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Are secondary mirror heaters not good?

Dew Heater Acquisition Deep Sky
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Oscar avatar
Oscar avatar
…(deleted section)…

I bought a "4tronix" secondary mirror heater, and I just tested it, it heats, and it also fits my secondary, however, I heard from another thread that secondary mirrors increased star size, and generally degraded image quality, and that a dew shield is a better option.

So my question is, should I still use it on my secondary, or should I use it to warm up my ASIair instead? because my ASIair also gets dewy, and I'm afraid the water can get inside and short something.

I got the heater because my backyard gets very humid at night, I think because I'm about 30 miles close to the ocean, and sometimes it's a little windy, and I'm thinking that windiness can carry the moisture onto my secondary.

I know already that my dew situation is so bad that even a 5.5 inch dew shield didn't stop dew from getting on a surface (my guidescope, which I'm not using anymore), from my location, and I heard 5.5 inches should've stopped dew formation.

I don't even have a dew shield for my Newtonian anyways.

I'm also afraid of fog, because summer is gonna be worse in humidity; last night I saw that the sky was clear, but then it got really foggy, and that's probably just the beginning; is there some accurate online fog/humidity forecast website?
Continuum - Laurent Lucas avatar
Continuum - Laurent Lucas avatar
Kudos to you for recognizing a mistake, that takes courage. How old are you?

Infrared satellite images (sat24) are useful to spot short-term fog.

A dew heater could distort the wavefront, but if it can save an otherwise unusable night, that's for the better!
Oscar avatar
Oscar avatar
Continuum - Laurent Lucas:
Kudos to you for recognizing a mistake, that takes courage. How old are you?

Infrared satellite images (sat24) are useful to spot short-term fog.

A dew heater could distort the wavefront, but if it can save an otherwise unusable night, that's for the better!

So far, I haven't had any secondary mirror dew problems; the heater was just precautionary.

Maybe I should use it for the ASIair, then if dew gets on my secondary, I get a dew shield?
Continuum - Laurent Lucas avatar
Continuum - Laurent Lucas avatar
The Asiair heats by itself, I would not worry about it.
Wes Schwarz avatar
Wes Schwarz avatar
You are going to need a dew heater and a dew shield. Pulse the heater on low. The thermal expansion and heat plumes from the heater can create issues.
Also i doubt anyone cares about the “snot nebula”? Own it. Run with it. Dont apologize. Dont do things you think you’ll later regret.
Mike Hamende avatar
Mike Hamende avatar
Some 5mm EVA foam makes good and easy DIY dew shields.  I've found that applying about 2 or 3 watts of heat to the body of my focuser prevents dew on the coma corrector and the secondary mirror.  I used to have severe dew issues even with a dew shield.  I did use secondary heaters of about 0.5 watts but I didn't like running wires across the spider as it altered the normally very clean diffraction spikes.
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Jeff Reitzel avatar
Jeff Reitzel avatar
I'll throw this one in for your weather question. Astropheric is one great app to use for our hobby. There are others like Clear Outside or even Clear Sky Chart. As far as fog goes you need to know the temperature and dew point at your site. When they get within 5 degrees F of each other the chance for fog is high. The closer they are to each other, the higher the chance for fog. You should be able to get that info from a quick internet search of your local weather or from an environmental sensor on your rig such as  focuser sensor, dew heater controller, or something like a Pegasus Power Box sensor. Many of them report both temperature and dew point. There are several other factors needed for fog to actually form so local weather stations/forecasts are usually the best resource. 
CS,
Jeff
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andrea tasselli avatar
andrea tasselli avatar
If the tube is well insulated by the dew shield with reflective layer outside then you shan't have many problems with dew (I never had any on either the 6" or the 12") even without a secondary heater (which are very no-no in my book). Best to use thick comping mats for this. It needs to run from the focuser out to 1.5x the aperture diameter from the tube end.

And if is foggy you just pack up and go.
Oscar avatar
Oscar avatar
Continuum - Laurent Lucas:
The Asiair heats by itself, I would not worry about it.

Thanks, didn't know.
Wes Schwarz:
You are going to need a dew heater and a dew shield. Pulse the heater on low. The thermal expansion and heat plumes from the heater can create issues.
Also i doubt anyone cares about the “snot nebula”? Own it. Run with it. Dont apologize. Dont do things you think you’ll later regret.

I just got a Dew Ninja dew heater (strip) for the telescope tube; where should I place it? The back of the scope, the mid, or the front?

Thanks for the advice.
Mike Hamende:
Some 5mm EVA foam makes good and easy DIY dew shields.  I've found that applying about 2 or 3 watts of heat to the body of my focuser prevents dew on the coma corrector and the secondary mirror.  I used to have severe dew issues even with a dew shield.  I did use secondary heaters of about 0.5 watts but I didn't like running wires across the spider as it altered the normally very clean diffraction spikes.

