184 replies to this topic

[Rustler46]

Over the last 49 years I've owned 3 Celestron SCTs of 5-, 8- and 11-inch aperture. My current favorite is a 1985 Super C-8 Plus, which has proven itself to have good optical quality. My observing experience and some sage advice from Cloudy Nights forums has led me to what I consider to be my optimum configuration. Of course opinions vary as to capabilities and best use of the Schmidt-Cassegrain design. But love 'em or hate 'em, there are many of these telescopes in use. With that in mind I offer what has worked for me. I hope this will lead to a discussion that will be of benefit to others who use this type of telescope.

 

So this is the standard, non-edge variety with all the compromises inherent in its design. Later versions of Celestron's SCT's have made some improvements in the basic design. While I have at times used this telescope for astrophotography, mostly it has been utilized for visual observations. Astrophotographers may find what I use to be of benefit. But here is a list of what I've done to enhance the observing experience and performance for the things I observe. 

• Mounted on a Losmandy G-11, go-to equatorial mount on a permanent pier
  While the stock fork mount of earlier years is more portable, a permanently mounted telescope, even in light-polluted skies has benefits in ease of use and time needed to get on with observing.
• Dew prevention with minimal heat and front corrector surface fan
  This I consider to be the one thing that has improved performance more than anything else. Applying heat does prevent dew. But this works counter to allowing the optics to adjust to nighttime air temperature. While everything else can be warmer than ambient temperature, the corrector lens can quickly fall below the dew point temperature with resulting dew. I won't at this time go into the physics of why the front fan works. But here is the proof. I have for years used the DewBuster system for dew prevention via controlled heat applied to the OTA to keep the corrector from dewing up. Here is what has worked:
  • Using DewBuster to keep the telescope's internal temperature to be 5-7°F warmer than the air keeps the corrector lens dew-free.
  • With front surface fan, keeping the interior 0-1°F warmer than the air keeps the corrector free of dew. Minimal heat has other positive effects, such as lesser internal tube currents.
    But in spite of the common paradigm that dew prevention requires large heat inputs, this front fan method really works, and provides improved optical performance. See Dew Elimination.
  • ​Covering optical tube with Reflectix aids in reducing thermal issues. The dew heater strip resides under this covering in contact with the metal tube.
• Working within the SCT design compromises to minimize negative effects.
  • Endeavor to keep the focal plane at the design distance of 100 mm from rear port. I believe this metric had its genesis in the 1970's world of film-based astrophotography with SLR cameras adapted to the SCT rear port. Focusing on the film at a distance of 100 mm worked in that instance. 
    Please note - this is for standard, non-edge Celestron's. Other models &  brands may have differing designs.
  • Ability to extend the focal point is one of the benefits of the SCT design. But it has some potential negative consequences in a design full of compromises. Some negative effects are:
    • ​Vignetting and reduction of effective aperture for light gathering. Here's why:
      Pushing the focal plane further back entails the focuser mechanism pushing the f/2 primary mirror forward toward the secondary mirror and its baffle. Doing so begins to cause the outer portions of the light cone to be blocked by the baffle. The effect is reduced light throughput or smaller effective aperture. Also the central obstruction is now a larger percentage of the aperture, resulting in reduced contrast.
    • Increased focal length - 1/
    • Increase of spherical aberration - 2/
    • The closer back focus is kept at 100 mm the better the SCT will perform. Binoviewers are particularly a challenge in the regard. 
    • See Focal Reducers and the SCT.
    • 1/ - For a C-8 every 1 mm increase from 100 mm design back focus causes the focal length to increase by 3.2 mm.
    • 2/ - As I recall from the above link, every 25 mm increase past 100 mm adds 1/23rd wave additional spherical aberration. For large increase in back focus, the increased spherical aberration can be considerable.
       
