the same angular size. You have to use angular size as the eyepiece image is at infinity. So I was right after all. Regards Andrew PS equations from "Telescope Optics" Rutten & van Venrooij Now corrected.

Yes I am confused now and I think I was wrong . Can't work on it now but will do the sums and get back with the results Regards Andrew

Visually, at the same magnification and aperture the size of the airy disk should be the same. If you take a prime focus image the linear size of the airy disk will increase in proportion to the focal length. That's it period. Trust me I am a physicist . Regards Andrew

I have seen some examples where they do and some where spikes were removed! There have been magazine articles on how to add them for Xmas cards if I recall correctly. It's a free choice, enjoy what you will. Regards Andrew

There should be no difference as focal ratio does not affect t the angular airy disk size. There may be second order effects due to the shorter fl telescope having larger aberrations, field curvature being the most obvious one. Regards Andrew

I am in shock. My wife just offered to buy me a telescope! I take it all back, whatever I may have implied in the past. Watch this space. Regards Andrew

I agree aesthetics is important that's why I love the diffraction spikes from the point spread function on my ex Newt and current ODK. Just as much an "image" of the star as that produced by a refractors circular aperture! Regards Andrew

I have never quite understood why small refractor owners wax lyrical about looking at the point spread function of their telescopes aperture stop. I guess it's because they can where as with large apertures it's too small to see. 🙃 Regards Andrew PS only joking, honest.

Only if you view them at the same magnification which would mean two very different fl eyepieces. Regards Andrew

If it were my me I would put all the money into the best mount I could afford and get the best out of the Newt. You can improve the image with a a coma corrector, pump up the focal length with a Barlow or PowerMate and turn it down with a reducer corrector. Regards Andrew

Nice to see your observation in the show us you kit at night thread. Well done too low for Castillejar. Regards Andrew