Good idea about the foam.

I wont use the secondary heater; I've been really wanting clean diffraction spikes.

About the focuser: how did you apply heat the focuser? A low profile one like this? https://www.teleskop-express.de/en/telescope-accessories-5/focusers-adapters-motorfocus-296/focusers-and-micro-transmission-92/ts-optics-2-low-profile-rack-and-pinion-newtonian-focuser-8-kg-payload-974

I don't see any place to put a dew heater on there...
Jeff Reitzel:
I'll throw this one in for your weather question. Astropheric is one great app to use for our hobby. There are others like Clear Outside or even Clear Sky Chart. As far as fog goes you need to know the temperature and dew point at your site. When they get within 5 degrees F of each other the chance for fog is high. The closer they are to each other, the higher the chance for fog. You should be able to get that info from a quick internet search of your local weather or from an environmental sensor on your rig such as  focuser sensor, dew heater controller, or something like a Pegasus Power Box sensor. Many of them report both temperature and dew point. There are several other factors needed for fog to actually form so local weather stations/forecasts are usually the best resource. 
CS,
Jeff

Thanks a lot for the info! Didn't know about Astrosperic.


andrea tasselli:
If the tube is well insulated by the dew shield with reflective layer outside then you shan't have many problems with dew (I never had any on either the 6" or the 12") even without a secondary heater (which are very no-no in my book). Best to use thick comping mats for this. It needs to run from the focuser out to 1.5x the aperture diameter from the tube end.

And if is foggy you just pack up and go.

I think I'll use foam, thank you for all the info.



----------------------------

And what do I do with my secondary heater? Maybe put it somewhere on the mount? Close to the RA/DEC motors maybe?
Oscar avatar
Oscar avatar
This is unrelated, but I'm making my dark and bias frame library right now.

Can someone tell me how many is a good amount?

So far I took:

200 Bias for -10C, 100 Gain

100 Darks for 10s, -10C, 100 Gain
100 Darks for 60s, -10C, 100 Gain
30 Darks for 300s, -10C, 100 Gain

I'm thinking waiting for 100 darks for 300s is way too long; Is 40-50 good enough?

I understand darks and bias are not really important for a 533 camera, but I'm thinking that darks/bias must benefit it in some way; I'm still waiting for more parts to be delivered (like the TS-optics focuser), so that I can do normal imaging, so I'm taking this opportunity to take calibration frames.
Matthew Proulx avatar
Matthew Proulx avatar
I've used reflectix around my tubes for a few years with wrap around dew heaters under them near the front. I've tried many a secondary heater and they all led to affecting the stability of the mirror. Primary dew heaters, the same. I've had a lot of dew otherwise with the heat bands turned off. I frequently have 90% humidity. My Pegasus astro box controls the temps automatically based on humidity.
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Arun H avatar
Arun H avatar
This is unrelated, but I'm making my dark and bias frame library right now.

Can someone tell me how many is a good amount?

So far I took:

200 Bias for -10C, 100 Gain

100 Darks for 10s, -10C, 100 Gain
100 Darks for 60s, -10C, 100 Gain
30 Darks for 300s, -10C, 100 Gain

I'm thinking waiting for 100 darks for 300s is way too long; Is 40-50 good enough?

I understand darks and bias are not really important for a 533 camera, but I'm thinking that darks/bias must benefit it in some way; I'm still waiting for more parts to be delivered (like the TS-optics focuser), so that I can do normal imaging, so I'm taking this opportunity to take calibration frames.

See John Hayes' post in this thread. Something like 16 darks is all you need.

https://www.astrobin.com/forum/c/astrophotography/deep-sky/lets-discuss-about-dark-bias-dark-flats/?page=1
Oscar avatar
Oscar avatar
Arun H:
This is unrelated, but I'm making my dark and bias frame library right now.

Can someone tell me how many is a good amount?

So far I took:

200 Bias for -10C, 100 Gain

100 Darks for 10s, -10C, 100 Gain
100 Darks for 60s, -10C, 100 Gain
30 Darks for 300s, -10C, 100 Gain

I'm thinking waiting for 100 darks for 300s is way too long; Is 40-50 good enough?

I understand darks and bias are not really important for a 533 camera, but I'm thinking that darks/bias must benefit it in some way; I'm still waiting for more parts to be delivered (like the TS-optics focuser), so that I can do normal imaging, so I'm taking this opportunity to take calibration frames.

See John Hayes' post in this thread. Something like 16 darks is all you need.

https://www.astrobin.com/forum/c/astrophotography/deep-sky/lets-discuss-about-dark-bias-dark-flats/?page=1

Oh, I remember that rule now.

