Andrew_B

Alcohol is absolutely fine, even on fluorite (which is more chemically resistant than many ED glasses), so long as you avoid stuff like surgical spirit which contains additives that would leave residues on the lens. Alkalis such as ammonia solution should be avoided.

Modern hard coatings are made of materials like zirconium dioxide which is very hard and durable - it's used as a thermal barrier coating on turbine blades in jet engines so it's pretty tolerant of harsh environments.

You can buy little disposable cotton cloths that work well. I bought a bag of 100 that wasn't very expensive at all.

I've got an FS-60CB and the green/magenta colouring is perfectly normal either side of focus but doesn't stop it performing very well and delivering high contrast images in focus. Sometimes when the seeing is bad I use that false colour to focus because it's hard to see the fleeting fine details that would normally be used to judge whether the scope is focused - essentially I'm moving the focus back and forth until green disappears without showing magenta or vice-versa.

On the subject of star testing, I found this quote by Roland Christen about its limitations when testing apos rather than achromats:

Quote

Identical inside/outside rings in an achromat is normal. This is due
to the nature of chromatic aberration, which take all colors, except
green, far away from focus, where they cannot interfere with the
diffraction pattern. In an Apo lens, red, yellow, green and blue are
all very close to focus, but not perfectly, exactly at the same
identical focus. These colors tend to interfere with the perfect
diffraction pattern. Every Apo lens I have ever star tested, even
ones that tested close to 1/20 wave P-V, shows different inside and
outside patterns of interference. This is normal. By contrast, I have
an 8" SCT that shows perfectly identical inside vs. outside
diffraction patterns, but tests only 1/4 wave.

The real test of an optic is not so much how the diffraction pattern
looks outside of focus, rather, how much extraneous junk is floating
around a star when it is in focus. (in my 8" SCT there is quite a lot
of that). In an achromat with perfect optics, there is a lot of
unfocused blue and red light around any object. The brighter that
object is, the more that light interferes with the image. One saving
grace for achromats is a set of filters. It does not "fix" the
chromatic aberration, rather it lets you see the image in one color
in monochrome very sharply.

7 hours ago, Highburymark said:

The question here is why should the average 60mm F/6 scope have better colour correction than a 100mm F/6 scope, assuming they are doublets and use the same glass? I have now read conflicting reports on this. One says the light bends less with the smaller scope - hence is less liable to false colour. Another claims that scopes with the same focal ratio should be equal in CA, no matter the aperture - as Andrew states in his first post above. 
If there is a difference in CA between the two scope sizes, I don’t think it’s because light is bending more. But perhaps it’s because overall, more light is bending? The larger glass is refracting a greater amount/surface area of light away from the central axis of the objective lens. 

Absolutely. More tests to do.

Just a small correction. My understanding is that if you have two scopes of the same design with different apertures and the same focal ratio then the smaller one will have less false colour. In order for the bigger scope to have the same CA as the smaller one, its focal ratio would have to increased by the same amount as the increase in aperture.

1 hour ago, Space Hopper said:

Interesting, but the Tak TSA120 is a triplet and all the others are doublets.

A well designed and figured triplet will always out perform a comparable doublet ?

Provided the optics don't differ in terms of using more advanced designs such as all-spherical vs aspheric or foil spaced vs wide air gaps, a well made triplet will be better than a doublet using the same ED glass. A triplet can also let you get away with using cheaper glass, so an FPL-51 triplet can be as good or better than an FPL-53 doublet of the same focal ratio. The downside is that you've got more work and cost to grind, polish, and coat 6 surfaces rather than 4 and the extra glass and larger lens cell can significantly increase the weight and cool down time.

Edit - aside from controlling simple chromatic aberration, having the additional element of a triplet can apparently be useful in correcting spherochromatism and might allow the designer to reduce other aberrations as well.

On 01/01/2022 at 02:16, Highburymark said:

I remember this diagram but didn’t remember the relative difference in CA control between big and small apertures. It certainly suggests my experience with the 60ED is not particularly unusual.
But why is a good question. Is it easier to manufacture smaller aperture lenses accurately to iron out false colour, or is the light being bent less? A quick Google search suggests it’s the latter. 

