Glass spider

[kalasinman]

@James. My 600mm prime focus lens is 75mm and f/5.6. I fail to see how buying a 600mm f/5 frac is getting me anywhere. I can catch a lot more photons per imaging session with 200mm aperture, than 80mm, yes? The GSO 200 would cost me 405 USD in my hands.

The ZEQ25GT has proven tracking ability beyond it's advertised capacity. Combined with it's modest weight and a moderate price, it's a steal. That's why they are selling so fast (except in the UK).

[long_arms]

It will definitely be interesting to see this project. Hope you post it on here  ! Will you be cutting the central hole yourself? 

I have to say coated optical windows, of very good quality apparently cost A lot. There was a very good topic on that other forum you've mentioned but unfortunately its unreachable atm. 

This quote from Texereau is regarding making one yourself but still quite interesting:

"On balance is the effort justified? The answer is NO if the telescope is not otherwise optically perfect in the sense already explained, and if the worker is not accustomed to judging stellar diffraction images in 10inch or larger instrument. More specifically, beginners or users of a simple 8-inch standard instrument have little reason to stray into this laborious project. Need we stress the sheer folly of capping the telescope with a commonplace slab of glass such as a Saint-Gobain porthole plate, in the hope of improving the image?"

I reckon that unless you spend a lot (too much) money on a window, it will have a negative effect on the light...BUT the improvement from lack of tube currents may give you a net gain. Plus no diffraction spikes! 

Therefore, curved spider vanes and some of this stuff over the end www.amazon.com/Baader-Planetarium-Turbo-Film-127x51cm/dp/B0075ZR7IG  vs a reasonably priced optical window would be a good experiment imo. Close call. 

I personally want to make my own 12 inch mirror/scope + an optical window to go with it one day. Although I haven't been brave enough to ask for a quote for the window blank, I expect it to be far more than the mirror blank. 

Dan

[RikM]

@James. My 600mm prime focus lens is 75mm and f/5.6. I fail to see how buying a 600mm f/5 frac is getting me anywhere. I can catch a lot more photons per imaging session with 200mm aperture, than 80mm, yes? The GSO 200 would cost me 405 USD in my hands.

The ZEQ25GT has proven tracking ability beyond it's advertised capacity. Combined with it's modest weight and a moderate price, it's a steal. That's why they are selling so fast (except in the UK).

How much light you put on each pixel depends only on the focal ratio of the optics used. If your GSO 200 is f/5 then yes, it will be a little faster than your f/5.6 lens. If the GSO is f/4, then I am glad you like DIY because these need some tinkering to get them working sweetly.

The ED80 refractor is insanely good for the price. I don't recall seeing any image from a comparable focal length camera lens that gives the same sharpness, contrast and colour correction. Perhaps an L series Canon prime lens might do it, but they cost an awful lot more than £600!

[Gina]

I'm afrad it's pretty much the usual case of you get what you pay for.  If you want the sort of quality you get from a Mak-Newt it will cost you at least that sort of price.  Probably more as it seems to cost more to make you own than buy a professionally made one.  Sorry to be negative and I sincerely wish you all the best for you project.  Personally, I like doing things for the fun of doing it rather than saving money and many others do too.

[timwetherell]

Another issue is the effect of the extra glass. The glass I am considering , according to published specs, passes 96-99% of light. Reflection less than .35% due to coating.

So a question that remains unanswered in my mind at least, is whether the 4-6 wavelength flatness of the glass will appreciably affect the quality of the image. As this is neither a lens or a mirror, I could speculate the the effect could be far less than one might think.

I think it will have minimal effect myself. The amount of refractive deviation is determined by the angle of the glass surface the parallel ray strikes, so if you're say 2000nm out over say 200mm I wouldn't think it would be all that bothersome because the angle will be miniscule. It's a very different situation to a mirror being 4 waves out of figure!

One reason refractors are very forgiving of thermal deformation is that the a refracted ray is deviated  by half the incident angle whilst for a reflected ray it's deviated double the incident angle. 

If it were me, I'd be tempted to try it as it comes before spending a bucket of money having it polished dead flat

As for the 99% transmission, make your primary mirror 1mm bigger and you won't care!

all the best

Tim

[Peter Drew]

I have a 12" optical window made by David Hinds. It's about 1/2" thick and has a 1" central hole, could suit someones project ?