• ​Collimation - see Ed Moreno's guide to SCT collimation.
• ​Baader Zeiss-spec prism diagonal improves performance for some objects like Moon and planets.
• Telrad finder with home-brew dew heater - the commercial dew shields are completely useless in my experience. The third photo shows the Telrad's window with the resistor heaters on the edges keeping it dew free. The DewBuster website has directions for doing this.
• ​Mechanical considerations - Losmandy dovetails
  • ​Dovetails are needed for mounting the optical tube on a GEM.
  • The less expensive, orange Celestron dovetails are a compromise due to being an extrusion. To keep weight down, the cross section is reduced to a minimum. This leaves it susceptible to flexure, particularly when using the SCT for long exposure photography. See 3rd photo below.
  • The Losmandy dovetails are much more solid, which is of great use when carrying a smaller refractor piggyback. Save yourself some money and buy the better ones first.

 

Celestron-8 in optimum configuration

 

Celestron-8 with AT115EDT on G-11 mount

 

Celestron dovetail on C-11

 

I hope this will lead to some beneficial discussions and greater enjoyment for those using  Schmidt-Cassegrain telescopes. No doubt there will differences of opinions. But sharing what others have learned by experience can perhaps be of benefit.

 

Best Regards,

Russ

Edited by Rustler46, 11 December 2022 - 06:51 PM.

[Xeroid]

Thanks for sharing your thoughts, I too have greatly enjoyed my C9.25 and for the many wonderful views I have seen for the first time in my life...

[Rustler46]

Thanks for sharing your thoughts, I too have greatly enjoyed my C9.25 and for the many wonderful views I have seen for the first time in my life...

That is a nice compromise between the C-8 and the beast C-11 and C-14. All of these will benefit from knowing some of the trade-offs involved. I'll be adding some hyperlinks to the Cloudy Nights discussions that have enlightened me on use of our SCTs.

 

A few nights ago I enjoyed using my old C-8 in observing some doubles in Perseus. Seeing was generally good enough to help the SCT show its capabilities.

 

All the Best,

Russ

[RedLionNJ]

Nice setup, Russ. I like it & agree (through experience mostly as a planetary imager) with a lot of these.

 

I do have a couple questions for you (actually, been waiting for someone to post something like this so I can ask!):

 

The dew shield - that looks like an 'AstroZap' semi-soft style. Do you use any special tricks to affix it in position so it doesn't move/slip?  And how long does it project in front of the OTA?

 

Focuser - I see you have the stock Celestron SCT focusing mechanism. Have you ever considered adding a 2nd, Crayford-style focuser? Or is that value of approx 100mm acting a limiting factor, since you also have the diagonal for ease of visual observing?

 

That 100mm - given your experience, how crucial do you think it is if you're only interested in about an arcminute diameter, directly on-axis (e.g. planetary imaging)?

 

Losmandy rail connected directly to the mirror cell and the front of the OTA - have you considered OTA rings instead of the "brick" connectors? Would the present an advantage or possibly a disadvantage?

 

I admit to having an incredible problem with dew/frost this winter on my 12-inch SCT, particularly when pointed high at Mars. My dew strip doesn't cut it. My 8-inch dewcap doesn't cut it (& frequently slips). Once dew or frost start forming, I have to use a hairdryer every ten minutes or so to extend my imaging period. By the time the corrector has re-cooled and I've refocused, I have just enough time for a single 5-minute video before I have to start the whole process over again.  Very frustrating.

 

Not intending to hijack your thread - feel free to respond privately if you wish.

 

Much appreciated - love the post.

 

Grant

[luxo II]

Grant, on my 10" I wrap a heater strap around the OTA about 15 cm back from the corrector, then apply insulation over that. The strap warms the metal of the OTA and the air inside, which seems to distribute heat nicely across the corrector, while the insulation cuts the thermal losses to the outside air. As little as 3-4W is sufficient. https://www.cloudyni...50631-img-1063/

 

If you aren't using insulation then yes I'd say it will dew, even with so much heat applied that you have thermals inside the OTA.

 

Admittedly it's a mak, not an SCT...

Edited by luxo II, 11 December 2022 - 02:30 AM.

[Rustler46]

Nice setup, Russ. I like it & agree (through experience mostly as a planetary imager) with a lot of these.

