94 replies to this topic

[Ihtegla Sar]

Fair enough, but before you give up on the C8 have a search for various threads on CN about solving thermal issues in SCTs and Maks by insulating instead of cooling them.

I love looking at globulars and planetary nebulae as they are relatively unaffected by light pollution but to resolve then I need more than 4". However, I agree that on planets I prefer the image from the refractor a lot of the time.

Yeah, I should look into reflectix. Maybe once I get a bigger mount. The Super Polaris that came with my C8 takes several seconds to dampen vibrations from focusing at higher magnifications, but I have a bigger mount on the way.

[Jon Isaacs]

Durability--there are threads here on CN stating that mirrors need to be recoated every six years, and if you wait too long it not only dims the image but also can cause damage to the surface of the mirror.

 

The coatings on my 10 inch Dob are 19 years old. Which scope do you think puts up the brighter images, my 120 mm Eon ED or the 10 inch F/5 with the 19 year old coatings?

 

Unlike Sketcher, I live in a location where a 10 inch typically shows more planetary detail than a 5 or 6 inch refractor. 

 

Jon

[Ihtegla Sar]

The coatings on my 10 inch Dob are 19 years old. Which scope do you think puts up the brighter images, my 120 mm Eon ED or the 10 inch F/5 with the 19 year old coatings?

Unlike Sketcher, I live in a location where a 10 inch typically shows more planetary detail than a 5 or 6 inch refractor.

Jon

Interesting that your coatings lasted more than three times longer than Don Pensack's. Is the air really that much worse for mirror coatings in Los Angeles than San Diego?

Edited by Ihtegla Sar, 11 January 2021 - 04:18 AM.

[Kunama]

The Galaxy mirror in the previous scope was coated in 1993 and has never been recoated, still looks better than many 5 year old mirrors in some Chinese GSO / Meade scopes I have had.  I guess it depends a lot on who coated it.

[dan_1984]

That's scaremongering.  Mirror coatings last a lot longer than six years unless you have weird contaminants getting on the glass (which would also get on a refractor lens).

 

An 8" mirror in need of recoating will still put up brighter images than a 4" refractor.  And it will still have twice the resolution.

Resolution is dictated by seeing. Above 8" you need very good seeing to benefit from the larger aperture, other wise you get a brighter blob and thats about it. I have a 12 inch SCT and the views are average, washed out, with a grey background, doesn't have that "wow" factor. After 3 or 4 years of using it I don't even know if they excel at anything..visually not impressive, imaging is a pain. If I had the opportunity to live under great seeing, my main scope would be a reflector, but for my skies refractors are the best tool.

[Jon Isaacs]

Interesting that your coatings lasted more than three times longer than Don Pensack's. Is the air really that much worse for mirror coatings in Los Angeles than San Diego?

 

I imagine Don recoated his mirror before I would have. And LA may be worse than San Diego..

 

Coatings don't split close doubles, aperture does that.

 

Jon

[213Cobra]

Interesting that your coatings lasted more than three times longer than Don Pensack's. Is the air really that much worse for mirror coatings in Los Angeles than San Diego?

It depends on where you are. Both cities are geographically spread, and have microclimates. But certainly Los Angeles air is more challenging, both because it is a vastly larger urban agglomeration than San Diego, and because LA County has the highest economic output of any county in the US.

 

I'm in Los Angeles. My one reflector is a Takahashi Epsilon 160 which I use visually. The scope and therefore the mirrors are 34 years old this year. I bought it new in the year it was made, 1987. It served its first 18 months in coastal Massachusetts, with observing forays also into New Hampshire, Maine and Vermont. Since early 1989, it's been used in California, mostly Los Angeles, with a smidgen of time in Orange County and also in the San Jose area. It's had occasional forays into the southern California desert.

 

The mirrors look new in terms of coatings showing no discoloration nor distress anywhere. Has it lost reflectivity? In principle, but it's not obvious. I haven't felt compelled to recoat but I'm sure I'll get to it sooner than later. Since moving west from New England, this Epsilon has seen very little dew or moisture of other origin. The scope has the Takahashi manhole cover on the objective end and the focuser is always capped when I'm not using it. Almost all my time in California has been in the arid San Fernando Valley portion of the city of Los Angeles, so that's probably a factor. And since the scope doesn't see daytime use, nighttime use isn't collecting the heavier business hours atmospheric fallout. Or maybe refractors have been cutting into mirror time.