From what I understand, all things being equal the focal ratio needs to scale with the aperture in order to maintain the same level of colour correction. That would mean a 100mm doublet scope would need to be an f/10 to have as little false colour as an equivalent 60mm f/6, which sounds similar to your experience.

The details are way over my head but I get the impression that when designing a lens you have a complex set of compromises and an optician needs to decide how to balance the different aspects of performance. If you were creating a scope aimed at the visual observer then you might choose to maximise resolution in green light to correspond to the peak of the eye's sensitivity and allow the lens to be much less corrected in violet or deep red due to our relative lack of sensitivity to those wavelengths. I suspect that might have been done with the FS-60 because although it's a superbly sharp and contrasty visual scope, it's obviously a lot less well corrected at short wavelengths, especially compared to newer designs from Tak like the FC-series. Correcting the g-line to get rid of blue bloat would likely appeal to imagers but it could slightly lower performance for the visual observer or might impact correction in the yellows and reds which could show up at high magnifications when doing planetary observing.

A cheaper alternative to the 24mm Panoptic would be the 24mm APM Ultra Flat Field eyepiece or one its clones sold under various other brand names. I've got one on order after reading a lot of positive reports about it on here and CN forums, and after being very impressed by the 10mm model in that range (which I got a pair of for an absolute steal). Orthoscopics are worth considering and can give incredibly good views without breaking the bank, although the FOV is relatively narrow and eye relieve can be an issue at shorter focal lengths.

I bought a couple of Svbony UFF 10mm eyepieces and I love them. They're sharp and bright, very compact, and you can pick them up for very little money - mine were only £75 for the pair including delivery.

The design is part of a range that were originally commissioned by APM and sold as "Ultra Flat Field" eyepieces, and are available in focal lengths of 10mm (60 degree AFOV), 15mm, 18mm, 24mm (all 65 degree), and 30mm (70 degree). Svbony only sell the 10mm and 18mm models under their brand but some or all of the other focal lengths are available from brands such as Celestron, Altair, Meade, Orion, Tecnosky, Sky Rover, and Artesky.

They were designed to work with scopes down to f/5 so they should be okay with your setup. I've not tried any of the other focal lengths in the range although I have ordered the 15mm and 24mm models (Sky Rover brand) so I should get a chance to test them soon.

21 hours ago, dannybgoode said:

The full clean, collimate and test is £100.  RVO have a full clean room now so saves sending it away for a test

That's not bad actually.

So the collimation service only relates to the alignment of the lens cell to the telescope? I'd been under the impression that collimation also adjusted the position and/or spacing of the lens elements but it makes sense that it wouldn't involve something as complex as that.

8 hours ago, dannybgoode said:

Wasn't it down to a mis-calibrated measuring tool?  Think the thing they used to measure the curve was reading wrong.  Luckily it was very accurately and consistently reading wrong hence it was the most accurate mistake ever manufactured!  A smoothness of 10nm over 2.4m is impressive!

And of course the blanks for both the PerkinElmer and Kodak blanks were produced by Corning.  Nearly everyone who has ever owned a modern mobile phone will have stroked and fondled Corning glass  

It was the setup of the measuring tool that was wrong and the fact that P-E ignored the results of other tests performed using different equipment that showed an error which is pretty poor QC and project management  on their part to only rely on a single instrument to do the final testing of such a vital component. NASA was also at fault for a lack of oversight and allowing a deficient testing regime to be implemented.

Back on topic,  is a zygo test expensive to perform?

20 hours ago, Don Pensack said:

That was a 2" Barlow with an ED glass lens from Barsta and sold under many many labels  (Knight Owl, Olivon, etc.).

It was one of the first Barlows in 2" that I saw that had a removable lens that could be threaded elsewhere.

