"Choosing and Using a Refracting Telescope" by Neil English - book

[ollypenrice]

Well, I think the original review was just what I look for in a review. Thanks to Ant for that. I also think that Sixela's technically informed contribution and Dave's warm personal endorsement of Neil the person are truly in the spirit of SGL and have been a pleasure to read.

It only remains for me to put in a last minute request to Santa...

Olly

[Rossco72]

I'll be getting mine after Christmas

[F15Rules]

Thanks Olly, I think you have hit the nail on the head: the spirit of SGL and of the season is surely goodwill towards each other, and a respect for each others' opinions, even if we don't share them.

I see Neil's book as a great fireside read for those all too common cloudy nights (sorry for mentioning the "other" site:D), when none of us can get our scopes out under the stars - whether they are apos/achros/maks/newts/dobs etc etc.

Most of the book (and I have only dipped into it so far), is actually about specific brands of refractor, their strengths and weaknesses, and the theoretical arguments are not the main theme: sure, Neil believes long focus scopes offer specific advantages over shorter ones (whether apo or achro), and I happen to agree with him.

This is because I have had better views of specific targets, ie double stars, through F10 and longer (especially F12 plus) scopes, than through shorter ones, and I also find it much easier to find best focus. In fact, I never felt the need for a Crayford focuser in the past, and some of the best Astro Physics/TV scopes had R&P focusers which worked perfectly. Rather, I think Crayfords are useful for using heavy 2" EPs, primarily for wide field views, but for close up double star work most people use standard barrel 1.25" EPs anyway. So, it's horses for courses.

For the record, my own best ever, or to be more accurate, most satisfying, stellar images have come through my Pentax J80 F12 Achromat..truly a virtually perfect image of double stars to my eyes. But that's only my opinion, and let's face it, when I am observing, that's the only opinion that matters:p!

And the same for you guys.

So, let's not get into the often rather snide and snobbish kind of exchanges sadly often seen on the "other" site, and instead have friendly debates and differences whilst all celebrating what a brilliant hobby we all share.

Happy Christmas to everyone!

Dave

[F15Rules]

I've now had the pleasure of reading 90% of Neil's book..it's a great "dip in and out" read!

It's a thoroughly informative book which revels in refractors of all types, and is very accessible to hobbyist astronomers from total beginners to reasonably fanatical types and everything inbetween..

..er, but their is one short section on Binoviewers which is basically, unintelligible to myself, and I suspect to many other readers, due to it's being riven by technobabble and technospeak - quite unnecessarily in my opinion.

Oh, and it was written by Chris Lord.

Say no more.

Happy New Year everyone, and Neil, thanks for a great book.

Dave

[ChrisLord]

I've shown the book to several recognised experts in the fields of the history of science and astronomical optics. The errors I have pointed out are only the more obvious ones. The first two chapters contain sweeping statements that are just plain wrong, and reveal a complete lack of understanding. This is not the forum to debate such matters. If it had been a paper, they could be debated through a journal, but it is a book, part of a series, so other than bring the errors to the notice of the author, the publisher (which I did) and the readership in general, there is little more I could have done.

Some may interpret what I have done as character assassination. It is nothing of the sort. All I have done is highlight some fairly egregious errors. All of them could have been easily avoided with a modicum of judicious research before writing the draft. Some of the errors were penned in the light of information supplied prior to putting the book to bed, as the author acknowledges.

If you are going to publish statements regarding astronomical optics in a practical astronomy book (may I remind you it is part of Springer's Patrick Moore Practical Astronomy Series) that can be shown, and in one instance proved, to be incorrect, it is unreasonable to expect no one to bring it to your notice, or the notice of the astronomical community.

With regards the so-called "F/Ratio Conjecture". If you read my paper, published in ATM Letters Journal, which can be found on the Index page of my website, and then read the article describing the observational test set up to either confirm or confound my theory, you will see that I have been objective in my approach. I have taken a hypothesis based on how seeing induced defocus affects the perceived focus of a stellar image, and how the system Strehl ratio influences that perception, and put it to an observational test. The outcome does not support the depth-of-focus hypothesis in any way.

Instead of having a go at the messenger, may I suggest you try to grasp the principles under discussion? If having read and comprehended the arguments you disagree, then say so, and tell us why you disagree. Leave personalities out of the discussion, and stick to the facts. There is also nothing to stop any of you repeating the observational test for yourselves. Rather than relying on instinct, try making some measurements.

One last point, concerning focusing mounts. The rackmount on my 3-inch f/16 refractor was made by the late Ron Irving. The rackmount on my TEC140APO is a Starlight Instruments 3545 Feathertouch. On the TEC140APO a 30º turn of the fine focus knob shifts the eyepiece 1 wave. Bearing in mind the 9:1 ratio, this equates to 1/108th pinion turn. 1 wave being 22 micro-inches a full turn of the main focusing knob moves the eyepiece 108 waves or 2376 micro-inches. At 1/55 wave RMS depth of focus = 1078 micro-inches, so ~1/2 turn moves the eyepiece through the DoF range. When focused midway there is ±1/4 turn leeway. The 3-inch f/16 achromatic with 1/6 wave P-V has DoF = 11264 micro-inches, ~10 times that of the TEC140APO. So it only needs a rack with 10 times the racking ratio. The rack pitch is 14tpi. One full turn moves the eyepiece 14 teeth or 7/8" or 78 times the DoF. Since there is no fine focus knob, to move the eyepiece through half the DoF takes only ~2º turn of the knob. Clearly there is no room for backlash!

