Takahashi FC-100DC: skewed Airy pattern

[pystab]

I bought this scope in January. Only in the last few weeks have I tested the diffraction limit by viewing stars at high magnification. Then what I saw was a biased Airy pattern, with each ring brighter on one side of the Airy disc than the other. With Arcturus in the centre of the field I saw 3 or maybe even 4 rings on one side, but only the inner one or two were still visible on the other side. It was definitely in focus: the central disc was small and snapped to focus, and the pattern expanded as you'd expect when defocused either way. Sorry, no photos - I don't have the kit.

The skewed pattern was obvious with my highest-power eyepiece, a 3.2 mm BST Starguider (230x, 59x per inch of aperture, 0.43 mm exit pupil). It was harder to see (because smaller) but still present with a 5 mm Vixen SLV (148x, 38x per inch of aperture, 0.68 mm exit pupil). Both eyepieces were bought new earlier this year. When I first noticed this I was using a diagonal (Baader prism) but it's still there when viewing straight-through without the diagonal. The pattern did not rotate when rotating the eyepiece, but did rotate when rotating the scope in its tube rings.

I have had no previous experience of quality telescopes, so I don't know whether this effect is within normal expectations or a sign of a real defect. I should add that I've been careful when handling the scope and it hasn't had any knocks, nor did it arrive with any sign of damage in transit.

Can anyone comment on what might be going on here - the nature of the problem, what might be done about it, and indeed whether it might just be normal for this scope?

Tim.

[vlaiv]

Sounds like collimation problem.

Repeat star test and observe star at high power at the center of the field. Do slight defocus and look at central bright spot in relation to rings that appear when you defocus - if it is not in center, you probably have problem with collimation of the scope. All rings that appear should be concentric.

If you have problem with collimation, you can either take it to someone who knows how to collimate it, or you can try to do it your self. I've never collimated refractor scope (have done newtonians and RC), but I would personally try to do it myself.

Here is a quick guide how to do it:

http://philjay2000.tripod.com/usefulstuff/adventures.pdf

You can also find other resources on the internet.

 

 

[Doc]

Was it bought new from a retailer or secondhand? If new I would ask them for help, it should still be under warrenty. I agree that all rings should be concentric and the centre disc should be in the centre. Could atmospherics cause this problem?

[Peter Drew]

 These Takahashi's seem to be nothing but trouble these days!    Seriously though, as others have said, it would appear to be a slight collimation error. There should be means provided to adjust it but it must be attempted carefully.

[25585]

Get it assessed professionally at least. If it's considered a simple job, still have a professional do it. Your Tak vendor might know where/how that is possible. Expensive glass, take no risks!

 

[andrew s]

Not sure how well this works with modern anti reflection coated lenses but if you have a Cheshire collimation tool you can check the lens in day light.

With the lens cap on and the Cheshire well illuminated you will (should) see a set of refections from the lens surfaces. These should all be concentric and centered. I used this method with an achromat to good effect.  

Regards Andrew

[Ben the Ignorant]

What you describe is like the patterns in the two photos titled "artificial sky test" in this thread, right?

http://r2.astro-foren.com/index.php/de/component/content/article/10-beitraege/02-ed-optiken-halb-apos-und-frauenhofer-systeme/793-b096a-skywatcher-150-ota-stratravel-fuer-kometen-sternhauf-und-fotografie

This is a lab assessment of a defective doublet, it was badly decentered before the adjustment but still skewed afterwards. Your Tak's pattern is probably like the second artificial sky pic.

Classical decentering. I think Taks have no adjustment screws, is that so? Then you have to send it to a qualified person.

[Timebandit]

 

Sounds like collimation

If new I would have a word with the supplier to see if they can help.

ES Reid is the chap for Fracs if you need it independently checked and adjusted.

Hope this helps

 

 

[Peter Drew]

@pystab.  If you have a laser collimator insert it into the eyepiece holder without the diagonal and check that the beam passes through the centre of the objective. There is an outside chance that the focuser is tilted which could produce a similar effect.

[Piero]

20 minutes ago, Peter Drew said:

@pystab.  If you have a laser collimator insert it into the eyepiece holder without the diagonal and check that the beam passes through the centre of the objective. There is an outside chance that the focuser is tilted which could produce a similar effect.