You could use it on the bottom of the eyepiece (1.5x), in its own tube (2X), on the front of the 2" star diagonal (2.6x) or in its own tube in front of the star diagonal (~3x)

And, of course, threaded to the bottom of a 2"/1.25" adapter for 1.5-1.6x with 1.25" eyepieces.

I agree, it was a good lens.  I haven't seen them for a while.  I wonder if they're being sold under other labels.

Looks like it's still being sold, assuming this is the same lens.

1 hour ago, dannybgoode said:

This is precisely what happened with Hubble's mirror. There is a misconception that the mirror was badly ground somehow when in fact it was essentially as perfectly smooth as they could make it given the constraints of the technology but was the wrong curve. 

Indeed it was because it was so perfectly wrong (it that makes sense!) that they could make the corrector lens. 

I thought about the Hubble example last night. Very accurately figured and polished but the wrong shape.

It was such a ridiculous piece of mismanagement. The Hubble spec was changed from a 3m mirror to a 96" (2.4m) one to take advantage of the existing production facilities and experience from the KH-11 spy satellite programme, but those mirrors were all made by Kodak (now L3Harris) and still are. Kokak made the backup mirror which was perfect and is now in the Smithsonian, but PerkinElmer who were given the primary contract had never built a space-qualified mirror larger than the 26" optics produced for the KH-9 Hexagon and it shouldn't have been the biggest surprise that they made a serious mistake.

I treated myself to a Scopetech Mount Zero a few weeks ago and I couldn't be happier with it. Very lightweight and simple but surprisingly sturdy and a real joy to use.

Like other posters have said, it is sensitive to the balance of the scope but it's easy to tell if that needs adjusting because you can feel that it takes more effort to move the slow motion controls in one direction than the other. It's also worth taking the time to adjust it to your liking which can be done via the clutches and also by adjusting the brass nuts on the ends of the slow-mo mechanisms. Be subtle and take your time with it because you only need tiny adjustments of a fraction of a turn to find the right level of friction and to take out any slack in the mechanism, but it's easy to do with a couple of 13mm spanners and makes a real difference to the smoothness and accuracy of the mount.

3 hours ago, John said:

I think green light measurement is a little harsher than red, at least where visual observing is the intended use.

The real proof is in the star test though and how challenging "on the edge for the aperture" targets are handled, IMHO.

 

 

Good point. Green light is obviously shorter wavelength so 1/50 lambda RMS at 532nm is therefore a more demanding standard than at 633nm. The highest specification I've seen quoted was 1/300 lambda at 193nm for the lenses used in lithography equipment which must be extraordinarily expensive to make (350mm fluorite blanks anyone?).

I get that Strehl ratio is a direct measure of the 'quality' of the image produced by the lens based on how the peak intensity of the Airy disc from a point source compares to the theoretical intensity from a similar perfect optic. What I'm not clear about is whether the RMS value is measuring the surface roughness of the lens surfaces or whether it relates to aberration of the wavefront. It strikes me that you could have a lens that's been figured and polished to an incredible level of smoothness, but it could also be the wrong shape to produce a good image.

It’s impressive that the Strehl was so high in res light given that you’d expect the lens to be optimised to deliver its best performance in green light. 
 

I doubt the little peak near the edge has much effect on overall performance. There’s a reason very high end optics always have their surface accuracy specified as the RMS value rather than P-V. 

I had a look at the different versions of the 2" ClickLock eyepiece holder and there seem to be three M54 thread fittings. One is female and 0.75mm pitch for Bressar, Explore Scientific and Omegon (part #2956253), while the other two are male with 0.75mm pitch for TS-Optics and Skywatcher (part #2956251), and 1.0mm pitch for Skywatcher and Orion (part #2956254).

From your other posts it looks like your adapter (#2956254) has a 1.0mm thread pitch while the extension rings are the finer 0.75mm pitch. If those rings fit onto the focuser draw tube then it sounds like you need the M54x0.75 version (#2956251) of the ClickLock clamp to match.

There's a chart on this page showing the different models and thread fittings.