Focusing a traditional high f/ratio achromatic refractor is a far more awkward task than focusing a modern low f/ratio apochromatic refractor. Of course, there is nothing to stop you fitting a fine focus precision rackmount to a classical achromatic. Just bear in mind that old achromatic refractors were supplied with basic rack and pinion focusers, that no matter how well made, give nothing like the focus control of a rackmount like the Starlight Instruments Feathertouch, or rackmounts with a similar specification.

[gman007]

my! it seems that this book has created quite a stir.

i shall have to buy a copy and see for myself.

i do get the impression though having looked through all of the threads - some quite to the point, some just plain techno rambling (Chris Lord) that someone is grinding an axe:icon_eek:

may the 4th be with you!

Gman

[John]

If Chris Lord has so many reservations about Neil's work, I can't help wondering why he allowed his piece on Binoviewers to appear in it

I'm sure Neil has not used it without Chris's permission.

[Peter Drew]

Come on folks, its a New Year now, time to get on with using telescopes and stop worrying about them. I've spent half my life making telescopes, including many long focus refractors, professionally, and using them astronomically as an amateur. I occasionally see errors in print but my first test is "how much does it matter". I currently own a 220mm f12 achromat, a 125mm f15 triplet apo a 150mm f5 achromat and a 90mm f5 flourite. I've not yet had any difficulty in bringing any of them to a good focus so an academic debate about the relative difficulties is of too little interest to me and would respectfully suggest that the protagonists continue, if they wish, via the PM option.

Happy New Year to all, specially Chris and Neil. .

[Steve]

Nicely put Peter

[barkis]

Nicely put Peter

Amen to that.

[GazOC]

Ditto. An apt time to draw a line under all this perhaps.

[RobH]

Nicely put Peter

I quite agree.

[russ]

Ditto. An apt time to draw a line under all this perhaps.

ditto that.

[steppenwolf]

I agree, this thread has run its course and today would be a pretty good time call a halt to it - well Put, Peter

[ollypenrice]

It has. Seems tough on the OP but there it is.

Olly

[A McEwan]

Oh don't worry about me, Olly! I have a piece of paper taped to my monitor now specially for when I'm thinking about posting a review. It praphrases the instructions from a packet of fireworks:

"Light fuse and withdraw to safe distance.... "

Ant

[hubalkj]

Chris Lord's assertions, espescially those about long focus achromats are just plain WRONG!! His remarks reflect an obvious lack of understanding about the subject(s) that Neil ws trying to cover in his well written book.

Like Chris Lord, I too have been involved in astronomy, telescope building and optics. I have owned and tested many different types of telescopes, especailly refracting telescopes. It is quite obvious that Mr. Lord is simply incorrect in his findings which are exhibited by his comments that further indicate a lack of understanding of the content of Neil English's book. If Chris Lord is going to level critique, it should be based on unbiased information and sound scientific principle. Case in point is his so called "focal length conjecture".

Chris Lord's vitriolic attack against Mr. English further solidifies his complete lack of understanding about many things, including the fine art of tactful scientific debate. Then again, how much tactfulness can be expected from those who hind behind the veil of the internet only to post vitriolic comments about a subject of which they possess little to zero knowlege and an irrational, illogical obsession with one particualr type of telescope!

Chris Lord and others who feel that they must consistently use "trash can" tactics when debating the merits of a particular subject should merely be toally and completely ignored!! I have read many of Chris Lord's incorrect forum postings and other "articles" and have repeatedly cited these errors to Mr. Lord in a tactful manner. In every case, I received not a single reply, which I fully expected to be the case given Mr. Lord's continuing predilection toward irrelevancy.

Neil, if you are reading this thread, congratulations on a well researched and thorughly enjoyable book!

[johninderby]

Why does this remind me of discussions I've seen on Cloudy Nights

I don't really know Neil other than having exchanged emails on a few subjects where I've always found him helpful and ready to discuss other points of view.

So in this thread I'm on Neil's side. Definitely a top bloke .

John

[Rossco72]

Why does this remind me of discussions I've seen on Cloudy Nights

I don't really know Neil other than having exchanged emails on a few subjects where I've always found him helpful and ready to discuss other points of view.

So in this thread I'm on Neil's side. Definitely a top bloke .

John

John,

On the occassions I've had the plesure of Neil's company he has been enthusiastic and passionate about his subject and I think that is reflected in the book.

It's obvious Neil's own leanings are towards longer focal length scopes but I don't think it stops him appreciating or giving sound opinion on faster APO's.

I'm about half way through Neil (Having obviously jumped a bit to my own scope!!) and it's an enjoyable read.

Cheers

[hubalkj]

Neil and I have also exchanged many pleasant and informative emails about the subjects covered in his book. I too am firmly on Neil's side. Having read over the content many times, I could find nothing in his book that was glaringly wrong. As I stated in my earlier post, Chris Lord's vitriolic review was filled with incorrect statements based purely on a clear and perceived bias instead of pure scientific knowlege. In my opinion, it is time for this subject to be put to rest!

[Whippy]

I thought this got laid to rest some time ago, clearly not so I think it's time to lock this and let it rest in peace.

Tony..