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What would cause this?

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#1 StrangeDejavu

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Posted 27 July 2013 - 11:49 PM

I just had my Dob outside and noticed something strange going on with Saturn. Through the entire session, Saturn had a glare; almost like "white" CA, if that makes sense. I brought the Dob outside from my house which is set at 76 F. Outside temperature is 75 F with 84% humidity. I let the primary fan run the entire time I was out (9:30 - 11:30). According to CSC, Seeing was 4/5. I switched around from 24mm, 11mm, 8.8 and 4.7mm before bringing out my 4" f/10 and trying it. Saturn came to focus nicely in my refractor, but I can't say the same for the Dob; reaching focus was tough. Collimation was spot on, I checked it about every 20 minutes.

What would cause this? Does it sound like the primary dewed up? Or do these sound like the signs of poor seeing?

#2 star drop

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Posted 28 July 2013 - 12:25 AM

Did you try the same magnification in both telescopes? That usually levels the seeing problem close enough except for the larger column of air viewed through the larger aperture instrument. Did you see dew on the optics while collimating?

#3 rnc39560

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Posted 28 July 2013 - 01:13 AM

What is CA? Didn't mean to intrude. Just not familiar with some of the terms yet.

#4 SeattleScott

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Posted 28 July 2013 - 01:23 AM

Chromatic aberration. Typically a problem with refractors since they have trouble bringing all the wavelengths of light to focus together due to red having a long wavelength and blue having a short wavelength. Looks like a halo around a bright star or planet, usually blue in color. Reflectors and SCTs do not typically have this problem because they employ mirrors instead, or in addition to, a lens.

That being said, I cannot think of anything offhand that would cause a white halo around Saturn using a reflector other than dew. So can't help the OP.

#5 rnc39560

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Posted 28 July 2013 - 01:44 AM

Thank you.

#6 AstroTatDad

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Posted 28 July 2013 - 01:53 AM

Mitch, the only thing I can think of off hand right now is dew. Since I have owned my Dob I can recal 2 times this has happened to me.

1. I was out viewing for about a hour and 45mins "Dob was out for a good hour before viewing" fan on the whole time. I viewed Saturn "good and sharp" then I went off viewing some DSO's. I was using filters at this time and started to notice my viewing wasn't that great "after 1 hour and 45mins" so I took my filter off and went back to Saturn. I get Saturn back in view and I was like "what?" Saturn was glowing/halo like fuzzy white. So I looked down fan was unplugged, lol. it's so dead quite around here I can hear the fan, but I had my ear phones on and didn't even notice it was unplugged. So I took my EP out and with a light looked at the secondary "Dew" :) looked down the tube "Dew" :) so I called it a night.

2. I will not go in detail with this one. but samething started to happen when I was out viewing. I looked down "fan was plugged in" I looked down the tube, primary was fine.. took the EP out to check the secondary "DEW" I was like "really??" so I packed it up for the night. I did check the bat's and they were very low.

All in all I never had any problems but those 2 times. But from what you are saying sounds close to my experience that I had. If it happens again check for dew on both.

#7 frito

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Posted 28 July 2013 - 01:55 AM

kind of depends on what you mean by a halo.

if its hazy around the planet and other bright large objects like say the moon for example that is often caused by moisture, not nessisarily moisture on any of the optics but could just be moisture in the air. fogged up optics can and does cause the same thing to happen along with a dimming and blurryness added to the image, its worse if its on the eyepiece rather than on a mirror but both will produce it, on a dob first thing to check for dew is the secondary, then the eyepiece, primaries on dob's rarely dew over unless they completely open truss and even then the mirrors are often too big to drop below the dew point quickly unless you have a ton of cooling fans on them.

the other type of halo i've seen on reflectors is a sort of ghost image type of effect on planets, this is caused by poor collimation, a star test would confirm if that is the problem. how are you collimating your scope? if you are using a laser it may have lost its collimation resulting in you thinking your collimated when in reality your laser is making your collimation worse than a simple sight tube or collimation cap would make it. if you have another way to collimate I would try that and see if it helps if you are seeing a ghost image, the discription white CA tends to make me think that is what you are seeing.

#8 AstroTatDad

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Posted 28 July 2013 - 02:02 AM

Yeah there you go Dwayne, I would say mine was more of a foggy-ish on my mirrors but mostly dew like. I collimate my Dobby everytime I take her out "haha that sounds funny" but like I said it has only happened 2 times "knock on wood".

#9 frito

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Posted 28 July 2013 - 02:03 AM

Mitch, the only thing I can think of off hand right now is dew. Since I have owned my Dob I can recal 2 times this has happened to me.