[timwetherell]

The window be a few waves from flat on each side since this forms a very long focus lens the surface muts be optical smooth.   What is critical is that the image formed this  "lens"  needs to be smaller then the Airy disk formed by the mirror. 

Yep, well said. Exactly !

[Ant]

I've heard a lot of opinions, and welcome them. What part of "I can't afford to spend over 1k USD on a scope" don't you folks understand? I've tried to make this obvious. Is humiliation part of SGL? Bah, nevermind.-thanks so much-Jack

Jack,

I have just read through this thread and cannot see that you have stated that you have a sub USD$1000 budget. We don't humiliate people on SGL, the only person in this thread that I can see that hasn't been pleasant is you!

[kalasinman]

My fault as I forgot to give a budget figure, although I somehow assumed since almost every post around this subject on this site I have made has been about cost that my position would have been obvious. Again entirely my fault. I also assumed that posting in a DIY forum would make my desire to do it myself obvious. This would , it seems to me, make postings about ready made scopes off topic, yes?

[Earl]

Well most off the shelf scopes need a fair biy of DIY to get it to where you want

[JamesF]

@James. My 600mm prime focus lens is 75mm and f/5.6. I fail to see how buying a 600mm f/5 frac is getting me anywhere. I can catch a lot more photons per imaging session with 200mm aperture, than 80mm, yes? The GSO 200 would cost me 405 USD in my hands.

The ZEQ25GT has proven tracking ability beyond it's advertised capacity. Combined with it's modest weight and a moderate price, it's a steal. That's why they are selling so fast (except in the UK).

Assuming the 600mm lens is of good quality then I agree that there seems little point in having another OTA that is broadly similar in terms of of optical specification.

I think attempting to replace the spider with an optical window could be a fun project, and if you do proceed I shall certainly follow your progress with some interest.

What I'm not convinced about is that it will give you the results you're after.  Let's say for the sake of argument that you put a reducer/flattener on an ED80 and compare that with the 200 newt so they'd be fairly close in terms of focal ratio.  Because the focal ratios are similar, photons would arrive at each photosite at broadly similar rates so there's probably no advantage in the larger scope there.  The larger aperture will give better resolution, but actually the ED80 will probably resolve close to 1 arcsecond and unless you're up a very high mountain or have an exceptionally steady atmosphere I'm not sure you'll really achieve much better than that.  What I think the newt might give you is a larger illuminated field at the camera, but I honestly have no idea how much coma correction you'll need to do or how flat the field will be to make that more useful than what you'd get with the ED80 and flattener.  I think I have things mostly right there.  I'm a little short of sleep today and not feeling entirely on the ball as a result.

Alternatively, if I really wanted 200mm of aperture without a spider (and this does push the bounds of your budget a little) I might be tempted to look at a C8 with the f/6.3 focal reducer (again I'm not sure quite how large a flat field you'd get with that -- it's a bit of a "left field" option and probably best avoided .

I wouldn't in the least want to dissuade you from building a newt with an optical window if  that's what you really want to do.  As I said, I'd be very interested to see how you get on.  Good luck with the project if that's the way you decide to go.  Please do post with your progress.  I'm struggling to convince myself however that it really achieves anything in terms of imaging that couldn't be done more easily and within budget (perhaps more cheaply) another way.

Interesting information about the iOptron mount.  It looks to me as though it should fit quite nicely between the EQ3-2 and HEQ5 which would make it tempting for me for taking on holiday.  The HEQ5 is a bit of a handful to take to France in the car with everything else.  I wonder why it doesn't seem that popular in the UK?  Too close in price and not close enough in spec. to the HEQ5, perhaps?

James

[kalasinman]

@James thanks for your support .

Technical points, a fair amount of surface non -flatness may not be significant if the surfaces are parallel, the imperfections of the surface will cause diffraction and the elimination of the spider will reduce diffraction. I'm guessing that regarding contrast, I will be no worse, and possibly better overall.