 

Phil

[Mirzam]

It’s really not that easy to generalize about coating lifespan on reflecting scopes.  Variables include 1) initial coating quality, 2) local conditions—I.e, acid dew, salt, air pollution, cryptic moisture condensation* etc., and 3) degree to which the telescope design protects reflective surfaces from exposure, i.e., Maksutov and Schmidt Cassegrain designs fare better than Newtonian and classical Cassegrain scopes.

 

* Thick mirrors have a lot of thermal mass.  When they chill during the night, they can act like a cold finger to condense moisture on the mirror surface when you are not paying attention.  This is a problem with observatory scopes in the east where we can experience elevated humidity levels during colder times of year.  This can kill a coating in only a few years.

 

JimC

[Jon Isaacs]

* Thick mirrors have a lot of thermal mass.  When they chill during the night, they can act like a cold finger to condense moisture on the mirror surface when you are not paying attention.  This is a problem with observatory scopes in the east where we can experience elevated humidity levels during colder times of year.  This can kill a coating in only a few years.

 

 

A thin mirror is more likely to be colder than a thick mirror.  

 

Jon

[Mirzam]

The lower thermal mass of the thin mirror allows it to track ambient temps more closely.  The thicker mirror can remain colder for longer creating a temperature lag effect.

 

I was visiting D&G optics a number of years ago and was shown one of the company’s 20” Cassegrain scopes mounted in a dome. We pulled up the cover, peeked into the scope and saw that the mirror was heavily covered with condensation even though it had not been used the previous night.  The mirror was the coldest object in the dome and pulled moisture from the humid Spring air.  It was dripping wet.

 

The same thing happened occasionally with a full-thickness 12.5” mirror in my home observatory. Once I noticed the problem I rigged up a heating strap on a timer to keep the scope interior warm (closed tube) for a few hours each morning during the cold season.

 

JimC

[Jon Isaacs]

The lower thermal mass of the thin mirror allows it to track ambient temps more closely.  The thicker mirror can remain colder for longer creating a temperature lag effect.

 

I was visiting D&G optics a number of years ago and was shown one of the company’s 20” Cassegrain scopes mounted in a dome. We pulled up the cover, peeked into the scope and saw that the mirror was heavily covered with condensation even though it had not been used the previous night.  The mirror was the coldest object in the dome and pulled moisture from the humid Spring air.  It was dripping wet.

 

The same thing happened occasionally with a full-thickness 12.5” mirror in my home observatory. Once I noticed the problem I rigged up a heating strap on a timer to keep the scope interior warm (closed tube) for a few hours each morning during the cold season.

 

JimC

  

 

I'm just glad I don't face such issues. The only times I've ever had condensation on a primary mirror was taking it inside when it was cold. Then it just dews up. 

 

I can see how a thick mirror in a high humidity environment could lag behind and dew up..   

 

Right now my little weather station says it's 53° F with a relative humidity of 22%.. The dew point is 15° F..

 

Jim

[Ihtegla Sar]

 

I'm just glad I don't face such issues. The only times I've ever had condensation on a primary mirror was taking it inside when it was cold. Then it just dews up. 

 

I can see how a thick mirror in a high humidity environment could lag behind and dew up..   

 

Right now my little weather station says it's 53° F with a relative humidity of 22%.. The dew point is 15° F..

 

Jim

You are lucky to live where you live.  I ended up bringing my 20" Obsession home for the winter (December through early February) because during those months the skies are mostly cloudy anyway and there is a lot of humidity at my dark site, but the air is fairly dry and we get a number of clear nights the rest of the year.  I didn't want to unnecessarily expose it to humidity during a season when I probably wouldn't be able to use it anyway.  After having no issues with dew all year, in early November the mirror completely iced over before midnight.  Not dew, but a layer of ice/frost and not in the morning, at midnight.

 

So I went up a week or two later in late November and decided to reverse my usual practice of setting the scope out early and putting a fan on it, I would leave it in the shed well covered to try to keep the mirror as warm as possible until just before I used it.  Well the sky was completely clear at 4:30 pm as the sun was setting, but you could see a line of fog hugging the horizon in the distance, miles away.  As the sky darkened, you could see the fog moving closer and all of a sudden, in the space of less than five minutes, the sky went from completely clear everywhere except for the distant layer of fog hugging the horizon, to completely fogged in.  The fog rolled in like a horror movie.  So, I packed up and headed home, deciding to bring the 20" Obsession with me for a few months.

 

My refractor is much easier to deal with in rapidly changing weather conditions and it cools and warms up very quickly so dew and thermal stabilization are never an issue.  

 

Ironically, I have found it much easier to reach thermal stabilization with the Obsession in the fall when things are getting quite cold than in the summer months.  Where I live, thermal stabilization is a pain in the summer but in the fall, the Obsession reached thermal stabilization quite quickly, without much effort . . . before it iced over completely.

