Refractor coatings

[moriniboy]

I have a refractor which is showing ghost images.

Basically there is one ghost above the main image and two below.

Its not the ep's as I have tried different orthos, hyperions, kellners etc.

Any suggestions about trying to correct this before I consider getting an anti reflective coating applied?

Also does anyone know of any reputable people who could carry out this coating work?

[Peter Drew]

Which model refractor is this?

[Ajohn]

You might try Oldham Optical or Knights Optical for coating. There are several UK companies about that make optical components.

If it's the tasco I was given something similar but smaller some time ago and it wasn't too bad. Then I took the lens out - big mistake. I don't know how they aligned the elements but I couldn't match it. I suspect there is a chance that this area might be your problem. In my case it wouldn't focus cleanly any more so if there was reflections I wouldn't have taken much notice.

John

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[ronin]

Not sure that the problem would be the coating the amount of reflection is fairly small even on uncoated glass (4%). I would first guess that the objective is not straight in the tube and is sat at an angle. Is it a seperated doublet ? If so one lens may have moved.

Recoating depends on the value of the objective as the original coating would have to be removed and then replaced, not sure if it is a cemented doublet of they would do just the 2 external faces or if whoever would want to seperate the doublet and redo all 4 faces.

[moriniboy]

Ok so I have checked the focuser, its well aligned.

Checked the colimation using a Cheshire and this photo shows the results.

Not sure what I am seeing but I would assume that both refelctions from the lamp should be together?

[moriniboy]

After my Son spent 15 mins carefully turning the front lens only in the cell with no difference to the reflected image I decided to take off the front lens and swap faces, attached is a photo of the result.

Reflections are closer together but as its cloudy (surprise) I can't check it out

Perhaps the previous owner put it back together wrong, apparently it was cleaned before I picked it up?

If the views are now ok I might just adjust the shims between the lenses to get it spot on.

[John]

In both pics the scope does not appear to be in collimation. Either the objective lens needs tilting slightly or the focuser is not aligned with the optical axis of the objective. Or a bit of both !

A collimated laser shone down the tube (no diagonal) should exit in the centre of the objective lens. If it's not central than the focuser needs to be tilted to make it so.

Once the above has been checked out, if the cheshire still shows as above, then adjusting the tilt of the objective can correct it. I've had refractors with the error similar to your 1st image and it's been relatively easy to solve.

The front lens of the objective lens may have been OK from the start to be honest !

[LeeB]

Hi the ghost image you see is the rebound reflection from the front of the back lens in the doublet to the back of the front element. They are common in telescopes where the two internal curves are almost , identical. I assume its the carton we are talking about. When I disassembled the lens i marked the elements both for orientation and for position on rebuild. I myself was considering a recoat with anti reflective coatings but these don't always improve things with older lens. (the older lens might have very tiny pits and marks which pick up excess coating) although in this case the lens are in great condition so may be worth a try. I found that the ghosting is vastly reduced on these early scopes, when the objects are less bright and that for jupiter or venus a low density moon filter helps. The collimation picture above looks a little off, it was spot on when tested just before xmas. reseat the doublet and try again (I find it easier to adjust in vertical position) I found that in one orientation in the cell the collimation was dead on. while in others extra spacers were needed to tilt the doublet correctly.

[Ajohn]

I can't help feeling this is a de centring problem relating to the 2 lenses in it. You could try deliberately moving the elements to opposite sides of the cell as you put them in and see what difference that makes. I think it's far too strong to be reflections but even if it is the reflections should surely coincide with the image if the elements are centred.

Seeing mention of coating, a subject that interests me, who in the UK will do this and any idea of costs? I was guessing who might in the earlier post.

John

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[moriniboy]

An update

This has now been sorted, steps undertaken included

Objective now fully multi coated

Fastening holes in objective cell elongated to allow whole cell to be moved for collimation.

Focuser aligned and re-felted.

Fully flocked internally (including baffles)

Focuser tube flocked

Improvements seen:

No more ghost images

Views of the moon better than I have ever seen before, through a 40mm Kellner, with no CA.

Great views of Jupiter, however light transmission is now that good that the light washes out the detail on full aperture so I use a mask over the objective to regain contrast, again with no CA

Cosmetic improvements

Stripped off Hammerite paint finish to cell and focuser, and re-sprayed in satin black.

Fasteners changed to stainless steel hex heads.

Quick release finder shoe fitted.

Photos to follow.

[nightfisher]

Look forward to the pictures, can you give details about the multi coating, like where and how much it cost?

[moriniboy]

New coated objective

Focuser

Spectrum Coatings (Manchester) carried out the coatings and I also got a before and after print out of the reflectivity readings.

http://www.ophthalmic-lens-coating.co.uk/

I had to get aluminium lens holders made specially to fit the objective (82mm) so the lens does not fall off in the coating machine, this was an additional expense.

Would budget a total of £75 to get an 80mm objective coated (all four faces) and the holders, but I think I got a deal as the factory was quiet after Christmas.

[AndyH]

Very interesting stuff

Any more details about the scope?

Cheers,

Andy.

[Astrobits]

Image Optics ( aka Galvoptics ) in Essex also do a lot of scientific coating. Almost 20 years ago, and at my request, they produced Light Pollution Rejection filters for 1 1/4" eyepieces which they sell now under their "Night Image" name. They do, of course, also aluminise our mirrors so they are used to dealing with small orders.

Nigel

[moriniboy]

Any more details about the scope?

Hi Andy

Its an 80mm x 1200mm F15 Carton

[moriniboy]

As the weather is being so kind tonight I decided to experiment with mask aperture to see just how large it could get before detail in Jupiter started to get washed out.

Used an old solar film mount and marked 35, 45, 55 and 65mm rings then cut out the smallest and observed, if ok I moved on to the next size and repeated. At 65mm the detail was staring to wash out so 55mm is the optimum for tonights viewing, this works out at F21.8.

Just need to observe on a few more nights to check if ok then make a permanent mask.

[LeeB]

Excellent job on the recoating, looks like a similar coating the the one bresser use (lovelly green). Glad the scope is performing well.

How long did it take for the recoat and what coating did you specify, I have one i'd like to do.

[moriniboy]

Hi Lee,

It took 1 week and they advised that broadband coating would be best.

[LeeB]

I have been playing with masks lately , 55mm on a 1200mm fl seems abit agressive (Ca index a smidge over 10) i did similar at 50mm for fl1000 , but found the extra benefit was marginal over Ca index 7 , ie stopped to 60mm. giving f16.67.

[Astrobits]

Can you post a copy of the reflectivity report you got for your objective?

Nigel

[moriniboy]

Before

After

[moriniboy]

I have been playing with masks lately , 55mm on a 1200mm fl seems abit agressive (Ca index a smidge over 10) i did similar at 50mm for fl1000 , but found the extra benefit was marginal over Ca index 7 , ie stopped to 60mm. giving f16.67.

The scope came with a mask, the centre hole was only 35mm! Like I said earlier, based on that nights viewing 55mm was the best. As each night is different I'm trying to find a compromise situation based on a few nights viewing.

[LeeB]

I understand , sorry if the comment sounded a bit sharp , it was'nt ,meant to be. I agree that the night makes a hugh difference. It might be prudent to make a couple of masks. I have found a different size works best depending on object brightness. so was going for masks that give 3,5 and 10 on ca index. for brightstar,bright planet and moon respectively.

[Astrobits]

Thanks for the reflective graphs.

I think that I will stick with the conventional single layer AR coating should I need to get any refractors done.

Nigel