But what happens when I go past 16, with bias and darks?

If that was talked about in the linked thread, I didn't understand it. 

And BTW, I did take at least thirty 300s dark frames, but I found, near the end of my darks Autorun session, that the cooler was using %100 power on my ASIair, and was actually not cooling to -10C; it was more like -8C, and my 533 was hot when I touched it; is everything okay with my 533? I didn't expect the hotness.

And are all my darks okay to use together? It's only a difference of -2C between my first 300s darks and last 300s darks...

If I'm not very clear about something, plz let me know.
Mike Hamende avatar
Mike Hamende avatar
Mike Hamende:
Some 5mm EVA foam makes good and easy DIY dew shields.  I've found that applying about 2 or 3 watts of heat to the body of my focuser prevents dew on the coma corrector and the secondary mirror.  I used to have severe dew issues even with a dew shield.  I did use secondary heaters of about 0.5 watts but I didn't like running wires across the spider as it altered the normally very clean diffraction spikes.

Good idea about the foam.

I wont use the secondary heater; I've been really wanting clean diffraction spikes.

About the focuser: how did you apply heat the focuser? A low profile one like this? https://www.teleskop-express.de/en/telescope-accessories-5/focusers-adapters-motorfocus-296/focusers-and-micro-transmission-92/ts-optics-2-low-profile-rack-and-pinion-newtonian-focuser-8-kg-payload-974

I don't see any place to put a dew heater on there...

I use adhesive backed polymide film heaters like this. If you have a dew controller with RCA outputs  you can use a pigtail like this. 

If you don't have a dew heater controller you can you a PWM controller like this with pigtails like these to adjust the power of the film heater.  

As far as where to put the heater, just stick it anywhere on the fixed cylindrical barrel of the focuser.  If you are not electrically inclined then tread carefully.
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Oscar avatar
Oscar avatar
Mike Hamende:
Mike Hamende:
Some 5mm EVA foam makes good and easy DIY dew shields.  I've found that applying about 2 or 3 watts of heat to the body of my focuser prevents dew on the coma corrector and the secondary mirror.  I used to have severe dew issues even with a dew shield.  I did use secondary heaters of about 0.5 watts but I didn't like running wires across the spider as it altered the normally very clean diffraction spikes.

Good idea about the foam.

I wont use the secondary heater; I've been really wanting clean diffraction spikes.

About the focuser: how did you apply heat the focuser? A low profile one like this? https://www.teleskop-express.de/en/telescope-accessories-5/focusers-adapters-motorfocus-296/focusers-and-micro-transmission-92/ts-optics-2-low-profile-rack-and-pinion-newtonian-focuser-8-kg-payload-974

I don't see any place to put a dew heater on there...

I use adhesive backed polymide film heaters like this. If you have a dew controller with RCA outputs  you can use a pigtail like this. 

If you don't have a dew heater controller you can you a PWM controller like this with pigtails like these to adjust the power of the film heater.  

As far as where to put the heater, just stick it anywhere on the fixed cylindrical barrel of the focuser.  If you are not electrically inclined then tread carefully.

Thanks for the advice
Arun H avatar
Arun H avatar
Arun H:
This is unrelated, but I'm making my dark and bias frame library right now.

Can someone tell me how many is a good amount?

So far I took:

200 Bias for -10C, 100 Gain

100 Darks for 10s, -10C, 100 Gain
100 Darks for 60s, -10C, 100 Gain
30 Darks for 300s, -10C, 100 Gain

I'm thinking waiting for 100 darks for 300s is way too long; Is 40-50 good enough?

I understand darks and bias are not really important for a 533 camera, but I'm thinking that darks/bias must benefit it in some way; I'm still waiting for more parts to be delivered (like the TS-optics focuser), so that I can do normal imaging, so I'm taking this opportunity to take calibration frames.

See John Hayes' post in this thread. Something like 16 darks is all you need.

https://www.astrobin.com/forum/c/astrophotography/deep-sky/lets-discuss-about-dark-bias-dark-flats/?page=1

Oh, I remember that rule now.

But what happens when I go past 16, with bias and darks?

If that was talked about in the linked thread, I didn't understand it. 

And BTW, I did take at least thirty 300s dark frames, but I found, near the end of my darks Autorun session, that the cooler was using %100 power on my ASIair, and was actually not cooling to -10C; it was more like -8C, and my 533 was hot when I touched it; is everything okay with my 533? I didn't expect the hotness.

And are all my darks okay to use together? It's only a difference of -2C between my first 300s darks and last 300s darks...

If I'm not very clear about something, plz let me know.

nothing prevents you from getting more. There is no downside other than time. A two degree difference is not material. Remember your lights will not be at the exact temperature either.
Liam Jones avatar
Liam Jones avatar
This is unrelated, but I'm making my dark and bias frame library right now.