I'd also go for a small apo refractor in this situation for the high quality views it can deliver as well as the convenience of a relatively compact and rugged package that shouldn't ever require you to worry about things like collimation. Pair it with a zoom eyepiece and a suitable mount and you've got a scope that can be left permanently set up to grab a view of something interesting at a moment's notice.

I've got a 60mm refractor which I keep permanently mounted to a Scopetech Mount Zero on an old but sturdy Manfrotto tripod. The Scopetech is a superb mount for this kind of dual-use setup with its simplicity and light weight, but it also gives very steady views and has the convenience of being able to instantly swap between using the slow motion controls and just turning the mount by hand so it only takes seconds to get lined up on a target. Only downside is that it's not cheap, but a mount like that should give many years of enjoyment and is a good investment if your budget can stretch to it.

My little scope is a Takahashi FS-60CB and I have an extender module for it so I can use it in either a 355mm f/5.9 or a 600mm f/10 configuration. With an 8-24mm zoom eyepiece I get a magnification range of around 15-45x or 25-75x which are both very useful, with the former being better suited to the winter months due to there being less light, and the latter giving that bit more reach when the days get brighter in spring and summer. If you get an 80mm scope then the larger aperture allows for proportionally higher magnifications while still enjoying a reasonably bright image so equivalent ranges would be 20-60x and 33-100x.

It's worth noting that while an f/10 optic is practical at 60mm, it's a bit too long a focal ratio when you get to 80mm due to the resulting size and weight of the instrument which would make it a bit unwieldy for use as a spotting scope. A decent f/6 to f/7 ED doublet should fit your needs perfectly.

On 04/12/2021 at 01:02, Rjbram said:

Yes, I agree, definitely worth learning from the youtube tutorials.  the software has not been the easiest. I am on a Mac (and run the camera using raspberry pi) and I know there is not as much astrophotography software for Mac OS. I have tried SiriL, which seems to crash or just fail to complete the process too much. Affinity Photo has a stacking routine which I am experimenting with, and getting mixed results. Colors look odd. The best, in my hands so far, is the ASI studio software, which is extremely basic, but seems to produce the best colors by default. I am considering purchasing a modest Windows laptop or mini PC, to see if the windows world is going to cause more joy in the long run. 

I use Astro Pixel Processor on my Mac which does a great job of stacking and has really good processing tools that can work wonders at getting rid of light pollution, gradients, vignetting, and Moon glow. For final enhancement I've found Affinity Photo works really well and does the same stuff I did with Photoshop but at a fraction of the price.

Insulating the tube apparently goes a long way to solving the thermal issues with Cats.

As to their reputation, I think it's suffered in part because high-end models (Intes Micro, A-P, TEC) have been very rare instruments that few will have had a chance to use while the opinions of most users have been  informed by their experiences with mass-market SCTs. Although modern ones from Celestron in particular seem to be pretty good, the ones being built in the 70s/80s/90s that cemented the reputation of the SCT were a lot more hit and miss due to poorer quality control that let far too many lemons slip out of the factory.

On 13/12/2021 at 12:41, Zermelo said:

If you particularly want a 10mm, you might have a look at this one:

https://www.ebay.co.uk/itm/284532210530?hash=item423f70fb62:g:MYgAAOSwLh1hRukg

Discussion here :

https://stargazerslounge.com/topic/386326-ultima-edge-eyepiece/#comment-4170293

 

I bought a couple of the Svbony 10mm Ultra Flat Field eyepieces a few weeks ago and I've been very impressed when using them in both cyclops mode, and with a binoviewer.

There very small and light with no undercuts (hallelujah!), but they feel solidly built and give great views. I got my pair direct from China for the princely sum of £75.40 delivered and I notice Svbony are doing an offer on their website with various products on sale including the 10mm and 18mm UFF eyepieces. I suspect you'd struggle to find better eyepieces around those focal lengths for the money.

The only caveat is that I think Svbony sell direct from China so delivery times are longer than through a UK reseller on eBay and getting a return or replacement in the event of a problem could be a bit more involved.