[mikeDnight]

Mike, what are the "most purposes" of which you speak?  On all but the brightest deep sky objects an 8" reflector will walk all over a 4" refractor.  Not saying the 4" frac wouldn't be nice but when I had one at the same time as my C8 it was the C8 that got used almost all the time.

Most purposes for me are mainly lunar and planetary, but I also enjoy using my 100mm refractor for deep sky too. I do recognise the advantages larger apertures provide, but I don't know what it is exactly that makes me enjoy my 100mm more than the 250mm F6 Dob that mostly stands unused in the corner of my observatory. I love the sharpness and purity of the views through my 100mm, and the definition it provides is simply glorious. The views and level of detail in my 100mm is jawdroppingly impressive when it comes to targets like Jupiter, Saturn and Mars. Even Venus is swathed with subtle cloud top detail. May be the Dob is just too bright for my eyes, while in the 250mm, although I can certainly see the detail, its just not as well defined. 

As regards the challenge of fainter deep sky targets, perhaps it's the challenge that I enjoy. So far the most difficult target I've successfully observed with the 100mm is IC 474, and a tiny black notch in the ghostly glow that I believe was the horse head, although I didn't see any shape to it. Brighter DSO's can be an absolute joy. I've attached a sketch I made some time ago of M27. Most people see it as a smudge, but that's because they don't take the time needed to really study it. Perhaps that's the key to getting the most out of a small aperture scope?

Unfortunately, or fortunately depending on your view point, I can only post the one image at a time. Otherwise I could bombard you with some beautiful deep sky sketches, each one taking 30 to 60 minutes to observe properly.

 

 

Stellar wise, the 100mm will reach an easy mag 13.2 and remains sharp on a night of steady seeing up to 1000X on double stars. To be honest though, although 1000X is a nice party trick, there is no meaningful advantage over 500X, so I don't use 1000X with any serious intent. It's just nice to know the stars are textbook perfect even at rediculous magnifications.

 

Of course the planet's put on quite a show even at medium powers. The attached Mars sketch from last year was made using a binoviewer and a magnification of approx 180X 

 

 

When it comes to the Moon, the 100mm could keep me occupied for several lifetimes. It resolves the central rille running along the length of the Alpine Valley floor nearly every lunation. And ultra fine rilles on the floor and up the terracing of the crater Werner that most seasoned lunar enthusiasts are blissfully unaware even exist, or can't even see in their larger apertures. 

 

So as you can see I'm pretty content with just a 100mm refractor, and not as content with the larger Dob's. Perhaps my UK skies have a lot to do with my lack of contentment with larger apertures. Although the 8.5" refractor at my local astro club has no difficulty whopping the reflectors no matter how big they get. But the SCT's just never make the grade here. They can't produce anything even approaching a textbook star image and so always give soft, lacklustre views. Again, it most likely our skies!

 

Mike 

Attached Thumbnails

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[N-1]

A nice Mars & M27 there Mike. You have skill and patience but also one of the best 4-inchers ever produced.

[BillP]

Each design has different strengths and weaknesses that the other design does not share.  So which is best really just depends on which strengths you like the most and which weaknesses you dislike the most.

[MortonH]

Most purposes for me are mainly lunar and planetary, but I also enjoy using my 100mm refractor for deep sky too. I do recognise the advantages larger apertures provide, but I don't know what it is exactly that makes me enjoy my 100mm more than the 250mm F6 Dob that mostly stands unused in the corner of my observatory. I love the sharpness and purity of the views through my 100mm, and the definition it provides is simply glorious. The views and level of detail in my 100mm is jawdroppingly impressive when it comes to targets like Jupiter, Saturn and Mars. Even Venus is swathed with subtle cloud top detail. May be the Dob is just too bright for my eyes, while in the 250mm, although I can certainly see the detail, its just not as well defined. 

As regards the challenge of fainter deep sky targets, perhaps it's the challenge that I enjoy. So far the most difficult target I've successfully observed with the 100mm is IC 474, and a tiny black notch in the ghostly glow that I believe was the horse head, although I didn't see any shape to it. Brighter DSO's can be an absolute joy. I've attached a sketch I made some time ago of M27. Most people see it as a smudge, but that's because they don't take the time needed to really study it. Perhaps that's the key to getting the most out of a small aperture scope?

Unfortunately, or fortunately depending on your view point, I can only post the one image at a time. Otherwise I could bombard you with some beautiful deep sky sketches, each one taking 30 to 60 minutes to observe properly.