1. I was out viewing for about a hour and 45mins "Dob was out for a good hour before viewing" fan on the whole time. I viewed Saturn "good and sharp" then I went off viewing some DSO's. I was using filters at this time and started to notice my viewing wasn't that great "after 1 hour and 45mins" so I took my filter off and went back to Saturn. I get Saturn back in view and I was like "what?" Saturn was glowing/halo like fuzzy white. So I looked down fan was unplugged, lol. it's so dead quite around here I can hear the fan, but I had my ear phones on and didn't even notice it was unplugged. So I took my EP out and with a light looked at the secondary "Dew" :) looked down the tube "Dew" :) so I called it a night.

2. I will not go in detail with this one. but samething started to happen when I was out viewing. I looked down "fan was plugged in" I looked down the tube, primary was fine.. took the EP out to check the secondary "DEW" I was like "really??" so I packed it up for the night. I did check the bat's and they were very low.

All in all I never had any problems but those 2 times. But from what you are saying sounds close to my experience that I had. If it happens again check for dew on both.


lol yeah Jeff, fans are good to have but running them for too long esp on smaller scopes in dew prone conditions/areas can be detrimental to an observing session. I've learned how to deal with dew because in the winter both here at home and even more so at a darker site my club frequents dew is a rampant problem, even in the summer right now whenever i see someone bring out a SCT without a dew shield i know they are going to end up with a dewed over corrector plate within a few hours, its that bad. for my dob i've made resistor network dew heaters for my secondary and finders and in such conditions i keep the eyepieces i'm using in my jacket pockets to keep them warm and i never run my fan for longer than a half hour or so after it gets dark and dew will rarely form on my primary by the end of the night. it also helps to make sure if you are not using the scope to keep it pointed somewhat horizontally rather than vertical, dew believe it or not forms on anything the sky directly above you "sees" its the reason why dew shields work to some extent at least. solid tube dobs are nice because they have a massive built in dew shield :) the secondary is much smaller though and more exposed so its almost always the first thing to dew over on your typical dob. on truss dob's a dew heater is a must have thing on secondaries, esp really open ones like obession UC's and similar.

#10 AstroTatDad

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Posted 28 July 2013 - 02:07 AM

very very true Dwayne. :) I have been thinking of doing the same for my Dob, my SCT is set with heaters. Dew hasn't been bad, so I haven't got around doing the Dob yet.

#11 frito

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Posted 28 July 2013 - 02:12 AM

yeah, the only thing worse than dew is frost! yep i've had a frosted over scope before.

here is an awesome picture of one of my dob's i traded to a friend that was out too long in the cold at our dark site this past january.

http://sanjoseastron...stronomers.html

#12 AstroTatDad

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Posted 28 July 2013 - 02:30 AM

WOW!! That looks like my windows on my car when I use to live in the mountains in the winter time. :) yeah when it was that cold, I often took our scope out.. If I did it was for a very short time haha.

#13 Tony Flanders

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Posted 28 July 2013 - 05:07 AM

yeah, the only thing worse than dew is frost! yep i've had a frosted over scope before.

here is an awesome picture of one of my dob's i traded to a friend that was out too long in the cold at our dark site this past january.

http://sanjoseastron...stronomers.html


It's entertaining to hear you Californians chat about frost as though it's something exotic. To me, the only "awesome" thing about that photo is that the telescope is, for all practical purposes, frost-free. Its a rare winter night for me when the tube has that little frost at the time I decide to knock off for the night.

I was expecting to see a quarter inch of rime.

Dew or frost, it's much the same thing. They're inevitable on the outside of the tube, but you don't want them on the mirrors.

Normally they form on the secondary mirror of a Newtonian but only in the worst conditions on the primary. In either case, they're very easy to see if you shine your red flashlight on the mirror.

Of course, condensation also forms on eyepieces all the time.

#14 frito

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Posted 28 July 2013 - 05:19 AM

haha yeah, we've got it good here weather wise for the most part thats for sure. Frost is pretty rare for us bay area folks, well not rare but certaintly not an everyday occurance most of the year and when it does get that cold here its usually just a few degrees below freezing is all so yup we got it good.

#15 Jon Isaacs

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Posted 28 July 2013 - 08:36 AM

It's entertaining to hear you Californians chat about frost as though it's something exotic. To me, the only "awesome" thing about that photo is that the telescope is, for all practical purposes, frost-free. Its a rare winter night for me when the tube has that little frost at the time I decide to knock off for the night.



We are spoiled.. no mosquitoes in the summer, summer days long enough to observe. In the winter, it does get cold enough to frost but it has to be clear and very dry so frost is generally not a problem.

The worst problem is that sometimes at altitude, a low cloud forms, probably radiation fog. There will be a wind so it can be perfectly clear one minute, three minutes later, it's fogged over and everything is dripping wet, 4 minutes after than it's clear again but by the time the scope is dry enough to use, it closed out and dripping again...

