andrew s

Yes, guilty as charged forgot about it. Make that 3 but I am not sure spotting scopes rearly count (and it's a correct image refractor).😳 Regards Andrew

I only have two. One at PixelSkies in Castillejar for the clear skies and one at home that I can stroke. Regards Andrew

Send it to a managed observatory and run it remotely? Regards Andrew

No, I ideally wanted mirrors only and the best optical quality I could get. When one came up second hand at good price it was a no brainer (which somehow seems strangely contradictory). Regards Andrew

Yes I was tempted by the IOptron Rumak before going for the 180 Tak Mewlon. Regards Andrew

Yes a lot depends on what you want to do and of course budget. How close a doubles tempt you? Personally I would go with a scope that, under good conditions, can resolve 1" i.e. at least 110 to 120 mm then you will mostly be seeing limited. Regards Andrew

Only with a binoviewer or rose tinted spectacles. 👀 Regards Andrew

How about a nice Maksutov? No diffraction spikes and the added bonus that the central obstruction enhances high frequency resolution compared to unobstructed due to narrowing the PSF at the expense of increasing the first ring. Touch paper lit 👹 Regards Andrew

Now don't damage it by breaking a bottle of champagne over it at the naming ceremony. 🥂 Regards Andrew

Modern magic . Regards Andrew

You don't need the aperture. You need the focal length and pixel size to calculate the plate scale in arc second per pixel. The are on line calculators. Regards Andrew

@paulastro ignore @mikeDnight you don't need a cure for sanity. Unfortunately, others are beyond helping. Regards Andrew

Yes for the vignetting. Best for the focuser to be squared on as accurately as you can otherwise collimation may change with focus position. Regards Andrew

You can have a perfect star test and still have offset vignetting if the secondary is not centered under the focuser. You need a sight tube to do this. Also did you check the new focuser was squared on to the tube? Just thoughts. Regards Andrew

Good to have some facts rather than opinion. Mitigated somewhat by the CFZ going as F^2. I think it all points to small and compact being simpler and less subject to various disturbances. Regards Andrew

So am I. Regards Andrew

But your scopes and eyepiece collection are. (Queue sinister music.) Regards Andrew

You have a position I can find a quote to justify it.🤼‍♂️ Personally I prefer measurement.🤪 Regards Andrew

I know, as an ex President of the BAA and current Variable star section Director I am used to seeing him as God rather than the devil. 😱 Regards Andrew PS I know where he lives.

Your a very naughty boy. Regards Andrew

It might be worth noting that Pickering commented that "refractors below 5" were only useful for variable star work and that 8" to 10" was required for serious planetary observations". You pays your money and picks your quote!😉 Regards Andrew PS @JeremyS I can't even find the Journal!

No google does not seem to find it. If you have a copy or a link please send it. The most common conclusion was that refractors were better than reflectors of the same aperture but that with good thermal management reflectors could equal refractors. Howverer, they were all subjective not based on measurements. Regards Andrew

Presentation heat transfer.pdfWell the weather was poor this morning so I have searched high and low using Google and found papers and articles from the 1800s to 2019 on the topic of tube currents. Many I had seen before but many were new. Interestingly the older ones comparing refractors and reflectors often considered the need to reduce them in large refractors as well as reflectors. My main conclusions was that total light path length in air of differing refractive index was key so reducing thermal gardiants or mixing the air were cures. In addition having the largest thermal mass at the bottom to the tube was worse that having it near the top! There seem to be two related issues. Initially, reaching or getting close to thermal equilibrium with the air causing bulk convection currents or unstable air columns with cool air above warm. Secondly, over cooling, wrt to the air, of some parts due to radiation to the sky which creates boundary layers. Differing strategies to overcome this were discussed but amount to reducing temperature gradients as much as possible and mixing the air to normalise the refractive index. Eventually I can across this discussion https://www.cloudynights.com/topic/624178-heat-transfer-in-and-around-a-telescope/ the second drop box link to the full presentation is live and well worth a read. It the most modern and comprehensive discussion I found. I am going to follow up some of the ideas including the low emissivity transparent film - even on a Takahashi. Regards Andrew PS Here it is for ease of access Presentation heat transfer.pdf

I have never seen anything like that can you or anyone provide a reference to that memory (about amplification not his sensitivity)? 😀 Are you referring to tube currents or atmospheric seeing? I understand the tube current amplification on multiple passes in the tube but find it hard to see how it is anything but linear on wave front errors from the atmosphere or elsewhere. If it were the case, then Cassigrain primaries would need very high tolerance as their wave front errors would be amplified similarly. Off to see what I can find. Regards Andrew

As @Peter Drew points out tube seeing is also important (it's covered in later sections of the link). Excellent, thermal management is needed in folded designs. In addition focus and collimation are also very important and are more easily distributed in some configurations than others. Regards Andrew