andrew s

So how many telescopes is the now @JeremyS ?I am sure you can get treatment on the NHS for this addiction. Regards Andrew

I tend to agree and my home made Newtonians always had an inch! However there is the school of thought that adds baffles down tubes to stop stray light. These must push tube currents into the light path. Yes my 180 has a front opening of exactly 180mm. So anything off axis will suffer vignetting but hay it's a narrow field instrument. I suspect this is in part to avoid sky flooding. However, I also suspect it is to add rigidity without spoiling the looks of the tube. Regards Andrew

Not seen your eyelids before? 🤪 Regards Andrew

And the winner is.....?. Regards Andrew

As the guide is placed close to the exit pupil it only needs to be the size of the exit pupil and not that of the eye lens to avoid vignetting. Regards Andrew Post crossed with @Stu

@Louis D just look at the diagram . At the extremes the lens is just large enough to pass the whole light cone. If it were any smaller it would not as part of the beam would miss it. I would have hoped you could see this for yourself. There is nothing spacious about this. Regards Andrew

Just to round of my view. If you take the equation in @Ruud post then: For an eyepiece only required to work on axis the desired exit pupil size defines the eye lens size alone. As you increase the afov then both the afov and eye relief gradually become more and more important. If you don't want to vignette the edge of the afov you must still consider the design exit pupil diameter. Regards Andrew

I missed @Ruud post where he gave the equation which includes eye relief, afov and exit pupil! Given he has provided it I won't repeat it. Regards Andrew

As you pointed out it would depend on the focal length of the telescope. As with other aspect of eyepieces design you need to target a range of telescopes. For example, focal ratio (how steep a rays must it accept without significant aberration) and telescope field curvature. If this were not the case we could all use the good old Huygen eyepiece that enabled our forbears to study the Universe with long focal length and ratio refractors. It is just not as simple as it may seem. Eyepiece design if far harder than that of objectives. Regards Andrew PS Iwill provide the equation after dinner.

You won't as that's not the issue. For on axis ray the exit pupil comes through the centre of the lens. For a ray at the extreme of the field of view it comes through the outer edge and if the eye lens is too small it vignettes it just as filters can do in imaging. Regards Andrew

While I agree the eye relief should be added. If you exclude exit pupil size you can get vignetting if the eye lens is not large enough. You could just as well say if the lens is large enough field of view or eye relief have no effect. Regards Andrew

Nigella is a girls name for black so how about "The Black Night ( or Knight) Observatory" Regards Andrew

The large angle of incidence on the Ha filter will shift if off band as you move from the centre of the field. Regards Andrew

It's down to two main factors. The field of view of the eyepiece and the size of the exit pupil. The bigger they are the bigger the eye lens needs to be. Regards Andrew

Brilliant. Regards Andrew

Google "cooling telescope mirror" it gets a number of interesting articles including some with video showing the air movement. Regards Andrew

Interesting indeed @Peter Drew, I was looking at this again recently where the boundary layer on spider vanes was being discussed. The issue in general is the formation of a boundary layer with a different and changing refractive index to the bulk air. To stop this it seems you have two options. Insulate to keep the mirror and enclosed air at the same temperature or cooling the mirror to match the falling air temperature. The big boys air condition the whole telescope enclosure to match the expected night time temperature! Regards Andrew

If it can light will leak in anywhere there is a gap or opening. Regards Andrew

Cover the back ( and front) or take your darks in the dark. Regards Andrew

@dph1nm thanks. I have taken several sets of sky flats pointing at a "blank" field over four nights. About 8hrs worth in total. My night assistant Voyager did a fine job and they were ready each morning. This has produced a good quality V flat that works well with my vignetted field. Now trying the SA200. Fortunately, my scope is in Castillejar Spain and I (Voyager) fits them in before my targets are hight enough to image or after they get too low. Regards Andrew

Such is life. Enjoy the scope. Regards Andrew

Good you tried. Any details on what you did. Regards Andrew

It's a pity as Wednesday is the same night as the BAA webinars just have to catch up on one or the other on YouTube. Regards Andrew

@JeremyS can't say I noticed a smell but it was not new. I think it is the one in the review you linked to. It's also not my first Takahashi, years ago I had a Sky 90 but was not impressed. In a bid to design a lenless system I found that you could now get mirror infinity focus microscope objectives (tiny Cassegrain telescopes in reverse) but they cost nearly the same as a new 180c ! If they were a tenth of the price I would give it a go. I did in the past use a microscope tube with objective and eyepiece with a planetary Newt and it worked very well. Only £1,500 to £1,700 from Edmund Optics. I am however, looking at playing with some ball eyepieces following up on a post from @John on William Herschel's eyepieces and finding the Couture "Ball Singlets" in W Paolini's "Choosing and using Astronomical eyepieces". Edmund seem to have all the bits needed at a very reasonable price. Time to fiddle. Regards Andrew PS don't know why the arrow heads in the diagram are the wrong way round! How about a Hasting triplet as a monocentric.

Yes it would, the longer the base line the more distance you can go for the same error. Regards Andrew