 

I do have a couple questions for you (actually, been waiting for someone to post something like this so I can ask!):

 

The dew shield - that looks like an 'AstroZap' semi-soft style. Do you use any special tricks to affix it in position so it doesn't move/slip?  And how long does it project in front of the OTA?

 

Focuser - I see you have the stock Celestron SCT focusing mechanism. Have you ever considered adding a 2nd, Crayford-style focuser? Or is that value of approx 100mm acting a limiting factor, since you also have the diagonal for ease of visual observing?

 

That 100mm - given your experience, how crucial do you think it is if you're only interested in about an arcminute diameter, directly on-axis (e.g. planetary imaging)?

 

Losmandy rail connected directly to the mirror cell and the front of the OTA - have you considered OTA rings instead of the "brick" connectors? Would the present an advantage or possibly a disadvantage?

 

I admit to having an incredible problem with dew/frost this winter on my 12-inch SCT, particularly when pointed high at Mars. My dew strip doesn't cut it. My 8-inch dewcap doesn't cut it (& frequently slips). Once dew or frost start forming, I have to use a hairdryer every ten minutes or so to extend my imaging period. By the time the corrector has re-cooled and I've refocused, I have just enough time for a single 5-minute video before I have to start the whole process over again.  Very frustrating.

 

Not intending to hijack your thread - feel free to respond privately if you wish.

 

Much appreciated - love the post.

 

Grant

Hi, Grant. Thanks for your comments and questions. I'll try to answer and shed some light on your concerns.

 

The dew shield is indeed an AstroZap model. I have removed the dew heater and added some extra padding inside so that it would still fit well with the cutouts for top & bottom dovetails. According to the DewBuster website locating the heater around the corrector cell is a poor location, since the lens is separated by spacers from the metal cell. Heat is very inefficiently transferred to the lens. Best to heat the air inside the OTA so the entire rear surface of the lens is being heated. MY dew heater resides about 1/2 way along the metal tube. Carbon fiber would be another issue to consider. My dew shield is held back tight against the dovetails by some small bungie cords attached to convenient locations on the dovetails and the outer end of the shield. This works great.

 

After 37 years the lens cap no longer stays attached to the corrector cell. So it is held in place as shown below with a small bungie cord.

 

 

Notice above the power cords - one with PowerPole connectors handles the larger current for DewBuster controller, dew heater and front end fan. The cord with an RCA audio connector handles the small current powering the fan.

 

This shows how two small bungie cords keep the dew shield in place.

 

In answer to your question - the dew shield extends out around 10 inches beyond the corrector cell.

 

I do have a nice JMI EV1 Crayford focuser that can be attached to the C-8 or C-11. I use that for lunar/planetary imaging for two reasons. First it has a fine focus knob that can change the focus in small increments. Second it has a numerical focus scale that allows for repeatable focus settings, that are difficult to impossible to achieve with stock focuser. So as the seeing changes and you observe brief moments of the best seeing you can determine if focus best was at the 0.34 or the 0.35 setting. If the former was better you can go back to the better setting. I believe that is a very valuable utility for a focuser to have a scale, either analog or digital.

 

This fine JMI focuser fits both my C-8 and C-11 as well as my 10-inch GSO reflector.

 

To be completely up front I just try to minimize changes from the design 100 mm, and accept the consequences as indicated. This becomes an issue with large deviations from design caused by a binoviewer's large back focus requirement. For bino- & regular mono-viewing there are other important decisions that are worth the tradeoffs. I will be using a direct connected Baader Clicklock eyepiece holder with the diagonal inserted first. For an off balanced binoviewer or imaging train there is value in being able to secure the device at a comfortable viewing angle, while resisting the torque of the off balanced load. That likely causes a slight deviation from the 100 mm design. But the tradeoff is worth it.

 

Tomorrow I'll post some links to Cloudy Nights discussions of SCT optimization. Forum member Eddgie has been most helpful with this. There are numerical factors applied to deviation from nominal 100 mm design. Focal length and spherical aberration both increase for focal distances beyond 100 mm. But truth be told, we've been unknowingly accepting these consequences without even noticing the change. 

