andrew s

Wow, did your pencil slip! 😃. Very nice sketch. Regards Andrew

@vlaiv has captured the core of the issue. If you want a fuller more detailed explanation see here https://telescope-optics.net/induced.htm and the sections that follow. Regards Andrew

Very impressive @vlaiv theory into practice. Regards Andrew

Hi @LDI can you point to some work of yours. It may help us to gauge how to respond to your request. Regards Andrew

You will get a lot of poor seeing due to the heat given off by your garage floor, wall etc. The building will not cool as quickly as the night air causing increased turbulence. It's not rearly a fair test of the telescope. Regards Andrew

Excellent, SGLers guilty of this own up. You know who you are. Just need to redraw it every few weeks, months, years with different scopes and mounts. Regards Andrew

Just did the YouTube catch up. Excellent talk. I would recommend it to one and all. Regards Andrew PS I would have loved to have asked why such a slow rotator suddenly produced star spots. Spots on red giants are not common and generally only seen on "fast" rotating examples.

Have you focused on a terrestrial scene was it ok, what about a star near the zenith in focus and inside and outside focus? This will help you decide what if anything needs doing. It could be seeing or the other issues mentioned above. Regards Andrew

A very delicate image. Regards Andrew

In the end you can't better laquired polished brass. I used to lust after the large Fullerscope's brass refractors of my youth. Regards Andrew

How about a campaign for pink Takahashi refractors. Regards Andrew

Yes quite right. You won't find it specified as it depends on the focal length of the objective, the Barlow and the eyepiece and exactly where they are position with respect to each other. For an objective at infinity the eye relief would be at the eyepiece back focus. As the objective gets nearer eye relief moves out away from the eyepiece. A stronger Barlow reimagines the objective close to itself (and the eyepiece) than a weaker one. But the detailed position will change to achieve focus. Regards Andrew

Yes it's exactly the same. Both the Barlow and Cassigrain secondary have negative focal length and so, I belive, in the paraxial approximation they are equivalent. Regards Andrew

A quick ray trace with pen and paper shows a Barlow produces a virtual image of the objective closer to the eyepiece. The eyepiece will then reimagine this further out from its back focus and hence increase the eye relief. Regards Andrew

I may well be recalling this incorrectly but my understanding is the the the exit pupil is the image the eyepiece makes of the entrance pupil or aperture stop i.e. the objective lens or mirror. (I can't recall which but they are the same on a normal telescope.) Any Barlow or Cassigrain mirror will change where this is. I would need to do the sums to see if this has the effect of making the objective seem closer to the eyepiece and so increase the eye relief. Regards Andrew

Just in. With out undercut but with cats paw. 🙀 Regards Andrew

It's ok your French now 🤪. Regards Andrew

Not explicitly as far as I know. However, the calculation was done as a function of the ratio of optic diameter to the atmospheric coherence length (Fried parameter) and the estimated variation about this average. So whatever the observed (actual) seeing it gives an estimated of the probability of capturing a "Lucky" image as a function of aperture. Regards Andrew PS the formula for the Fried parameter derived from turbulence theory does include a zenith angle term.

Thanks @mikeDnight in the "Amateur Astronomer's Handbook " Sidgwick gives examples of Pickering, Steavenson and others seeing extended detail at 1/2 to 1/15 of the classic theoretical resolution depending on what it was. So you are in good company. Just to help with my sums could you estimate how long you would be willing to wait for a good seeing moment before giving up as a night on a night as not worth observing ? Thanks Andrew

Thanks @JeremyS I will have to catch up with it on you tube as it clashes with the Chester Astro Soc. webinar. Regards Andrew

While the weather prevents me making any practical steps forward in this area I have been researching the science and looking at what the observers of old achieved. One area I have found no data for is how frequently you would expect those" moments of exquisite seeing" to be for you to continue observing rather than going in for a nice cup of tea or something stronger. For example would it be once a minute, once every ten minutes ? I have no idea. The reason for asking is that calculations have been done for the probability of getting a lucky image based on telescope aperture and seeing and I would like to convert this to a visual session. However, unlike imaging the acceptable rate is I suspect psychological rather that technical. Also if you have any estimates on the size in arc secs of the finest detail you have seen please comment on that too along with the size of telescope used. Thanks in advance. Regards Andrew PS If I do make any sense of all this I will post it up here.

Someone who makes a good living from presbyopia. Regards Andrew

Now that's an existential question. Regards Andrew PS nothing.

Yes, but tins are normally aluminium or steel not tin! Words are so, well so Regards Andrew

For the small planetary fields I don't think you need a coma corrector. A good Barlow would be better to allow longer fl eyepieces to be used. Regards Andrew