Science behind seeing (SCT C5 vs 80mm APO)

[andrew s]

16 minutes ago, Alan White said:

I felt for you Andrew, I could see you were taking Jeremy seriously.
Easily done, he has that serious look about him 😉

 

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.

Edited August 1, 2020 by andrew s

[JeremyS]

39 minutes ago, andrew s said:

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.

I’m not in 🤣

[mikeDnight]

4 hours ago, andrew s said:

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!

He did say that, which tells me he didn't observe much, or his telescopes/seeing conditions/skill as an observer wasn't upto much.

W.F. Denning carries much more clout in my book as he was an undeniably excellent observer. Denning made many excellent quotes, one of which states " What one man sees through a 5" glass, another needs a 10"."  In another he stated "There is little point in having a glass of great capacity at one end of a telescope and a man of little capacity at the other." 😆

Warmest regards, Mike 

[mikeDnight]

1 hour ago, Alan White said:

I felt for you Andrew, I could see you were taking Jeremy seriously.
Easily done, he has that serious look about him 😉

 

And who wouldn't, he's a Doctor.

[andrew s]

20 minutes ago, mikeDnight said:

He did say that, which tells me he didn't observe much, or his telescopes/seeing conditions/skill as an observer wasn't upto much.

W.F. Denning carries much more clout in my book as he was an undeniably excellent observer. Denning made many excellent quotes, one of which states " What one man sees through a 5" glass, another needs a 10"."  In another he stated "There is little point in having a glass of great capacity at one end of a telescope and a man of little capacity at the other." 😆

Warmest regards, Mike 

You have a position I can find a quote to justify it.🤼‍♂️

Personally I prefer measurement.🤪

Regards Andrew 

[andrew s]

26 minutes ago, JeremyS said:

I’m not in 🤣

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

Regards Andrew 

[andrew s]

13 minutes ago, mikeDnight said:

And who wouldn't, he's a Doctor.

 

So am I.  Regards Andrew 

[Alan White]

Well in that case chaps, 

I have this boil on my ......... 🤣

[Peter Drew]

Back to the topic and putting my pedant's hat on over my Beanie.  The shift in focal plane in a SCT due to variation in the inter-mirror separation is around the change of distance times the amplification squared, hence 25x times in the case of a typical SCT.  The effect of this is that due to thermal expansions and contractions the image plane moves much more than that of a Newtonian and a refractor.  The reason one is always fiddling with the focuser on a SCT a lot of the time.   🙂

[JeremyS]

1 hour ago, mikeDnight said:

W.F. Denning carries much more clout in my book as he was an undeniably excellent observer.

Spot on Mike! Sometimes I picture you going out to your wonderful Tak DZ with a complete set of Vixen HR eyepieces - the best that money can buy - attired as Burnley's latter day Denning, dragging your giant globe.

Edited August 1, 2020 by JeremyS

[andrew s]

1 hour ago, Peter Drew said:

Back to the topic and putting my pedant's hat on over my Beanie.  The shift in focal plane in a SCT due to variation in the inter-mirror separation is around the change of distance times the amplification squared, hence 25x times in the case of a typical SCT.  The effect of this is that due to thermal expansions and contractions the image plane moves much more than that of a Newtonian and a refractor.  The reason one is always fiddling with the focuser on a SCT a lot of the time.   🙂

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

Edited August 1, 2020 by andrew s

[mikeDnight]

2 hours ago, Peter Drew said:

Back to the topic and putting my pedant's hat on over my Beanie.  The shift in focal plane in a SCT due to variation in the inter-mirror separation is around the change of distance times the amplification squared, hence 25x times in the case of a typical SCT.  The effect of this is that due to thermal expansions and contractions the image plane moves much more than that of a Newtonian and a refractor.  The reason one is always fiddling with the focuser on a SCT a lot of the time.   🙂

Thanks for the explanation Peter. It's good to know I'm not as dumb as I look and I haven't lost it completely. I just couldn't remember where I'd read the info in the first place as it was so long ago. But that's what books are for, so I don't need to remember. 

2 hours ago, JeremyS said:

Spot on Mike! Sometimes I picture you going out to your wonderful Tak DZ with a complete set of Vixen HR eyepieces - the best that money can buy - attired as Burnley's latter day Denning, dragging your giant globe.

That does look like me Jeremy, but all my astro friends look and dress like that too. My globe's aren't as large though! At just 5" pathetic in fact!!

[paulastro]

I'm really pleased  I dont have a 3.9 inch ( or even a four inch! ) super expensive apo which costs an arm and a leg. 

Otherwise, I too might feel the need to try and prove to anyone who is bothered to listen that it is by far the best and only truly worthwhile telescope in the world. 😄

Edited August 1, 2020 by paulastro

[mikeDnight]

If you have a psychological problem that's arisen due to some inadequacy, possibly a size or performance issue Paul, there are two doctors on this thread that will be happy to diagnose you. They may not have a clue about medicine but the're good enough for the NHS. 💉

Edited August 1, 2020 by mikeDnight

[andrew s]

@paulastro ignore @mikeDnight you don't need a cure for sanity. 

Unfortunately,  others are beyond helping. 

Regards Andrew 

[Ricochet]

19 hours ago, andrew s said:

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 3.77 MB · 4 downloads

That presentation is a very good find. I like the idea of the film, as it provides an improvement without adding weight or bulk to the scope. I think that I may have inadvertently done something similar by flocking the inside of my dob. It doesn't prevent the radiative cooling of the tube, but I think that it probably prevents or lessens any conductive exchange between the tube and the air inside of the tube. I've not got any temperature sensors to confirm this, but the flocking feels "warm" and the inside of the tube does not dew up like the outside does. However, there is also a primary fan so the resulting small air current may be helping with respect to dew formation on the inside of the OTA. Now that I have properly isolated the fan from the tube with respect to vibrations, I have not noticed any difference visually between leaving the fan on all night, or turning it off for high power observations (nut perhaps I need to do longer testing on this).

[paulastro]

1 hour ago, andrew s said:

@paulastro ignore @mikeDnight you don't need a cure for sanity. 

Unfortunately,  others are beyond helping. 

Regards Andrew 

Thanks Andrew. However,  I've, no cocerns in this regard.  I'm actually a good friend of Mike's and he's well aware that I worked for the NHS for 25 years, most of them in psychology services!  

It's just a shame there isn't an antidote for Mike's 'sense of humour'  😄.