 

As for dovetails versus tube rings, I haven't considered the rings. The only rings I use are on the top rail mounted refractor.

 

As for my experience in deviation from 100 mm design for planetary imaging, I was never aware of the issue until relatively recently. According to Eddgie it will have an effect if pushed too far past design. But my best planetary image ever was before I knew of this. The following photo is Mars from 2020 on a C-11. I have no idea how far the focal plane was pushed out. The 100 mm design metric is just something to be aware of. Small deviations from that have minimal consequences. Where possible we do what when can when balancing all the factors that affect our results.

 

 

As for dew prevention the front fan has been a game changer for me. I'll try posting a link to the CN discussion on that. Here it is - Dew Elimination. I'm a believer in the heresy of low heat, front-fan dew prevention. I hardly ever get out my hair dryer any more.

 

Thanks Grant for the questions. Those are just what I was hoping for. I'm looking forward to what others have to say on the subjects. My opinion is one of many that can be of value.

 

Best Regards,

Russ

Edited by Rustler46, 11 December 2022 - 06:37 PM.

[luxo II]

As for dew prevention the front fan has been a game changer for me. I'll try posting a link to the CN discussion on that. Here it is - Dew Elimination. I'm a believer in the heresy of low heat, front-fan dew prevention. I hardly ever get out my hair dryer any more.

Agreed, it is an effective solution, though not one I need; a corrector 30mm thick holds a lot of heat.

 

OTOH on the rare occasions when it did dew, I was not prepared to try a hairdryer for fear of splitting it by thermal expansion (its BK-7). SCTs should be OK though.

Edited by luxo II, 11 December 2022 - 02:41 AM.

[whizbang]

Russ,

 

I followed the dew elimination thread and experimented with fans with poor results.  I think my fans don't move enough air.

 

May I ask what is the CFM rating on your fan?

 

And, why did you decide to mount your fan flush on the dew shield rather than at an angle like Alex (maroubra_boy) demonstrates in the dew thread?  Did you test the fan in both orientations?

[charles genovese]

On my mid 80's C8 (with superb optics)I have 2- 1 1/4" holes cut into the tube about 1" in front of the rear cell (just in front of the mirror) plus reflectix on the tube and the Kendrick dew cap. . I have a small fan with a contoured adapter made of JB weld but I found the C8 cools to ambient very rapidly without the fan and there are zero tube currents. (My C14 has fans on the rear and 3 holes in the front of the tube (just behind the corrector cell and also has zero tube currents). I don't use heater strips, just hair dryer when necessary.

Edited by charles genovese, 11 December 2022 - 01:08 PM.

[ABQJeff]

Nice set-up Russ!  What are your thoughts regarding covering the dew shield in Reflectix as well?  I did it per recommendations on CN, hasn't hurt and keeps black dew shield from frosting over since it is now 'silver' (so doing something).

[JMP]

Thanks for posting this, Russ. Coos Bay has to be a high dew environment!

[davidc135]

 

As for my experience in deviation from 100 mm design for planetary imaging, I was never aware of the issue until relatively recently. According to Eddgie it will have an effect if pushed too far past design. But my best planetary image ever was before I knew of this. Here's my Mars from 2020 on a C-11. I have no idea how far the focal plane was pushed out.

 

 

I believe that, for a C8, 1/24th-ish wave of over-correction is added for each extra 1 inch of back focus  I expect vignetting issues would sooner become an issue. If the optics were a little over-corrected to begin with it would matter more. Otherwise they could even be improved a touch. It would be worth a star test to check.

 

David

[Rustler46]

I believe that, for a C8, 1/24th-ish wave of over-correction is added for each extra 1 inch of back focus  I expect vignetting issues would sooner become an issue. If the optics were a little over-corrected to begin with it would matter more. Otherwise they could even be improved a touch. It would be worth a star test to check.

 

David

That's interesting, David. I knew it would affect spherical aberration. But I didn't know that this would be over correction. So if initially under corrected, extending back focus would be an improvement to some degree.