Can someone tell me how many is a good amount?

So far I took:

200 Bias for -10C, 100 Gain

100 Darks for 10s, -10C, 100 Gain
100 Darks for 60s, -10C, 100 Gain
30 Darks for 300s, -10C, 100 Gain

I'm thinking waiting for 100 darks for 300s is way too long; Is 40-50 good enough?

I understand darks and bias are not really important for a 533 camera, but I'm thinking that darks/bias must benefit it in some way; I'm still waiting for more parts to be delivered (like the TS-optics focuser), so that I can do normal imaging, so I'm taking this opportunity to take calibration frames.

 I believe that this can indeed improve the image quality. By the way, if you're looking for a convenient tool for writing tasks, you should check out https://chatgpt.com/g/g-a0ULgFcuZ-essay-helper  It has many useful features, especially for creating high-quality essays. I used it frequently in school, college, and still use it when I need something done quickly and efficiently.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
In early autumn at the very beginning of the season there is plenty of fog around my parts as well. The downside of being not too far away from the coast. Other than that it's mostly snow/rain and cold temperatures. Parts of my setup freeze over all the time but a few things never do, even in temperatures as low as -30 degrees or so. The Asiair, mount head, camera and scope usually always keep the frost away. I keep power on at all times for dew heaters and mount/asiair, as long as it's very cold, and they do a good job of keeping the frost away. I do tend to use the cap on my telescope on off time and cover it with a jacket to protect it from the environment though.

I don't use a reflector, but I figured I'd pitch in about the Asiair. If it's very humid around your location things may be different, but I have yet to experience any problems with it in my particular environment at least. On my refractor I only use two dew heaters, one close to the front element and a second around  the edge of the dew shield in order to prevent frosting on the end that may cause reflections or frost falling onto the glass. When I use the Samyang I keep one on the dew shield and one on the guide scope, for my SCT I only use a dew heater ring and a dew shield (non metallic).

At some point I simply settled on 60 frames each for calibration and it just stuck. So that's what I do. Flats and dark flats are done in no time, the darks I usually do whenever there's a down time anyway so I don't care how long it takes really. People do differently, but I usually stick with the same calibration frames until I swap the optics and it has yet to fail me. At least with the refractor and lenses. I suppose reflectors may be different and more prone to collect dust, I know I need to start taking flats every session with my SCT at least.
Oskari Nikkinen avatar
Oskari Nikkinen avatar
I wouldnt use the secondary heater unless it was the only possible solution. Heating the scope is kind of the opposite of what you want to do, since thermal equilibrium is very important for high quality data.

I image from a location which is about a kilometer or two from the sea, and humidity is often around 80-95%. I use a dew shield and a primary mirror fan with my 8" newtonian, and so far these 2 have kept the secondary dry on all but the worst nights, and in those worst nights only the outer ~3mm of the secondary froze. The primary fan helps keep dew and frost away with how it circulates air inside the tube (and of course carries heat away - the main reason for it).

Typical winter conditions look like my profile picture, so frost everywhere, but the dew shield + fan combo has worked well. You need not worry about electronics dewing/freezing, their internal heat will take care of that issue on its own.
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Dale Penkala avatar
Dale Penkala avatar
I use my secondary mirror dew heater ONLY when necessary! I use my 12” dew shield I made out of kydex and it works extremely well. The humidity can get heavy here in Michigan USA and if I notice any hazing at all that is when I use the secondary heater.

I’m not familiar with the one your using but I have used the Kendrick units as well as the Astro Systems units and they work very well. The Astro Systems is actually adjustable temp wise in that you can set it to only allow the heater to heat to within .5-1.0º of the outside temp if memory serves me correctly. As someone above had mentioned “pulse” this is how its achieved.

In my case with my 12” I used my spider vanes as the conductor so that I didn’t have to run a wire to the dew heater across one of the vanes.
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Alex Nicholas avatar
Alex Nicholas avatar
It really depends on focal length, your pixel scale and the local seeing conditions…

If you were in a location where the seeing was INCREDIBLE (1 arc-second) and your camera and scope was giving you 0.8" per pixel, yeah, you'd probably be affected by a heater in the optical path.

As you're close to the ocean (relatively) and you're both humid and windy, I think you'd find that you're never going to see the effects.

Definitely, on a scope like an RC where the scope has a 2000+ mm focal length, any distortion from heat in that optical path is going to have *some* effect… But if you're using an 8" f/4 or 5, and siting somewhere around 1.5~2 arc-sec / pixel, I really wouldn't worry about the optical effects of adding a few watts of heat to the secondary.

I'd rather lose a touch of resolution than have to pack up at midnight due to a completely wet secondary mirror….
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