 

2020-08-24 12.38.49.jpg

 

Stellar wise, the 100mm will reach an easy mag 13.2 and remains sharp on a night of steady seeing up to 1000X on double stars. To be honest though, although 1000X is a nice party trick, there is no meaningful advantage over 500X, so I don't use 1000X with any serious intent. It's just nice to know the stars are textbook perfect even at rediculous magnifications.

 

Of course the planet's put on quite a show even at medium powers. The attached Mars sketch from last year was made using a binoviewer and a magnification of approx 180X 

 

2020-10-03 00.51.21.jpg

 

When it comes to the Moon, the 100mm could keep me occupied for several lifetimes. It resolves the central rille running along the length of the Alpine Valley floor nearly every lunation. And ultra fine rilles on the floor and up the terracing of the crater Werner that most seasoned lunar enthusiasts are blissfully unaware even exist, or can't even see in their larger apertures. 

 

So as you can see I'm pretty content with just a 100mm refractor, and not as content with the larger Dob's. Perhaps my UK skies have a lot to do with my lack of contentment with larger apertures. Although the 8.5" refractor at my local astro club has no difficulty whopping the reflectors no matter how big they get. But the SCT's just never make the grade here. They can't produce anything even approaching a textbook star image and so always give soft, lacklustre views. Again, it most likely our skies!

 

Mike 

 

Beautiful images, Mike.  I've had the pleasure of seeing your work before on Stargazers Lounge. 

 

Sounds like I have better seeing than you, or perhaps the objects I'm observing are higher in the sky in the southern hemisphere.  My C8 and Newt were great for deep sky but for lunar and planetary the most pleasing views definitely came from refractors most of the time.  The FS-102 I used to own was particularly special on Jupiter and Mars.

[Bomber Bob]

Why your refractor and not the newt?

 

Dakin 4" F10 Refractor vs. Criterion RV-6 Newtonian:

 

 

About the same age (50 yrs), about the same focal length (1000 :: 1200).  Same ASI120MC, same GSO 2.5x APO Barlow.

 

The refractor is sharper.  The reflector shows more belt colors.

[Rick Runcie]

I know this is blasphemous, but I usually only set up a refractor and not also a reflector because the seeing is poor to average or I haven't used it in a while. My 10" Teeter/zambuto usually will show more detail on planets than my FS-152. Let me be very clear FS 152 has exceptional Optics and shows exceptional planetary detail but every time I think it is the best image I have ever seen I walk over to my 10in reflector and notice something that was not immediately visible on the refractor. The only time this does not occur is when the seeing is subpar. As a result I set up the 10 in far more often because it's quicker, easier and less hassle. I will never part with the fs 152 because the images are Sublime and the are so sharp and contrasty, but I usually set the two telescopes up side by side and compare them endlessly. It really doesn't take much for the seeing to improve to see the difference and the finer detail that is visible in the 10 in not to even bring up the image scale difference. Both will never leave my side they are to me the best in each class.

Best regards,  Richard 

[CHASLX200]

I know this is blasphemous, but I usually only set up a refractor and not also a reflector because the seeing is poor to average or I haven't used it in a while. My 10" Teeter/zambuto usually will show more detail on planets than my FS-152. Let me be very clear FS 152 has exceptional Optics and shows exceptional planetary detail but every time I think it is the best image I have ever seen I walk over to my 10in reflector and notice something that was not immediately visible on the refractor. The only time this does not occur is when the seeing is subpar. As a result I set up the 10 in far more often because it's quicker, easier and less hassle. I will never part with the fs 152 because the images are Sublime and the are so sharp and contrasty, but I usually set the two telescopes up side by side and compare them endlessly. It really doesn't take much for the seeing to improve to see the difference and the finer detail that is visible in the 10 in not to even bring up the image scale difference. Both will never leave my side they are to me the best in each class.

Best regards,  Richard 

All of my Zambuto's would beat any smaller APO's in my steady seeing.  600x no problem with a 10" while a 6" Tak is getting dim at 600x for Jupiter and Saturn. But that Tak is a 100x per inch killer for the moon and doubles.

[nowhere]

Honestly? It's because I think long focus refractors look beautiful. Since I live in the Vancouver area I put up with long periods of cloud and rain where I end up looking at my scope more than looking through it!

 

When it comes to the rare occasions I get a clear sky the trade offs between my refractor and a larger Newtonian are nicely illustrated by Bomber Bob's pictures. Each has it's own pros and cons. I prefer the view through a refractor usually. I hope to put together enough cash to get a 6 to 8 inch Cassegrain variant in the future so I can use the same mount as the refractor and have the option of going for the larger aperture when I feel like it.