Celestron SCT new 'breather vents'

[PeterAnderson]

I suppose my enquiry is whether others have had this problem with the new Celestron SCTs with the 'breather vents'.

I recently sold my Cave 16" Newtonian and have gone over to SCT's.- All Celestron. Okay, the 16" F6 plus supporting stuff that I had from 1980, looked impressive (Ooh! Aah!) but now that I am 74, I don't feel like clambering up and down ladders.
I have an 8" (C8) from 1990, a 9.25" from exactly a year ago, a CPC 11" from late 2011, and a C14 from late 2014. (Okay, they all do different things) - and I have a F6.3 focal reducer permanently mounted on the C11. They are all the standard optics - I am wary of the 'Edge' variety with a very strict position for focus, plus lenses in the setup that are not easy to access/clean.

I am not telling you all this to show how much I have, merely to say that I can do a comparison. Now the 8" initially probably had some low paid worker swipe over it with a dirty rag. It was well known that the insides of the corrector plates of these were often smeared and mine was no exception. I have removed the corrector plate and cleaned it possibly three times over the years, the last time about six months ago. I had stupidly kept it in a box and not only was the corrector plate getting smeary, but the secondary was mottling with fungus. Fortunately, Canon lens cleaner plus tissue removed all the marks and restored these to pristine condition, though I was not game to tackle a few small fungus marks on the primary.

The other instruments came with very bright clear reflective coatings and they have remained this way. I keep a UV filter screwed into the back cells, except for the C11 where I use the focal reducer, so the cells are sealed, airtight.

These telescopes all live in the observatory with thrown cloths over them, a concrete block and tin roof shed structure so with the tin roof temperatures can get quite warm - no hot! (I am now getting it painted with white heat reflective paint.)

Okay, now I come to my question. The C14 has these breather vents. In fact the whole 'EDGE' range have them as well. Initially upon unpacking, I thought that the optics were not quite as reflective, but they were 'clear'. Then after about six months I thought there was a bit of 'haze' which was on the INSIDE of the corrector plate. Whilst I was happy to tinker with the old C8, the C14 was a different matter and so I tried making sure the breather vents were perfectly clear, and tried putting the scope in different orientations, blowing fans, a hair dryer, whatever on it. All to little avail. (I went through a 'yes -no state because when it was warm the haze virtually disappeared.)

Recently I purchased some dessicant in an eyepiece fitting from Bintel, but it soon became apparent that most of the moisture was entering from the breather vents and then happily settling in the dessicant and on the corrector plate. Some interesting phenomena: When a change of orientation place the breather vents on the sides of the tube , a fogged band, on the level of the vents, was observed on the inside of the corrector plate!
When the telescope was pointed upwards and the air behind the corrector plate warmer nearer the tin roof (say on a hot day), the fogging just about completely disappeared. With the tube pointed downwards in cooler weather, back it came. However when night observing, well before any question of the dew point approaching, the fog returned quite noticably.

These last 10 days or so I have put sticky tape over the breather holes, and the dessicant seems to be slowly winning, with the fog fading - again somewhat subject to attitude of the tube. It has never been a thick fogging, but quite apparent when seen against the light.

From what I gather, trawling the internet, the dehumidification may take some time by this method given the volume involved, but I am prepared to do it, inserting the unit after each session. When I finally have it clear I will insert the UV filter to seal the back.

The position of these breather vents seems to be such that they only provide a cross-flow to the base of the tube, and at certain orientations, since they are 'top and bottom when the tube is above the mount.
Personally, I feel they are the problem, and may only be a solution if coupled with some active ventilation system. Maybe my situation is different...

I would sincerely like to hear from other on this. Am I alone???

[Camelopardalis]

I have fitted the TEMPest fans to both of my Edge HD scopes (8 and 11) and have only had a problem with one particularly moist evening where the fan had sucked the most air in a dewed up the inside. I learned from my mistake and now only power on the fans when there is a temperature gradient, to do as was intended, to somewhat equilibrate the internal/external air temperature.