 

I'll have to learn how to do a star test on my Celestron-8 to see if there could be an improvement in the error present. That's nice to know. Thanks for sharing your insight.

 

Russ

[Rustler46]

Thanks for posting this, Russ. Coos Bay has to be a high dew environment!

Like many climate regimes, dewing is a problem for many of us. But at least here there are near zero problems with nighttime biting insects. Nevertheless if there are any mosquitos around I use full strength, 100% DEET repellant - strong enough to melt their legs off should they be so foolish as to land on me. Seriously, I don't want to acquire any insect-borne disease.

 

Russ

[luxo II]

Make sure you're downwind and well clear of your optics when you apply DEET - it can damage coatings permanently.

 

Here its a real necessity but we try to make sure everyone uses a roll-on rather than spray.

Edited by luxo II, 11 December 2022 - 07:28 PM.

[Rustler46]

Nice set-up Russ!  What are your thoughts regarding covering the dew shield in Reflectix as well?  I did it per recommendations on CN, hasn't hurt and keeps black dew shield from frosting over since it is now 'silver' (so doing something).

There has been extensive discussion on different threads about how to best employ Reflectix. I wish I could find the thread entitled something like "Reflectix on SCTs". In any case my C-11 has a metal dew shield covered with Reflectix. It also has a dew heater wrapped around so that the metal radiates heat back towards the corrector lens. The dew shield Reflectix helps keep the dew shield warmer and a more effective radiator. I'm unsure whether that is good in that it might heat the air in front of the corrector, causing degradation of nearby local seeing.

 

Having the dew shield insulated has a benefit in keeping it warmer than the night sky. The corrector lens "sees" the inside of the dew shield and thus can radiate heat away in that direction. So kind of thinking out loud, Reflectix on the shield would help. My C-8 has none but still stays dew-free with just the front fan and minimal heat applied.

 

The main utility of the dew shield is to reduce the amount of heat-sucking sky seen by the corrector lens. I'll be replying to my OP giving some of the physics behind why the front fan is so effective. I know for many amateurs application of heat is ingrained as the best way to prevent dew. It works, but my experience is the fan is much better.

 

Russ

[Rustler46]

Make sure you're downwind and well clear of your optics when you apply DEET - it can damage coatings permanently.

 

Here its a real necessity but we try to make sure everyone uses a roll-on rather than spray.

Thanks for that tip! I didn't know how it affects optical coatings. Maybe I wasn't too far off with my humor about melting mosquito legs. In any case, I spray the DEET while inside my garage before coming in the vicinity of exposed optics. 

 

Russ

[Rustler46]

Russ,

 

I followed the dew elimination thread and experimented with fans with poor results.  I think my fans don't move enough air.

 

May I ask what is the CFM rating on your fan?

 

And, why did you decide to mount your fan flush on the dew shield rather than at an angle like Alex (maroubra_boy) demonstrates in the dew thread?  Did you test the fan in both orientations?

I don't know the CFM rating of my fan. But it is just 35 mm in diameter running at 12 volts, 1.08 watts. I would agree the fan needs to move air at a rate to bring enough of it into contact with the corrector surface to replace the heat being radiated to the night sky. The larger the lens, the higher the CFM rating needs to be.

 

My flush mounted fan was ineffective in preventing dew at first. It was mounted that way for mechanical stability. I didn't want to make a tilted attachment point like Alex did on my dew shield. His dew shield, if I remember correctly, was made of corrugated plastic. Mine is a thick sheet of plastic. In any case to get the spiral flow pattern, I just added a deflector inside that changed the radial airflow into one with more of a spiral flow. That is what was needed to make the same fan work in my circumstances.

 

This was my original attempt.

 

This modification enabled a more spiral flow of air that worked as Alex recommended.

 

Russ

[Rustler46]

• Dew prevention with minimal heat and front corrector surface fan
  This I consider to be the one thing that has improved performance more than anything else. Applying heat does prevent dew. But this works counter to allowing the optics to adjust to nighttime air temperature. While everything else can be warmer than ambient temperature, the corrector lens can quickly fall below the dew point temperature with resulting dew. I won't at this time go into the physics of why the front fan works. 

