andrew s

@vlaiv I think we are quite close. On point 2) we still disagree. The dip size is a continuous monotonic function as the PSFs get closer. There will be a dip at 40% , 20% 10% and so on to zero. I don't see what you mean by you can't see what is not there? Do you mean the dip jumps from 20% to zero ? Or something else. I can envisage a photometer recording a 10% dip etc. I will also review the link again to get back up to speed on his thinking so we can discuss it if you wish to. I am just off for a few days holiday so posting will be intermittent. Regards Andrew

We will have to agree to disagree. The basic points I am making are three fold. 1) The Rayleigh criteria is arbitrary. The is no physical law dictating the 20%. It is a practical place to put the cutoff but it could have been set at 18% or 22% . It was a free choice. Hence it is possible to detect "dips" at less than 20%. 2) Humans may or may not be able to detect dips at 20% depending on their eyesight. 3) I don't think humans when judging if something is resolved or not applies the 20% limit. The reason I pointed this out is that just restating the Rayleigh limit as what can or cannot be resolved is misleading when considering visual astronomy. The point about the obstruction in the link was that the second diagram in the section on "Theoretical inconsistency " shows how the MTF is cut of at 1.1 rather than 1.0. See the red square 32% CO solid plot. If you disagree with my 3 points fine, however, in my understanding they are correct. Regards Andrew

Sorry I don't agree. Have you looked at the link I posted? Regards Andrew

Grant if your using Voyager to control the rig it's very easy to limit the imaging on astro dark and or object altitude. I do this all the time and it works very reliably. Regards Andrew

Not at all I am looking at the sum. The Rayleigh criteria refers to the size of the dip being 20% of the peak hight. Humans may or may not be able to see the dip at the prescribed 20% some may not be able to see 30% other clearly 10% . Remember this is subjective reporting. Can anyone reliably estimate 20% drop in intensity? Technical , if you narrow the individual PSFs ( e.g. by adding a central obstruction) then they can be closer before the dip is at the 20% limit. Regards Andrew

It's not even true for a measured system. A central obstruction narrows the PSF (at the expense of increasing the intensity of the rings) and so even by the technical definition the cut of frequency of the MTF should not be the classical "Airy disk" limit. For details see https://telescope-optics.net/central_obstruction_effect.htm Regards Andrew

It is there. If you use a photometer you can measure the exact depth of the dip. If you use the human visual system you can't. If you look at the old post war papers you will see they used very large samples to determine human contrast detecterbility. Basically humans vary in visual acruity. Regards Andrew

Just a word of caution. The theoretical resolution is a convention i.e. a definition. It is used in optical analysis for example to set the cut off of the MTF. However, if you presented a telescopic image of two perfect disks separated at the limit then some would see the split and others not. Similarly some could "split" closer pairs others would need them to be wider. Regards Andrew

Good to know your limitations @JeremyS. I have been spending my time building a cat pen (6ft x 6ft X 12ft) as we are having to foster my daughters two Burmese cats for 3mth as the are moving from London to Manchester. Our lady Burmerse only just allows her brother in to feed so it's isolation for the boys. I had planned to buy one but it was 8 weeks delivery . How I wish our garden was not on a hill! Regards Andrew PS intended to say proper job on your dome.

Having watched Dave's talk I am sure glad I do spectroscopy and not imaging. Far too hard. Great talk David. Regards Andrew

Planispheres are available on the FLO site under books (see link at top of page) Regards Andrew

Indeed. I was trying to be amusing. Regards Andrew

But that is why I was confused. I got a refractror for solar and you said " not for solar surely " Maybe you were being ironic. Regards Andrew

My advice would be to start by adding 1mm. If this proves unsatisfactory experiment about +/- 2mm of the optimal distance Regards Andrew

@david_taurus83 I don't doubt you experimental results. If you change the distance from the objective to the reducer then you will change its performance. If a filter is thin enough not to add aberration then it will just push out the focus. I don't think optical v physical distance is the issue it is the simultaneous move ment of focuser and back distance change due to the filter that confuses the situation. If you have a built in corrector (I.e. before the focuser) then I belive you would have to push our the camera to obtain focus with a filter compared to without. However, I would always recommend experimenting to find what works with your particular set up. Without detailed ray tracing of the entire system it is impossible to predict exactly what will happen if you change the objective to corrector and correctror to CCD distance. Regards Andrew

I don't agree as per my diagram in the linked thread you need to increase the back focus distance by 1mm. Regards Andrew

I found both in the 80mm F15 refractor I just got for white light solar. It confirms why I love reflectors. Regards Andrew

As a Cheshire as long as its stable in the focuser length does not matter. Using it as a sight tube to place the secondary under the focuser you should adjust it so the the end of the tube just encloses the secondary. Length might matter. Using the cross hair to adjust the secondary tilt is more accurate with a longer distance between the peep hole and the cross hairs but I doubt it would be significant. Regards Andrew

I think that captures the reason why "small" refractors have such a strong following. Couple that with ease of use and stable collimation then you have a winner. Especially with modern examples with little or no chromatic aberration. Regards Andrew PS I still prefer reflectors. 😜

If Olbers paradox were true this is what it might look like. Rather abstract. Regards Andrew

Thanks, @Geoff Barnes just seen it. The explanation has gone from dust to spots and now dust again. It will be interesting to see where the consensus finally settles. Regards Andrew

@John I agree In the end visual astronomy has to consider the human visual system even as its power wains. Regards Andrew

Spot on @chiltonstar the Airy disk is not the image of a star but the convolution of a point source with the telescope aperture function. I sometimes wonder what these debates would be like if the "natural " form of a lens had been square or hexagonal rather than round. Regards Andrew

Your first sentence quoted above seems to be a logical contradiction. Even a mirror can have a Strehl that depends on wavelength. However, I think both are true. Since frequency is related to wavelength Strehl depends on wavelength. I suspect we agree but for wording Regards Andrew

Sorry I intended to say the Airy disk depends on aperture so you can have a larger aperture with a small PSF than a smaller one even if its Strehl ratio is not as good. Hence I don't think you can use it other than comparing like with like if real world performance is the consideration. It does depend on wavelength indeed Takahashi quote it as a function if wavelength for some of its scopes. You can easily have a higher Strehl ratio in the IR compared to the UV. Regards Andrew