I used this approach to check any misalignment of the focuser in my TV60. Dead easy with the Glatter laser and square attachment. 

[John]

If you bought the scope new I would get in touch with the vendor.

I have the Tak FC-100DL and the star test has been exemplorary from new. The diffraction patterns on mine seem more or less identical either side of focus both in terms of the number, relative brightness and concentricity of the diffraction rings around the airy disk.

Doing the star test with a green filter in place can help get clearer perceptions of the diffraction patterns I believe.

 

[FLO]

Just now, John said:

If you bought the scope new I would get in touch with the vendor.

It isn't one of ours (we weren't retailing Takahashi back then) but I can help. 

I suspect Peter Drew's suggestion will most likely find the cause of the problem but if not you will need to contact your supplier. 

Worth noting Takahashi don't consider collimation to be a warranty issue but they will recollimate a telescope free of charge within five years of it's purchase. Here in the UK that means returning it to Takahashi's service centre in France. A good retailer (i.e. FLO and Ian King) will handle and cover the cost of returning / redelivering the telescope, if purchased from them, during it's first two years. Beyond that you will need to return it yourself. After five years Takahashi will still recollimate a telescope but charge for the service (I don't know how much). 

HTH, 

Steve 

[Doc]

16 minutes ago, FLO said:

It isn't one of ours (we weren't retailing Takahashi back then) but I can help. 

I suspect Peter Drew's suggestion will most likely find the cause of the problem but if not you will need to contact your supplier. 

Worth noting Takahashi don't consider collimation to be a warranty issue but they will recollimate a telescope free of charge within five years of it's purchase. Here in the UK that means returning it to Takahashi's service centre in France. A good retailer (i.e. FLO and Ian King) will handle and cover the cost of returning / redelivering the telescope, if purchased from them, during it's first two years. Beyond that you will need to return it yourself. After five years Takahashi will still recollimate a telescope but charge for the service (I don't know how much). 

HTH, 

Steve 

That's really interesting information Steve. Do other manufacturers such as Skywatcher offer the same service?

 

 

 

[25585]

If an ES Reid (or any other so qualified) test and service could be bought/provided/channelled through FLO, that would be a great service to all refractor owners. We can often ascertain that something is not right, but DIY beyond that is risky.

[FLO]

6 hours ago, Doc said:

That's really interesting information Steve. Do other manufacturers such as Skywatcher offer the same service?

None that I am aware of. 

6 hours ago, 25585 said:

If an ES Reid (or any other so qualified) test and service could be bought/provided/channelled through FLO, that would be a great service to all refractor owners. We can often ascertain that something is not right, but DIY beyond that is risky.

We only offer the pre-dispatch Es Reid test for telescopes purchased from FLO (I don't think Es could handle more work than he already has). 

Steve Collingwood at SCTelescopes has done good work for us in the past and is trustworthy, if your telescope needs attention you could give him a call. 

HTH, 

Steve 

[CraigT82]

32 minutes ago, FLO said:

Steve Collingwood at SCTelescopes has done good work for us in the past and is trustworthy so if your telescope needs attention, give him a call. 

+1 for Steve Collingwood, very friendly and knowledgeable chap based in Kent.  

[mikeDnight]

5 hours ago, pystab said:

I bought this scope in January. Only in the last few weeks have I tested the diffraction limit by viewing stars at high magnification. Then what I saw was a biased Airy pattern, with each ring brighter on one side of the Airy disc than the other. With Arcturus in the centre of the field I saw 3 or maybe even 4 rings on one side, but only the inner one or two were still visible on the other side. It was definitely in focus: the central disc was small and snapped to focus, and the pattern expanded as you'd expect when defocused either way. Sorry, no photos - I don't have the kit.

The skewed pattern was obvious with my highest-power eyepiece, a 3.2 mm BST Starguider (230x, 59x per inch of aperture, 0.43 mm exit pupil). It was harder to see (because smaller) but still present with a 5 mm Vixen SLV (148x, 38x per inch of aperture, 0.68 mm exit pupil). Both eyepieces were bought new earlier this year. When I first noticed this I was using a diagonal (Baader prism) but it's still there when viewing straight-through without the diagonal. The pattern did not rotate when rotating the eyepiece, but did rotate when rotating the scope in its tube rings.