Here's the physics.

 

The corrector lens, like all objects exposed to the night sky is engaged in a two way exchange of heat energy. The night sky despite being very cold is radiating some heat toward the corrector. The corrector being much warmer is radiating much more heat back to the sky. The tradeoff is the lens loses more heat than it gains and begins to cool below ambient air temperature. When the lens cools to below the air's dew-point temperature, it causes the air in contact with it to deposit as dew the moisture it can no longer hold onto the cool corrector lens.

 

Without any intervention on our part, the lens is in a losing contest involving heat transfer. Eventually dew can form, depending on circumstances. With warmer, dryer desert air dew may never form. Under my cool, moist maritime air dew very quick forms. But as they say, "your mileage may vary" - YMMV. We all have different circumstances. But the physical principles still hold. So keep the above two-way heat flow scenario in mind. Now how can we tilt local circumstances to discourage dew formation?

 

The first step to be taken is to employ a dew shield. This reduces the amount of heat-sucking sky "seen" by the corrector lens. The result is less heat lost in that direction. It replaces that shielded sky with a warmer dew shield, that in turn radiates some heat in the direction of the lens. Covering the dew shield with insulation serves to keep it warmer than otherwise. In some climates, a dew shield is all that is needed to keep the corrector free of dew during an evening observing session. In other cases more is need to keep the corrector warm enough.

 

Well, the brute force method is to dump heat on the corrector lens to keep it above the dew-point temperature. This works to prevent dew. But if not judiciously done, there can be negative consequences due to excess heat. So one method to heat the lens is via a heated dew shield. This employs a heater strip wrapped around the front corrector cell casting. In most cases this is an inefficient way to get heat to the lens. The lens is separated from its cell by spacers, which inhibits heat transfer from the front casting to the lens. Much of the heat goes toward heating the air above the lens, with the consequence of degraded local seeing. The newer Celestron heated lens retainer ring would likely be an improvement in getting more heat to the lens.

 

The Dewbuster system recommends heating the corrector lens from behind by heating the air inside the optical tube. This method does work, and is in my opinion a great improvement over the heated lens cell. For those who prefer the heat application method this works quite well. I've had success in this way for years.

 

But what about the front fan method? Here's how it functions - here's the physics. If the air in the immediate vicinity of your telescope is not fog (or not soon to become foggy), it is above the dew-point temperature. When you blow this air across the corrector lens it will work toward keeping it also above the dew-point. So as the lens radiates heat away, the air blowing across its surface transfers heat to it to replace the radiated heat. In practice this will allow the lens to remain just a bit below ambient air temperature. As long as that air is enough above dew-point temperature, the lens will also remain above dew-point temperature. Once fog begins to form around the telescope, you are just blowing foggy air on the lens, and dew will begin forming. All this dew-preventing heat flow will continue to work with minimal application of heat from external dew heaters. How much external heat is required to make this work depends on local circumstances. In my dew prone environment minimal electric heating is needed. How much?

 

As indicated in my OP what follows is my experience with the Dewbuster system. I expect the Kendrick system will also work as well. These allow for keeping the internal temperature of the OTA at a set number of degrees above ambient air temperature. This heated air heats the lens from behind. So here is what I found to be the case for dew-free optics, either with or without the front fan:

• Without fan - keeping the OTA 5-7°F above ambient temperature is needed to prevent dew.
• With fan - keeping the OTA 0-1°F above ambient temperature is sufficient to prevent dew. Keeping dew free with less heat is possible. 

There you have it - proof the low heat, front fan method works. It took me some time to really believe would. For those still bit hesitant, I recommend the DewBuster system. But for those thinking maybe there is some benefit to this low heat "heresy", there's some other things to consider.