My guess is the first poster had a dewed over secondary.

Jon

#16 Seldom

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Posted 28 July 2013 - 09:27 AM

The worst problem is that sometimes at altitude, a low cloud forms, probably radiation fog. There will be a wind so it can be perfectly clear one minute, three minutes later, it's fogged over and everything is dripping wet, 4 minutes after than it's clear again but by the time the scope is dry enough to use, it closed out and dripping again...


I've found my truss shroud soaking or frosted on nights where it was way above dew (or frost) point. Please elaborate more on "radiation fog".

#17 David Pavlich

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Posted 28 July 2013 - 09:49 AM

Considering your location, dew would be at the top of my list.

David

#18 StrangeDejavu

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Posted 28 July 2013 - 10:19 AM

Thanks everyone, the secondary probably was dewed over. I'm still new to the Dob world, having just bought it on May 9th. Toward the end of May, we were getting thunderstorms, tropical storms, water spouts and/or thick clouds all day every day. Last night was the first night i've been out since around May 29th... :( The eyepieces were clear, I checked them before trying them out in my refractor. I collimate with a laser, which is actually spot on. First night I spent with my Z10, we had excellent seeing and I was able to see the greenish-gray hexagon on the pole of Saturn (no hexagon shape, just a circle, but the seeing was that good).

I guess next time it happens i'll bring an extension chord and a hair dryer. Thanks again everyone, I learn more and more each day. :waytogo:

#19 Jon Isaacs

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Posted 28 July 2013 - 10:20 AM

I've found my truss shroud soaking or frosted on nights where it was way above dew (or frost) point. Please elaborate more on "radiation fog"



My elementary understanding is that Radiaton fog is fog that forms in a low lying place and forms because the ground cools from radiation.

Radiation Fog

Jon

#20 newtoskies

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Posted 28 July 2013 - 10:28 AM

Yeah Mitch, it has to be dew. I had the same thing in June when the club had the outreach. Called it a night real quick.
I only had my dob out during the winter to get just a light frost on it. That was enough for me and I packed things up and called it a night. It was down to the 20's.

#21 StrangeDejavu

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Posted 28 July 2013 - 10:50 AM

Yeah Mitch, it has to be dew. I had the same thing in June when the club had the outreach. Called it a night real quick.
I only had my dob out during the winter to get just a light frost on it. That was enough for me and I packed things up and called it a night. It was down to the 20's.


Boo. :thumbsdown: I must say though, I would rather deal with the cold any night of the week. Summer observing in Florida is just plain miserable. When you get those clear nights though, you just suck it up and go out there. It was so humid last night, I had sweat rolling down my nose, into my eyes, all over my glasses, etc. :lol: I did discover one thing I loved last night though: ThermaCell. Oh my goodness, I will never be without again. On the average, 2-3 hour Summer observing night, I come in with around 20-25 mosquito bites. I had 2 last night! I was ecstatic. :jump:

But yes, I think I would rather deal with the cold than the heat. Winter observing here in Florida is as close to perfect as it gets. My favorite part of the sky comes out around early/late Winter, it's around 50 F at night, no bugs at all, good clear skies, low humidity which means less dew; I can't wait. ;)

#22 newtoskies

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Posted 28 July 2013 - 11:11 AM

I hear you Mitch. This summer has been really bad with the humidity and bugs, not just skeeters. The beetles and nats get on my nerves. For some reason the beetles only land on me.

I can't wait for Nov when it cools down but not too cold. Clear nights and no bugs.

#23 frito

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Posted 28 July 2013 - 12:04 PM

I've found my truss shroud soaking or frosted on nights where it was way above dew (or frost) point. Please elaborate more on "radiation fog"



My elementary understanding is that Radiaton fog is fog that forms in a low lying place and forms because the ground cools from radiation.

Radiation Fog

Jon


I know what you mean, we get that in the winter at our darker site we often use, it often forms right after sunset and usually clears up but it can take an hour or two sometimes to dissapte probably because ground temp starts to equalize with the temp above.

#24 GlennLeDrew

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Posted 28 July 2013 - 02:36 PM

The description of 'white chromatic aberration' is very suggestive of spherical aberration. A prominent 'fuzz' extending less than a planet diameter does not seem like fogging; dewing up has more of the effect of overall dimming the image.

How does the scope perform at other times? Or is this a new scope?

If the indoors and outdoors temperature are rather similar, there should be no need to run a fan.

#25 AstroTatDad

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Posted 28 July 2013 - 02:37 PM

@Tony, yeah I lived in Big Bear Lake for 10 years think the coldest it got was 13 degres and biggest snow storm was 4 feet of snow. Best part being up there at 7,500 elevation was the nice skies, miss it. I had a lot of good seeing when I use to live up there.

@Mitch, same here this is my first Dob as well. :)






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