I have had no previous experience of quality telescopes, so I don't know whether this effect is within normal expectations or a sign of a real defect. I should add that I've been careful when handling the scope and it hasn't had any knocks, nor did it arrive with any sign of damage in transit.

Can anyone comment on what might be going on here - the nature of the problem, what might be done about it, and indeed whether it might just be normal for this scope?

Tim.

Don't try collimating the refractor yourself!

If you bought it here in the UK, send it back to the vendor. There's a five year warranty on every Tak, so touching the collimation may void it. Also, you don't want to risk injuring the fluorite element in the process.

Some UK Takahashi vendors are very capable at re-collimating the lens cells, but if not its something they can pass back to Tak who will correct the problem for you, or if absolutely necessary, may replace the scope.

[Saganite]

4 hours ago, mikeDnight said:

Don't try collimating the refractor yourself!

If you bought it here in the UK, send it back to the vendor. There's a five year warranty on every Tak, so touching the collimation may void it. Also, you don't want to risk injuring the fluorite element in the process.

Some UK Takahashi vendors are very capable at re-collimating the lens cells, but if not its something they can pass back to Tak who will correct the problem for you, or if absolutely necessary, may replace the scope.

Wise advice Tim.

 

You have there one of the finest 4" refractors money can buy, and you owe it to yourself to have it working as it should. What you are seeing is certainly not normal, and so recourse to the supplier if purchased new, and if not an expert such as E S Reid.

[jock1958]

First thing I did when I received my FC100 DL was check it with a Cheshire eyepiece and laser through the centre of objective lens.

The star test on my Tak is pretty good inside and outside of focus, however each time I try the test out the results change depending on what the atmosphere is doing.

I concur with all previous advice and contact the vendor if collimatiion is suspect.

If we all lived a bit closer to each other we could get a FC100 appreciation society going and do side by side comparisons!

 

[Ben the Ignorant]

1 hour ago, jock1958 said:

If we all lived a bit closer to each other we could get a FC100 appreciation society going and do side by side comparisons!

 

11 hours ago, pystab said:

 With Arcturus in the centre of the field I saw 3 or maybe even 4 rings on one side, but only the inner one or two were still visible on the other side. It was definitely in focus: the central disc was small and snapped to focus, and the pattern expanded as you'd expect when defocused either way. Sorry, no photos - I don't have the kit.

Actually, the comparisons are easy to do, and no special kit is needed. I tested my achro and semi-apo with a pinhole artificial star, short focal eyepieces and a cell phone. Magnifications were up to 140x. I just hovered the phone above the eyepiece, resting its side against a wall for stability. One in four or five pics was good enough, and I shared the results here.

 

Ronchi patterns are even simpler to image because no eyepiece or high power is required. A plastic ronchi film (13 lines per mm) I got for free with an optics catalog is accurate enough for a basic assessment of aberrations, including chromatism:

Just keep the camera centered and one in two or three pics will be good.

 

[Ben the Ignorant]

Just so everyone sees it, this is the pic from optician Wolfgang Rohr's site:

Your Airy disks look like that, Tim?

[John]

Performing the star tests is one thing but interpreting the results correctly is something else altogether and I think there is a tendancy to be pessimistic.

There is some advice on the subject on the Altair Astro website which might be helpful:

http://cameras.altairastro.com/2018/03/20/star-testing-a-fast-modern-ed-doublet-refractor/

 

[Tim]

9 hours ago, Timebandit said:

 

Sounds like collimation

If new I would have a word with the supplier to see if they can help.

ES Reid is the chap for Fracs if you need it independently checked and adjusted.

Hope this helps

 

 

Yes, it was Es that told me he'd get a Skywatcher Esprit

[mikeDnight]

If Altair Astro consider those ronchigrams acceptable, I can only say I'm thankful I don't have one of their scopes. There should ideally be 5 or even 3 ronchi lines which should be straight indicating a perfect sphere, unlike the 7 lines showing a raised centre and a whopper of a turned down edge. It may be colour free but there will be significant SA.

[Davey-T]

General advice seems to be, make sure the scope is well acclimatised, allow a couple of hours cooling, no diagonal, use high magnification 200X, use a green filter, use a bright star nice and high in the sky and don't rely on one test as conditions vary greatly from night to night.

Dave