 

Adding excessive heat can have some negative effects:

• It works counter to getting the mirrors closer to nighttime air temperature. The newer designs with rear mounted fans are a benefit in this regard.
• Internal tube air currents can play havoc with optical performance.
• A heated lens with relatively stagnant air above can form performance robbing boundary layers, that would be otherwise broken up by a front fan.
• For those operating portable with a battery power source, the excess electrical use is a consideration.
• Likely some of you can think of other negative effects.

Don't get me wrong. Heat works. But even at home with unlimited electrical power available, my dew-blasting hair dryer is never brought out to the telescope. And the DewBuster is either turned off or set to just 1 degree above ambient temperature.

 

So that's it. Believe it or not, this is one physics-backed way toward an "optimized Schmidt-Cassegrain telescope".

 

All the Best,

Russ

 

Russ

Edited by Rustler46, 11 December 2022 - 09:51 PM.

[luxo II]

Dew Prevention-2.jpg

 

Main problem I see with that is that you're wrapping the heater strap around the corrector cell - which is almost pointless because all you're doing is warming the corrector cell - and glass is a very poor conductor of heat - so most of that heat is lost to the external air without warming the corrector.

 

Put the heater strap around the OTA, about 15cm behind the corrector so that it heats the metal of the OTA, Then apply insulation over the heater strap and around the OTA. I found that 3-4W is enough to keep the OTA warm to the touch and this means that heat is transferred to the air inside and hence well spread over the corrector.

 

And yes arranging the fan to create a vortex over the corrector is the key to getting that to work.

Edited by luxo II, 11 December 2022 - 11:00 PM.

[Rustler46]

Main problem I see with that is that you're wrapping the heater strap around the corrector cell - which is almost pointless because all you're doing is warming the corrector cell - and glass is a very poor conductor of heat - so most of that heat is lost to the external air without warming the corrector.

 

Put the heater strap around the OTA, about 15cm behind the corrector so that it heats the metal of the OTA, Then apply insulation over the heater strap and around the OTA. I found that 3-4W is enough to keep the OTA warm to the touch and this means that heat is transferred to the air inside and hence well spread over the corrector.

 

And yes arranging the fan to create a vortex over the corrector is the key to getting that to work.

Luxo II, thanks for that insight. It is much appreciated. I agree 100% with your thoughts. The dew shield has had the heater strip removed and placed where you have suggested. The compartment that used to carry the dew heater has been padded as you see, so that the dew shield fits the front of my C-8. This Astrozap (formerly) heated dew shield has cutouts to match the C-8's top and bottom dovetails. Once the heater was removed the dew shield had a slightly smaller diameter that no longer lined up with the dovetail cutouts. The padding was added to restore the previous inner diameter, so that it lined up correctly. Here's another photo showing what the dew shield looks like now, with no dew heater in place. And the fan has an internal baffle that redirects the air flow in a spiral, non-radial direction. As you mention the spiral vortex is necessary for the front fan to be very effective.

 

 

With so many people convinced that heat is the best way to conquer dew, many of those ineffective, heated dew shields are still being sold. 

 

Best Regards,

Russ

[luxo II]

there's still money to be made selling snake oil...

Edited by luxo II, 12 December 2022 - 02:14 AM.

[RedLionNJ]

Couple words of thanks to Russ et al:

 

I took off my Reflectix and placed a dew strap about four inches back from the black "end piece" which holds the corrector on my Meade 12-inch, then replaced the Reflectix again. The dew strap is attached to a variable-output power source from Thousand Oaks. We'll see what that can do, in conjunction with an AstroZap-type dew shield which protrudes about 8 inches in front of that same end-piece at the front of the OTA.

 

In addition, I have a pair of variable-speed USB-driven micro-fans which, if the current setup is insufficient, I can attached to the dew shield to encourage airflow across the corrector.  Baby steps.

 

Thanks, folks!

 

Grant

[Rustler46]

Couple words of thanks to Russ et al:

 

I took off my Reflectix and placed a dew strap about four inches back from the black "end piece" which holds the corrector on my Meade 12-inch, then replaced the Reflectix again. The dew strap is attached to a variable-output power source from Thousand Oaks. We'll see what that can do, in conjunction with an AstroZap-type dew shield which protrudes about 8 inches in front of that same end-piece at the front of the OTA.

