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Safely Check a Used Coronado SolarMax II 40 mm?

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#1 NeilEPerlin

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Posted 18 May 2024 - 04:48 AM

Hello,

 

I run a small astronomy group in northern Vermont. People keep offering us used equipment, often because they inherited it but don't know how to use it or even what it is but don't want to throw it out. Someone just offered us a used Coronado SolarMax II 40 mm. It appears to be in very good condition, but I obviously want to check it out before we use it.

 

So, my question - is there a safe way to test it without risking a look through it or sending it back to the manufacturer? Thanks for any advice.



#2 LarryAlvarez

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Posted 18 May 2024 - 05:52 AM

The SolarMax II filters require a rear blocking filter diagonal at the back of the telescope, prior to the eyepiece, to operate safely and to check the function of the filter with the Sun.  These blocking filters come in 4 sizes where each is numbered to correspond to the largest size focal length they can support without vignetting.   They are named BF5, BF10, BF15, and BF30 where BF stands for "Blocking filter" and the 5, 10, 15, 30 stand for 500mm, 1000mm, 1500mm, and 3000mm focal lengths.  In order to use the SMII filter, you would either require an adapter to fit the filter onto the front of a telescope or have a Coronado telescope but either way would still require you to have a rear blocking filter to make it safe.

 

If you have a working Coronado PST, you can screw the filter onto the front to act as a double stack filter and test it that way.  The PST has a blocking filter built into the rear section that will make using the filter safe.  Since you are in an Astronomy group it might be easiest to see if one of the members has a PST who can test it.  Coronado also made a 40mm scope in the past but stopped making that model soon after the PST came out. 

 

Having a Coronado blocking filter at the rear is a must when considering safely testing the filter with the Sun.  I'm sure there are ways to test it using a monochromatic light source but I'm no expert on that sort of test.  Perhaps someone here can chime in on that sort of test.   As far as alternative physical checks you can do without having a blocking filter or looking at the Sun, you could check for missing parts, alignment of the T-max and etalon spacer health. 

 

The worse thing that can happen to one of these types of filters is for it to become "decontacted".  The inner etalon filter is made up of 2 glass plates separated by ultra thin spacers.  When decontacting occurs, usually through bumping or dropping the filter, the plates will separate and usually break one or a few of the ultra thin spacers.  To check them just take a flashlight and carefully examine the filter from the back and look for the small thin spacers near the outer edge of the filter.  The spacers are irregularly shaped and that is normal.  If you don't see areas that look like the spacer is broken with dust around them or an odd look to them you are ok.  Another check is for the small stop pin.  SM II filters have a small pin on the upper section of the filter that serves to stop the rotation of the front Rich View tuner.  When looking at the top of the filter you'll notice a lot of small holes that are threaded.  The stop pin should be in one of them.  It sticks up from the main filter and threads through a slot on the top Rich View tuner.   Its important to have that pin to keep from over tightening the tuner.  Another simple check would be at the back of the T-max de-tuner back place.  Look at it and make sure it has all the screws and that when it opens and closes, it does so evenly on both sides.  Having one that has missing or loose screws can be a problem and having one that opens at an angle can also be a problem when tuning the T-max.  Attached are some pix of the physical checks.  Good luck.  smile.gif

 

 

 

 

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#3 MalVeauX

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Posted 18 May 2024 - 08:58 AM


So, my question - is there a safe way to test it without risking a look through it or sending it back to the manufacturer? Thanks for any advice.

Hi,

 

The safest way is to inspect its imaging pathway with a camera, so that your eyes are not involved.

 

Ultimately, unless its physically missing elements, it will be safe visually if its a complete filter system (ie, the etalon and its associated ITF+blocking filter module, usually in a diagonal). Most of these are designed to be safe even without the ITF+blocking filter so that any mishaps without them are "very bright" and "uncomfortable" but you will blink and look away at the first sight of it and be safe. I'm not saying to do this. Just pointing out that you will not go blind if you accidentally forget the blocking filter and put a normal diagonal in there. But, it's always a concern to keep up with good habits and visual safety with any solar filter system, so, always do a check before doing any visual that all elements are accounted for and that its secure and properly setup.

 

The etalon chamber of the SMII series is a rich view tuner, so it should tilt with a single knob and a wheel around the entire chamber should twist to pressure tune it. Just ensure the full range of each tuning mechanism works and goes through its range to its limit and back without any problems, smoothly. Unfortunately, SMII series (and III series) largely have issues with rich view tuners specifically because of the central spacer they use and the compression of the chamber so if the pin is slightly off center or if the etalon plates are slightly tilted and not orthogonal you get uneven tuning, banding, off band sections, etc. I've only seen a few samples of SMII and SMIII series etalons that have good uniformity, they're rare, almost all of them instead have the worst uniformity I've seen in a sample pool over time. Sadly. But, this may not matter to you if the price is right (ie, free, or deep discount).

 

Even if the etalon were decontacted, it will be safe, it just won't show h-alpha isolation and will just show an orange unfocused blob of a disc with a smeared side.

 

The blocking filter should be fine.

 

The biggest concern on these systems is actually the ITF (interference transmission filter) that preceeds the actual blocking filter. Coronado's ITF filters are notoriously undercoated on the sides, so they allow O2 and moisture in and it degrades the filter rapidly over time (people call it "rust" but its not actually rust). This is the biggest thing to look for. Thankfully its fixable if you find it. Visually or imaging wise, you will notice a dimmer image with less contrast or possibly an off band image that is dim if the ITF is fully compromised.

 

Very best,
 


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#4 NeilEPerlin

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Posted 25 May 2024 - 06:08 PM

Thank you both for the suggestions. I plan to get to the inspection this week. Hoping for the best...

 

Regards,

Neil




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