Mould on Optics:

Forums General Discussion Mould on Optics:

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    Martina McGovern

    Hi folks, hope you can help me.
    I have had mould on my 60mm Guide scope for over a year now – on the inside – it didn’t worry me too much as the guiding was fine. But now I have noticed a few spots of mould on the inside of my Celestron 8” EdgeHD SCT, a few approx. 3mm dia spots– been there for a few months now.
    I don’t feel confident enough to attempt taking my ‘babies’ apart to try to clean them.

    1. Does anyone know of somewhere that cleans scopes?
    Ideally within an hour or two drive from Cambridge city – so I can take it, I don’t have a carry case for the 8”….. Or a longer drive if you know of somewhere.

    2. What might have caused the mould to grow? {I’ve been watching the TV show “The Last of us” and mould frightens me 😊}
    I store my kit in a Shed – I put on the electric Dehumidifier every few days for about 4hrs. I always store the 8” with dust caps on but the 60mm guide scope has had the dust cap off for over a year, perhaps this is a bad practice. Also, I rarely take them out during the day – if I do it is only for about 1hr before sunset… I have just read that leaving the scopes out exposed to the suns radiation (not directly pointed at it) is good – well the article was actually talking about a camera lenses…..

    3. What are the best practice tips for avoiding the problem in the first place?
    Here is a photo of my 8” scope – guide scope is a lot worse…

    Thanks in advance for your help

    Trevor Emmett

    Hi Martina,

    I’m firmly of the view that this sort of problem is the ‘elephant in the room’ with regard to SCTs. It’s been a constant issue for the 10 years I’ve owned my CPC1100.

    Small specks on the corrector won’t be a big problem and, unless it’s obviously affecting your images I would leave well alone.

    Steve Collingwood of SCTelecopes (and Pulsar Observatories) did a good job cleaning and collimating my telescope about 5 years ago. I’m not sure he still offers this service but try

    With the recent demise of David Hinds I think Tring Astro might be able to advise.

    I would appreciate knowing how you get on. Good luck!!!!


    Trevor Emmett


    Martina McGovern

    Thanks for the info Trevor – I will let you know how I get on 🤞🙂

    Dr Paul Leyland


    Alas poor Yorlik, I knew him backwards.

    Doug Edworthy

    I saw Steve Collingwood today at Astrofest to ask him about repait to a pair of binoculars, but he tells me he doesn’t do service work anymore having got rid of his workshop and optical benches.


    Doug Edworthy

    Grant Privett

    There are camera specialist photographic companies that will attempt to dismantle lenses and clean mould off them. Given the number of elements in a DSLR lens, a telescope lens is a stroll in the park by comparison. I will ask camera enthusiast friend for a recommendation.

    I noticed the inside of my 300mm F/4 starting to smell slightly of damp so went over to forced ventilation – have not had condensation on the mirror since.

    One thought, I recall hearing of an SCT user inserting a tube containing packets of silica gel into the focuser (the tube had a wire mesh end so the air from the telescope got to the gel). You could also buy a couple of large packs silica gel from Amazon and put velcro tape on to them and place a fresh one on the inside of the lens cap at the end of every observing night. You then warm the spare up and store in an air tight jar so its ready for use on the next clear night.

    Martin Mobberley

    You may be thinking of a short piece I wrote in the May 2009 Astronomy Now, about Dave Tyler’s C14 dessicant arrangement? Dave made such a tube to slide into the focuser of his Classic C14. The tube was 64cm long and 5cm diameter, with the end drilled with 200 8mm holes. Silica Gel bags were stuffed into the end and a cap used so that the bags could not fall out. At the focuser end the tube was more than 50mm across to prevent it falling inside the SCT. I attach Dave Tyler’s photo of his system. Unfortunately this would NOT work with an EdgeHD design due to the corrector lens inside the drawtube.


    Martina McGovern

    Thanks for your feedback Grant and for trying to find out any contacts from your friend.

    I’m buying one of these from a contact in an Astro Facebook Group – he makes them himself – I’ll try it out then get more if they fit my scopes;

    Also, I ordered the bigger Silica bags this morning, with self indicator and are reusable – that as you say, I will tape them securely to the inside of the scope’s dust-cap.

    I have Baader Wonder fluid and Pec Pads that I use to clean the front of my scopes and the CMOS cameras – so I’m starting to think I might try to take the Corrector plate off the Edge HD – I’ve found two YouTube videos and will watch them closely and decide it I’m brave enough to give it a go…..😉😊

    When you say, “.. so went over to forced ventilation..” what do you mean by this? and how did you achieve it – did you buy a special kit?

    Thanks again

    Grant Privett

    Martin: Damn, had assumed the corrector lens in the Edge was up at the secondary. Sorry, bad advice on my part!

    Martina: Have emailed friend. Will post here when he responds. The “forced ventilation” referred to keeping the small fan in my 300mm f/4 Newt going 24/7 – I am tempted to run it at 9V rather than 12 to spare the bearing a bit. Closing the tube or leaving open seemed to make little difference.

    I did that in response to repeatedly finding condensation (water droplets and a fine dew plus a sheet of ice one night) on the mirror. Does the Edge have fans like the RASAs do?

    Full dew conversation here….

    Martina McGovern

    Grant, thanks again for you help.
    No fans with my 8″ EdgeHD.

    Grant Privett

    The recommendations I was sent for people willing to service DSLRs, SLRs and lenses are:

    This is an active list of good repairers – my friend has used a couple of them:

    The two links below also get good reviews, but he has no personal experience of their work.

    However, it must be remembered a 60mm lens may be out of their comfort zone of expertise.

    He also warns that there are people out there who will do a poor job and charge you a lot, so you may prefer to watch a lot of youtube videos and then do it yourself.

    Martina McGovern

    Thanks Grant for sharing this information – it is starting to look like I need to have a go at cleaning my scope myself – Fingers crossed 🙂

    Martina McGovern

    Success with cleaning the mould off the 60mm Guide scope lens.
    It unscrewed at the front of the scope and I got easy access to the lens – happy days.
    I used Baader Optical Wonder cleaning fluid and some PEC pads wipes

    Now I need to be brave and tackle my 8” EdgeHD……. Need to watch the YouTube videos again first…..

    Martina McGovern

    Update on 8″ SCT:

    Well, I summoned up enough courage yesterday to take on the task of cleaning the inside of my Corrector plate on my 8” EdgeHD SCT– I was very anxious about it – I didn’t want to damage my baby. In summary, it was a success 🙂

    I watched several YouTube videos (you gotta love YouTube videos – you guys that do them are brilliant) and jumped in…..
    e.g. found this one very helpful

    I have bought some Wisedry 20g rechargeable Silica Gel desiccant packets to tape to the inside of the Dust caps of my biggest scopes.
    I also bought a Nosepiece Desiccant holder i.e.

    Here are two images after I cleaned the scopes.

    Thanks to all on this ‘thread’ for your support and guidance.

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