Other than that, a dew heater strap works wonders on dewey nights. If you put the heater strap in the right place it shouldn't matter whether the moisture is on the inside or outside of the corrector, it should clear. I usually place the heater strap behind the thick rim at the corrector end so the heat is transmitting through the thinner OTA and warming the corrector from behind. You do have to be careful of course so as not to introduce a new set of thermal currents :D

Why are you wary of the EdgeHD? They 'suffer' from no noticeable coma and practically no field curvature :shrug:

[PeterAnderson]

Thanks for your thoughts about the heater. This is good advice but in my view if a manufacturer introduces an innovation, they should ensure that it performs well under all normal operating conditions. It demonstrably does not.

If this innovation did require some extra accessory, that should be explained and the accessory supplied. The innovation was heralded, but silence on the rest. The poor punter was led to believe it was stand alone.

If a SCT manufacturer wanted something extra, they could try designing a retractable dew shield or something really useful like that. Hmm...might cost too much... Hmmm might be able to sell it separately...

Why am I not keen on EDGE. Well, the corrector lenses are tucked away and difficult to access or clean. What (shudder) if they should dew up?

Then, in order to use to full advantage, the focus is very critical at a precise spot. Now we often stick some filters or other 'stuff' in the optical train, say an ADC (Atmospheric Dispersion Corrector) unit and this means the focal point most likely needs to be adjusted. Okay it is not optional for a standard SCT either, but the difference in that case is minimal.

For myself a totally flat field is not a major issue. I use the very centre of the field, or maybe close to it - only. (Lunar occultations and stuff.) True, the field is curved and a full prime focus image of the Moon on the C11 adequately demonstrated that. (The Moon was sliding past a star, and the image of limb and star were not pretty!) Using the F6.3 reducer and field flattener on the same instrument with an APS-C SLR (smaller sensor) sorted that out at much the same picture scale. Okay, it is a generic Celestron reducer and undoubtedly not as precise as the EDGE or its dedicated reducer, but it does me.

The strength of the EDGE is the perfect images for scientific wide field work. In effect, research grade optics at an amateur price, and it is superlative in this application. I know at least one fellow with a C14 EDGE doing top line research work. A superlative instrument, but not for me.

[Camelopardalis]

Fair enough, everyone is entitled to their opinion...

The vents do work...their purpose is to aid exchange of air between the inside and outside of the tube. If there is appreciable difference between the two temperatures equilibrium will take its course. I chose to add the fans to aid the process in when there are rapid temperature changes, like at a dark site up in the range. That was my choice to enhance a feature already provided. With the conventional SCTs the air is trapped and the air inside the tube only grudgingly gives up some of its heat. That is a much worse situation IMO. I'd actually like them to make the vents larger than they did, but horses for courses. Sounds like you've found the solution that works well for you.

You could argue that having a corrector plate out there at the end of the tube, exposed to the atmosphere and all its moisture is fundamentally flawed, and you'd probably be right...but that hasn't stopped you buying them repeatedly over the years ;) the requirement for dew heaters with an SCT is well documented in all but dry climates.

While your concern about the lens is real, it's only really justified if in practice they do indeed get dirty and require cleaning. I've had my scopes for over 5 years now and there's no visible signs of that being the case. Is it safe to assume you also don't own a refractor? That would no doubt throw up the same concern because unless you take the lens cell apart, most of the surfaces would be otherwise impossible to clean. I've also not experienced them dewing up, and I've lived in both the UK and Brisbane in the time I've owned them. They're just not exposed to the elements as they're in the baffle tube.

I'm not sure where your impression comes from regarding the focal plane...it's not critical for visual observing under normal scenarios. I use a variety of eyepieces with my scopes from short focal lengths for planetary observing all the way out to 40mm extra wide designs and have never seem any issues with what the scope/eyepiece is presenting that can't be corrected by my putting on my glasses :D using barlows, flip mirrors, ADCs...none of it causes an issue when looking at the centre of the image.

For imaging, the focal plane is quite specific...the same is true for many imaging scopes, however, regardless of designs. The optics involved in the imaging train and the flat nature of the imaging sensor chip up the ante. Using a DSLR on the f/6.3 reducer is equally picky about the lens-to-sensor distance if you want anything like round stars across the field. The bonus with the Edge HD is that you actually DO get rounds stars across the field :lol:

Btw, I'm not trying to sound awkward/funny, it just sounds like you needed to get something off your chest and I'm trying to give you some perspective...