 

In addition, I have a pair of variable-speed USB-driven micro-fans which, if the current setup is insufficient, I can attached to the dew shield to encourage airflow across the corrector.  Baby steps.

 

Thanks, folks!

 

Grant

I hope that solves your dew problem, Grant. Progress is being made!

 

Russ

[Rustler46]

 

• Working within the SCT design compromises to minimize negative effects.
  • Endeavor to keep the focal plane at the design distance of 100 mm from rear port. I believe this metric had its genesis in the 1970's world of film-based astrophotography with SLR cameras adapted to the SCT rear port. Focusing on the film at a distance of 100 mm worked in that instance. 
    Please note - this is for standard, non-edge Celestron's. Other models &  brands may have differing designs.
  • Ability to extend the focal point is one of the benefits of the SCT design. But it has some potential negative consequences in a design full of compromises. Some negative effects are:
    • ​Vignetting and reduction of effective aperture for light gathering. Here's why:
      Pushing the focal plane further back entails the focuser mechanism pushing the f/2 primary mirror forward toward the secondary mirror and its baffle. Doing so begins to cause the outer portions of the light cone to be blocked by the baffle. The effect is reduced light throughput or smaller effective aperture. Also the central obstruction is now a larger percentage of the aperture, resulting in reduced contrast.
    • Increased focal length - 1/
    • Increase of spherical aberration - 2/
    • The closer back focus is kept at 100 mm the better the SCT will perform. Binoviewers are particularly a challenge in the regard. 
    • See Focal Reducers and the SCT.
    • 1/ - For a C-8 every 1 mm increase from 100 mm design back focus causes the focal length to increase by 3.2 mm.
    • 2/ - As I recall from the above link, every 25 mm increase past 100 mm adds 1/23rd wave additional spherical aberration. For large increase in back focus, the increased spherical aberration can be considerable.

Yahoo!  

 

After some research and excellent customer service from AgenaAstro, I was able to reduce the back focus on my Celestron-8 by 55 mm. Utilizing the above information this further "optimized" my SCT by the amounts listed below:

• Shortened the focal length by 176 mm (9%). See 1/ above.
• Reduced spherical aberration by 0.10 waves. See 2/ above.
• There should be no vignetting or reduction in aperture.

Before contacting Agena Astro I had a 55 mm long, 2-inch adapter attached to the rear port of my C-8. For a number of reasons I wanted to use the Baader Clicklock 2-inch eyepiece holder followed by the Baader prism diagonal. The ClickLock has a 2-inch nose piece that fits into the 55mm long adapter as shown below.

 

Here is the extra 55 mm back focus arrangement.

 

Agena Astro is very quick in answering email questions - like within minutes. After two back and forth question/answers I discovered the Baader ClickLock 2-inch nosepiece can be unscrewed, uncovering a female SCT thread. Thus the ClickLock is directly attached to the SCT rear port, saving 55 mm of back focus.

 

 

Every little bit of improvement helps with our SCTs.

 

Edit:

My rationale for using the stock focuser, Baader ClickLock & T2 prism diagonal is for long unbalanced loads like a binoviewer or APM Superzoom. These can present a considerable torque on the connection at the diagonal, when these eyepieces are oriented at a comfortable viewing angle. While my JMI EV1 Crayford focuser has a wonderful close connection with fine focus knob, it has a weak connection to resist torque from unbalanced loads. This is just a small diameter screw pressing on the internal compression ring. In contrast the ClickLock has a robust connection in this area. Both Clicklock and JMI EV1 have a similar back-focus burden. But for lunar/planetary imaging the fine focus of the JMI focuser wins out. For all other uses I prefer the Baader ClickLock.

 

For now my observing focus is on double stars, not LP astrophotography. But I believe the preferred arrangement shown in the second photo above has a back focus very near to the Celestron's 100 mm design. So that's where I'm at in optimizing my C-8. 

 

Best Regards,

Russ

Edited by Rustler46, 13 December 2022 - 02:37 AM.