P.S.T milky filter

[gajjer]

Hi all, not sure if this is the right place but here goes. 

Had a go at solar imaging earlier with my Colorado PST. I couldn't see the Sun as clearly as I had previously so started taking it apart. The eyepiece section seems to be fabricated in two parts. The part that actually takes the eyepiece or camera, that houses a red filter approx 5mm diameter. The other part that screws into the body of the telescope has a larger filter around 10 mm diameter. It is this larger filter I have concerns about. The outward face of the filter appears silver - fine. However the inward face looks a milky red; I would cal it a greenish bloom over a red base.

It seems to be reducing the transmission of light significantly.

Any thoughts?

Cheers gaj

 

[Peter Drew]

Sounds like the common gradual transmission failure of the 10mm ITF.  This is apparently caused by the ingress of damp.  The problem usual starts from the outer edge and works inwards.   As you say, it dims the image but it is still usable safely.  There is a drop in replacement available from Maier Optronics .     🙂

[Bignerdguy]

This is the infmaous "Rust" problem for these scopes.  The ITF filter is not sealed and since it uses metal as the coating, this metal over time "rusts" and causes a milky appearance to occur blurring images.  If you want to replace it, go here for the replacement, its the only one of two and the cheapest option by far:

 

https://maierphotonics.com/656bandpassfilter-1.aspx

[Johan L]

I have the same problem with my PST that I bought second hand and Im going to change the ITF.  Seems like all of these scopes get this isue in time?

[Merlin66]

Bignerdguy,

The original "rust" problem with the PST was associated with the energy rejection coating Coronado applied to the front objective. They replaced the objective with an AR coated lens and added the "mini erf" - the ITF ( induced transmission filter) in the bottom section of the eyepiece holder.

Unfortunately the design of this ITF filter (and the one used by Lunt (BG 38)) can and do suffer failures - we think due to moisture ingress (these filters when not in use should be stored in a sealed container with desiccant)

For the Coronado PST and the Coronado blocking diagonals (BF10/15) there's a great "drop in" replacement itf filter available from Maier Photonics in the US. These replacement filters seem to be almost indestructible, I have some which are now almost ten years old and still 100%. Highly recommended.

 

Edited March 26, 2020 by Merlin66

[Pete Presland]

Plenty excellent advice here already, hope you get it sorted. The rust problem has been around for ages.

[tico]

The current models also suffer from this problem?

Tico.

[Merlin66]

Tico,

Unfortunately yes..

The ITF (or the Lunt BG38) filter may last for months or years, you never know. Lunt currently offer free replacement for failed filters. Only warranty support from Coronado.

 

[gajjer]

Thank you all for your responses. That has been most informative. Are there any instructions for fitting the replacement? Do you think I will be able to get a replacement in the current environment?

Cheers

Gaj

[Merlin66]

Gaj,

I have two PST in for repair at the moment...just waiting on the filters to arrive from Maier Photonics in the US (They still seem to be working over there)

The filters are a "drop in" replacement for the PST.

Unscrew the top section of the eyepiece holder....looking inside the lower tube you'll see the ITF filter. You need a small lens pin tool to pick up on the holes in the retaining ring, unscrew the retainer and the old filter will just fall out. Place the new filter into position and re-attached the retaining ring (do not over tighten!). Job done.

 

Edited March 28, 2020 by Merlin66

[Rusted]

I don't have a lens ring, pin tool. So I used a small pair of screw adjusting, workshop dividers bridged between the holes.
The Maier replacement ITF is well worth having judging by my secondhand PST's ITF.

Though you must expect a bit of import duty and then VAT on top of literally everything.
US retail price + postal charges + import duty + customs clearance charges x VAT at 20%[?]

It is just possible "they" will rush through a trade agreement to get the economic world turning again, post 19.
Though I wouldn't bet on it. Get your order in while you still can and be grateful when it arrives.
Remember to maintain your social distancing from the postman.

[gajjer]

Thanks guys I thought she was a goner! Before I part with $75 can someone confirm that it is the Maier Photonics 656 ITF band pass filter.

Thanks

Gaj

[Rusted]

Correct: You have a choice:

Maier ITF from the US $75 US dollars, plus customs + taxes + custom clearance + VAT on top of everything.

Or Beloptik from Europe at a higher, initial, retail price : €149 Euros + P&P but NO additional taxes.

Having read around the subject the Beloptik is considered optically superior by some users.

I bought a Maier in 2018 and have no complaints.

Both have offered excellent service in the past. Pre- Cov-19.

ITF replacement filter for Coronado blocking filters

ITF H-Alpha 656 nm filter

Do not touch the delicate surfaces of the replacement filter you buy.

Ensure correct orientation in the telescope.

EDIT: The Beloptik price has risen.

Edited March 29, 2020 by Rusted

[bomberbaz]

An interesting topic this, I was looking at a second hand one recently.

I have a query, is the front etalon rust problem only with older models?

And does anyone have a diagram of the inner working which show where mini ITF filter lays?

[Elp]

The brown/red rust affects older models, you can tell as the front objective is a reddish brown colour, you'll see speckles within the objective if affected, you'll also see it when looking through the scope, also as an off band effect like it's clouded over in places.

The newer objectives are a mostly blue colour, and the general consensus is if it's a post 100K serial number model it's a newer one. The blue/green ITF in these was moved to the bottom of the eyepiece holder, you will see the bottom of it if you can completely screw the eyepiece holder off the prism box housing or if you take the side plate off the prism box housing and look up above the pentaprism up into the eyepiece holder, the ITF is visible though to get a proper assessment it's best to screw off the eyepiece holder so you can angle it a bit more and get a better view.

Within the eyepiece holder, at the bottom is the ITF, above that a red BF one then the clearance cylinder for your eyepiece to go into.

Note newer scopes may be caked in loctite so removing these bits might take some effort. Be careful with the pentaprism box hex screws also as they're very soft and can strip in the heads.

The ITF will need replacing periodically depending on usage and storage humidity, but it's self maintenance rather than sending the unit to Meade whom I believe bought Coronado's assets.

Edited August 16, 2023 by Elp

[bomberbaz]

2 hours ago, Elp said:

The newer objectives are a mostly blue colour, and the general consensus is if it's a post 100K serial number model it's a newer one.

I have emailed the seller to ask the serial number, if it is a post 100K I might just go for it, cheers for the info.

[Merlin66]

The serial number is just a guide.

A better question to ask would be - does the front objective look "blue" or "brown/ gold"

[DaveMcC]

What is the part number or name to ask for to get the drop in Pst filter from that company in USA please. 

[Elp]

Youd have to email Maier, doesn't look like it's on their website. It's the 656nm hydrogen alpha ITF filter. Tell them it's for a Coronado PST, they'll know.

[Elp]

Or get it from Oliver Smie at Beloptik.

 

[DaveMcC]

Message sent, thank you.