I notice Svbony are doing a sale on their website with discounts on all sorts including many of their eyepieces. The 7-21mm zoom isn't part of the offer but it was pretty cheap to begin with.

The real bargains are the 10mm and 18mm Ultra Flat Field eyepieces which are selling for a fraction of the cost of the same optics sold under other brands.

18 hours ago, Trentend said:

Thanks all. I’ll perhaps let it defrost (it was very cold last night) and pick up a strap wrench from screwfix tomorrow to try. The reducer comes in two parts. The bit with the lens fortunately screwed out easily enough. What’s left is an extension adapter thing on the SCT thread. 

Temperature is often the issue in cases like this. If the scope was cold and the reducer was warmer then the metal of the focuser tube will have contracted relative to the reducer. When you thread the reducer on, that small change in size can be enough to allow you to tighten it further than normal so when it cools down to the same temperature as the scope it contracts onto the focuser tube and the threads lock tight.

Differential heating can solve the problem and it sounds like you used that method when you blasted it with the hairdryer. The important thing in this situation is that you want to try and heat the reducer (or whatever else is screwed tight) while not heating the focuser tube - another method would be to use a hand warmer or microwaveable heating pad applied to the surface.

19 hours ago, Louis D said:

Put it on dirt/lawn to get decent dampening if you don't have any pads.

A lawn is also helpful if you drop anything. Things are much less likely to be damaged by a fall onto grass than onto concrete (and that includes the human body in case you trip over in the darkness!).

Secondly, a lawn doesn't heat up as much in warm weather as bare ground, concrete, or asphalt so the air above it should be a bit more stable.

My first ever view of Saturn was through an 1869 6" Cooke refractor shown in the photo below (sadly I'm not the dapper gentleman in the image!). It's an amazing feeling to look through even a small scope and see detail on another world with your own eye rather than just looking at photos from professional observatories or space probes.

7 hours ago, mikeDnight said:

I think that Tak expect Tak users to use Tak eyepieces, Tak diagonals and a Tak barlow or extender-Q. Then presumably all will work flawlessly.  Back in the real world, I've found the 2X SW Delux barlow to be excellent (and cheap). Removing the blue spacer tube may well require the use of the black extension tube, and so doesn't resolve all potential focusing issues. You'd imagine that by now Takahashi would have had the intelligence to notice there is a problem with the FC100DC, and shorten the spacer while increasing the length of the draw tube. It is possible to iron out most focusing issues by careful choice of eyepieces and reduces etc. I removed the 1.25" visual back and replaced it with a short Tak 2" back while using a SW low profile 2" to 1.25" reducer, which gave me a little more inward focus.

In effect that's what they did with the FC-100DF.

My FS-60CB has the same focuser as the DC and while I can understand the engineering reasons for its short travel, it can be annoying at time to have to fiddle with adapters to reach focus with certain eyepieces or diagonals.  Having said that, the FT2025BCR on my other scope also needs an extension tube if I want to use my 1.25" prism rather than the 2" mirror diagonal which was a bit of a surprise. I found a really nice 50mm extension and 2" eyepiece adaptor for only 20 quid that matches the Feather Touch aesthetic perfectly but some people might object to having to spend extra money to get their diagonals and eyepieces to work when they've already shelled out so much for the focuser.

Tak obviously reuse components where possible so presumably they don't have a suitable focuser that's small and light enough for the DC but with a longer draw tube, and the next model up is the Sky-90 focuser in the DF and DZ. They could always design a new focuser but that's an expensive proposition, especially if it would only be used in that single model of scope.

The ClickLock reducer has a T2 thread inside and one option would be to thread this onto something like the Baader 2" / T-2 (M48) nosepiece (2458130). I've got both items and the combination gives a measured extension of 27mm so I don't know if that's a bit too much but you'd need to check the range of focus on your scope and be sure that it wasn't creating too much back focus for some of your eyepieces.

I've not been able to find anything significantly shorter. I thought there might be a 1.25" eyepiece holder with an extension that would do the job but they all seem to be longer than the option of threading a 2" nosepiece onto the ClickLock reducer.