[PeterAnderson]

Yes, indeed I did want to get something off my chest. I fully appreciate what you are saying.

In my observatory, my sessions are normally quite short and I don't know that I ever had dew on the outside of the corrector plate. True, I sometimes use a dew shield. If I have several events to time in an evening, I might either cover the top of the tube between them or roll back the roof to cover it. I live on the top of a ridge and there is generally some movement of air. Dew is not a problem.

I remember many years ago (say 1980), once having dew on the mirror of the 16" pointing vertically, imaging the lagoon nebula... but that was all.

So I was b.......annoyed when the inside of my 14" corrector plate started fogging when the other SCT's simply did not. Hence my statements.

I made a conscious decision not to purchase the EDGE for the reasons I stated. Certainly for objects in the centre of the field or close to it, there should be no difference. Where the EDGE shines is the flat field with near perfect images right to the edges. As I said, these images can get pretty ugly with a standard SCT near the field edge unless you use a corrector...

[Camelopardalis]

:thumbsup: sounds like you've got some great scopes!

Down here nearer the water there can be quite a bit of moisture. My sentiment is to usually stick the dew strap on the scope on low power as a preventative measure. Nothing worse than having the evening spoiled by some dew!

The Edge are exactly as you say, as is in the name, I wanted my scopes to be more multi-purpose. With a long focal length eyepiece it's possible to eek out a FOV of about a degree with my C11. Like you say though, if you don't care much about the surroundings, there's nothing to separate them near the centre of field.

[PeterAnderson]

Just as a footnote - I wanted to check out how temperatures drop in the evening. Now let us assume - and this is a big ask - that the telescope is close to ambient temperature at dusk. What happens then? Below is a rambing explanation of my investigations:
TAYLOR RANGE OBSERVATORY – Night time cooling – Peter Anderson

Taylor Range Observatory is situated on a 171 metre high ridge just over 1km to the WNW of the summit of Mt Coot-tha and nearly 100 metres lower. Its position is: -27° 27’ 42.3” S 152° 55’ 57.9” E Alt 171 metres.

As part of my lunar occultation observations I record the temperature at the time the observations are made. By tracking a number of days where I made observations from early to late evening, I was able to establish the pattern of cooling. Two of these records also show a mid-evening increase (notable in spring) as the inversion layer settles over the ridge.

My records from this same site stretch back over 40 years and I use the same two thermometers, mounted on the observatory wall at around 1.5 metres high. The observatory is of the double roll off roof variety and the temperatures would effectively be the ambient outside temperatures.

Whilst I have decades of records, for this exercise I have merely selected those evenings that were appropriate over the last five years, 2012 to 2017. I consider the results produced to be sufficient, as there were17 evenings in all, tending to group at the time of year when clear skies are more prevalent. I did not ‘cherry pick’, selecting all evenings that met the criteria of having observations over a number of hours.

For each of these selected dates, there were many more temperature observations than those shown on the table and for each selected date I did not restate except when there was no change in temperature for a considerable period including the final observation.

I use Universal Time (UT) in the table since the observations are recorded in UT. 8 hours UT is 6pm local time and 13 hours UT is 11pm.

Analysis of the results:
(1) For my site the maximum drop (one result only), is 4 degrees centigrade in a little under 5 hours.
(2) Three drops of 3.5 degrees are recorded in three to four hours and
(3) Six drops of three degrees over the same timespan.
(4) The remaining seven show drops from 1.5 to 2.5 degrees.

Conclusion
Without over-analysing these statistics, it is reasonable to assert that the ambient temperature drops at an average rate of around one degree centigrade per hour, perhaps slightly faster in the early part of the evening and certainly less towards 10pm and 11pm. However it appears that if the temperature starts slightly higher the overall drop will be greater, and less if the start temperature is lower. (I suppose this is pretty obvious.)
None of the selected evenings were during extremes of temperature.

The rate of drop will vary with the local conditions at the time – for example, there are differences in the drop rate in otherwise similar observations, but I do not have humidity or other records to compare/factor in.

Further, since I usually limit my observations to no later than 11pm, I have absolutely no reliable records later than this time in the evening.

Observation
Sites that are well away from the coast, or with different local topography, will undoubtedly experience a different cooling profile.