As a proof of concept, if I make this and the results give cause, I can have a window of high quality out of stock from another supplier for under $300

I've done a risk assessment, and I just don't see a downside, other than potentially wasting $48. On the plus side, I'll have fun regardless of the result, nothing that will have been done cannot be undone easily, and either way, I'll learn something that apparently there isn't a common body of knowledge about. This will benefit myself and hopefully others.

Please keep in mind, I only posted here to discuss, and possibly get some help with the issue of a DIY optical window to replace the spider.

[kalasinman]

@James, one other thing . The idea that you can make a small scope the same as a large scope with a focal reducer is a common myth. Yes by using a focal reducer you can add more photons, but these photons won't be of the object of interest. They will result from a wider field of view. Great if you are having trouble getting above the read noise, but not helpful if all those photons have to be cropped out to get the desired image. If it was that simple we wouldn't need to build the Keck and others like it.--Jack

[michael.h.f.wilkinson]

If the surfaces of the window are parallel but not plane, the window may well introduce problems (by acting like a meniscus lens locally). It may not be better than using curved vanes ultimately. If the radius of curvature is long enough at each point, the effects may be small. This is one problem with specs of float glass. If the entire glass is ever so slightly curved (say, spherical with the centre 4 wavelengths below the edges), I would not expect problems. If the surface has many shorter wobbles of the same amplitude (4 wavelengths) it is probably unusable.

A really good quality optical window should see improvements, however.

[kalasinman]

@michael...

"...  the imperfections of the surface will cause diffraction and the elimination of the spider will reduce diffraction. I'm guessing that regarding contrast, I will be no worse, and possibly better overall.

As a proof of concept, if I make this and the results give cause, I can have a window of high quality out of stock from another supplier for under $300

I've done a risk assessment, and I just don't see a downside, other than potentially wasting $48. On the plus side, I'll have fun regardless of the result, nothing that will have been done cannot be undone easily, and either way, I'll learn something that apparently there isn't a common body of knowledge about. This will benefit myself and hopefully others."

[kalasinman]

'Been trying to think of a low tech way to compare one method with the other. Comparing star pictures ? Seems difficult due to variations in conditions. I was thinking that using a DSLR at prime focus to image a sheet of news paper at a distance and compare might be useful.-Jack

[Gina]

Yes, I've used that method to assess cameras and lenses in the past.  Plus special charts to measure resolution of various parts of the FOV. Lines at various spacings and radial lines to assess astigmatism.  But yes, a newspaper makes an easy way of getting an empirical idea of lens quality.

[kalasinman]

Assuming the 600mm lens is of good quality then I agree that there seems little point in having another OTA that is broadly similar in terms of of optical specification.

I think attempting to replace the spider with an optical window could be a fun project, and if you do proceed I shall certainly follow your progress with some interest.

What I'm not convinced about is that it will give you the results you're after.  Let's say for the sake of argument that you put a reducer/flattener on an ED80 and compare that with the 200 newt so they'd be fairly close in terms of focal ratio.  Because the focal ratios are similar, photons would arrive at each photosite at broadly similar rates so there's probably no advantage in the larger scope there.  The larger aperture will give better resolution, but actually the ED80 will probably resolve close to 1 arcsecond and unless you're up a very high mountain or have an exceptionally steady atmosphere I'm not sure you'll really achieve much better than that.  What I think the newt might give you is a larger illuminated field at the camera, but I honestly have no idea how much coma correction you'll need to do or how flat the field will be to make that more useful than what you'd get with the ED80 and flattener.  I think I have things mostly right there.  I'm a little short of sleep today and not feeling entirely on the ball as a result.

Alternatively, if I really wanted 200mm of aperture without a spider (and this does push the bounds of your budget a little) I might be tempted to look at a C8 with the f/6.3 focal reducer (again I'm not sure quite how large a flat field you'd get with that -- it's a bit of a "left field" option and probably best avoided .

I wouldn't in the least want to dissuade you from building a newt with an optical window if  that's what you really want to do.  As I said, I'd be very interested to see how you get on.  Good luck with the project if that's the way you decide to go.  Please do post with your progress.  I'm struggling to convince myself however that it really achieves anything in terms of imaging that couldn't be done more easily and within budget (perhaps more cheaply) another way.

Interesting information about the iOptron mount.  It looks to me as though it should fit quite nicely between the EQ3-2 and HEQ5 which would make it tempting for me for taking on holiday.  The HEQ5 is a bit of a handful to take to France in the car with everything else.  I wonder why it doesn't seem that popular in the UK?  Too close in price and not close enough in spec. to the HEQ5, perhaps?

James

@James, you might find this interesting.

Model               £       Mount kg   Payload kg   Objects      Resolution     PE (peak)

HEQ5                 £759        10              11-15         42,000       0.144 Arcsec       30 – 6*

ZEQ25                £849        4.7               12            59,000       0.14 Arcsec         15 – 2*

Because of the unique design, the 12kg is available for imaging not a % of that.

The weight of the mount with tripod is less than the weight of the HEQ5-Jack

[rfdesigner]

Just to to all this.

I own a 12" windowed newt.   The window was not cheap.

I had the window ground to special order many years ago and it was a little more than a comparable mirror, but not as heavy as it can be thinner.  (mine is 18mm thick, I wouldn't want thinner as it would warp and bow causing optical problems)..   the biggest advantage is that any surface eddies in the air at the top of the window have only half the optical effect as those above a mirror as the light passes that point only once.  With the rest of the scope sealed and settled this can improve high resolution image quality..  but if it's all warm it takes much more time to cool down.

If I was starting over again I would put all the money into a better/bigger OTA..  bang for buck conventional newts are very hard to beat.  That said, it does work fairly well on controlling tube currents once cooled down so it's best to leave it in an obsy but suffers like most SCTs from dewing (unlike newts).

Derek

[rfdesigner]

Additionally, I just remembered, Glass refractive index varies across a piece, when grinding you are trying to make the optical thickness an equal number of wavelengths at all points in the window, that is not necessarily a constant thickness, although you'd never notice just by looking.

point is: making sure both sides are flat isn't enough.

[kalasinman]

@Derek, point taken. Thank you. At this point I have no interest in grinding glass. I'll settle for what I get with the piece from Edmund, do some testing and take it (or not) from there.-Jack

[Astrobits]

Additionally, I just remembered, Glass refractive index varies across a piece, when grinding you are trying to make the optical thickness an equal number of wavelengths at all points in the window, that is not necessarily a constant thickness, although you'd never notice just by looking.

point is: making sure both sides are flat isn't enough.

Glass does not vary in refractive index unless there is some variation in composition or there is some frozen-in strain. It is for this reason that glass for lenses and optical windows has to be vary carefully made and annealed. An optical window will take as much effort to make as a lens of the same diameter, hence the cost.

The old method of making glass sheet--PLATE GLASS-- often resulted in some areas of the sheet being good enough for some optical applications but the new FLOAT GLASS does not have the same optical quality. You can select some FLOAT glass for polishing by examining it under crossed polars. This will show up any strain and those areas can then be avoided.

Nigel

[timwetherell]

Additionally, I just remembered, Glass refractive index varies across a piece, when grinding you are trying to make the optical thickness an equal number of wavelengths at all points in the window, that is not necessarily a constant thickness, although you'd never notice just by looking.

point is: making sure both sides are flat isn't enough.

If the local refractive index change was over the full thickness then it's probably not a big issue because the rays are parallel and it won't change the refraction angle. But if there are blobs that vary with depth (which is more likely) then yes, what you have is mini lenses within the window. I know TEC stopped making some big refractors for just that reason, many of the optical blanks weren't homogenous and of course you only find out once you've finished the polish and tested the lens. Guess they have a lot of expensive coasters kicking around the shop  

[kalasinman]

I really welcome any on topic opinions. As there is only one contributor who has any hands on experience with this idea, and his remarks were conditionally  positive (thanks again Derek), I am leaning towards the conclusion that I should risk the princely sum of $48 and a few hours of my time.

Obviously, there is little agreement on much of anything in this hobby, so I might think results are worth the effort after testing, while another person would find them unacceptable.

The big surprise to me, as a result  of starting this topic, is that there is so little hands on information available. To me, that in itself makes the endeavor even more attractive.--Jack

[Gina]

Go for it I say   If you don't try you'll never know.  Many discoveries have been made by trying seeming daft ideas