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Post by mikey cee on Mar 12, 2013 10:28:03 GMT -5
Just wanted to hear from others with R30 lenses in the range of 5"- 9" and who live in cold climates. When I start my Jupiter observing sessions on cold nights say below freezing everything looks OK for the first few minutes. Then a green "cloud" begins to encircle the planet. When this occurs I must fine tune the focus inwards to elliminate this anomally. This can and does continue on for an hour or so. I mark my drawtube with black marker pen at the beginning and end of the observing period. I find that my inward focus range is 3/4"-1" difference after elliminating the green "cloud" effect. Do you other folks experience a similar thing? This is merely an observation in cool down time and not a negative statement just curious that's all.  Mike |
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gord
Full Member
Posts: 82
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Post by gord on Mar 15, 2013 21:02:06 GMT -5
Hi Mike,
That's an interesting observation. 3/4 - 1" of focuser travel is a lot, but not out of line for a large achromat, especially when you are looking at the focus points between red/green/blue and where the red hangs out on these.
The green "cloud" you are seeing is likely the out of focus green channel. You may be focusing or had been focusing on details more in the red or blue range and then were just noticing the green.
One thing with the eye/brain combination is that it's quite good a filtering information. This is why the classic purple haze often seems to fade from view over time. We just get used to it and ignore it.
In the case of the green though, pay attention to that one. It's the most important channel and the one we are most sensitive to. Focus for the best, sharpest image in green, and let the others fall where they may. That is unless you are using filters, specifically the narrowband ones.
To get the clearest idea of what the lens is doing, you can test it out on a star with various filters (narrowband ones) and see where the focus points are. Just measure where the focuser position is for each color and it will tell you everything. You will need 1. Baader solar continum (for green), 2. H-alpha (for red), and 3. H-beta (for blue).
Clear skies,
-Gord
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Post by StephenEveleigh on Mar 24, 2013 16:47:54 GMT -5
Hello Gents,
This is an interesting find. Only last night (March 23, 2013), I was out with my baby (127/f12/R30) and it was at freezing - coming from a heated room of about 70F. Now I was not going for a long set up, just a quick one and the scope had not been placed in the garage for its usual 1-2 hr cold bath.
And yes focusing on jupiter was difficult; you could focus the red bands clearly and have the rest of the planet mushy - but I would have called it yellowish more than green. You could adjust focus, making trade offs for either the red bands which are normally sharp, or bring the rest of the planet into focus.
I thought nothing of it - attributing it to wisps of high altitude cloud blowing by and the fact the scope did not have an oppurtunity to cool. The scope does need to time to stabilize and is excellent once that has occurred.
Stephen
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gord
Full Member
Posts: 82
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Post by gord on Mar 25, 2013 20:56:31 GMT -5
Hi Stephen,
Well, we certainly haven't been having much luck with weather lately. And by lately, I mean 6 months! Yikes, what a bad run. At least we've had a few clear early evenings recently to be able to catch the comet, even if the seeing is dreadful. I wanted to get out Sat. night as well, but am a bit under the weather.
What you are describing sounds like what we have been discussing for some time regarding the red de-focus and how it will affect the image on some targets. It's like you describe. You can focus on one, the other, or in-between. I don't believe it has anything to do with temperature (although that can have it's own issues).
Here's a test for you. Get things setup and well acclimated and then do the test again. But, be sure to check in on Jupiter after not viewing it continuously when looking to see if you get the same effect. The reason I say this is if you are viewing continuously (the target) and then try to evaluate if you are seeing the effect, it isn't as clear of a test. The eye/brain combo adapts itself over time to filter out the "noise" and so you won't be seeing things the way you were in the test you report above. I find it's the same way for looking for the classic purple haze. If I view for a while continuously, it kind of fades away and isn't so apparent. But if I go away, use a different scope for a bit, etc. and then come back, it's really noticeable.
BTW, I sent you a message offline a while ago and was wondering if it ever made it to you?
Clear skies,
-Gord
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Post by Ales - iStar Optical on Mar 26, 2013 4:33:53 GMT -5
Dear Stephen,
make sure to let the scope cool down, at least 1,5 hours for this mass of glass. Then test again. You should not do any kind of legitimate testing while your scope is not temperature stable and if you take ANY scope into extreme cold you will see some very ugly and strange image shifting, boiling, color changes, etc.. so what you are describing here is still a pretty mild case of that. Also make sure that your scope is perfeclty collimated. Basically, wait for warmer weather and share your tests and visual experiences. Because posting test results obtained with a non temperature stable optics only adds fuel to a negative comments about Istar and our products by some people. Thank you again and stay in touch,
best regards,
Ales
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gord
Full Member
Posts: 82
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Post by gord on Mar 26, 2013 20:39:46 GMT -5
Hi Stephen,
I don't think the cooling requirements are as strict as being made out here for the basic tests you are doing, but certainly give it a few minutes. It's only 5" after all. I disagree with Ales when he says to "wait for warmer weather". It's shouldn't be necessary and in our climate, frankly, we have to take what we can get!
If you were doing a critical star test evaluation on the other hand, then the 1.5hrs that Ales mentions would be a good recommendation. The star test is *extremely* sensitive and is where things like even slight variations in temperature will be easily seen. It can be done in any weather, you just need to work according to the conditions. It wouldn't hurt to even do the star test while cooling down to see if there is a noticeable change in appearance. That could give some indications of the amount of cooling required or if it really is important. Test right away, and then later.
I would also say that with most testing, it's always best to try at least a couple of times (different outings) to see if the effect/result is consistent. I had one night this winter where I was seeing astigmatism in my IStar 6" F10. Odd I thought as I didn't remember seeing it previously, but it was definitely there. Next outing it was gone. Temp was -15c that night. It's one of the colder nights I've ever used this scope, but it could also even been that and the position I had the scope in for that object. There are certainly many variables to consider.
I thought of another couple of tests that would be useful for you. The moon is a great target and one where the discussions around the various color corrections are going to have little to no effect. If you are seeing nice images of the moon, but still softness and difficult focus on Jupiter, then it will give a good indication. Saturn is also a target where a red shifted design is going to have less penalty and we are soon going to be getting a good view of that.
The other is one that was discussed on CN, that is using a prism diagonal. The prism will improve the correction of red and worsen blue. This will help balance the red hanging out somewhat. The effect has been reported by James Ling on his new APM ED which also has red de-focus. I believe bino-viewers will also contribute to this effect since they use prisms. A mirror diagonal will not affect the correction.
Clear skies!
-Gord
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Post by Ales - iStar Optical on Mar 30, 2013 11:13:31 GMT -5
I hope that most of you agree that any scope, and especially large refractors with a great mass of optical glass do require a fairly long time to cool down. No serious observing can be done during this time. Especially no testing of any kind should be attempted during the cooling down time. The image is changing constantly so test results obtained during the glass temperature shifting can not be taken seriously by anyone. Information supporting my statement can be found in any book about optics.
Same must be said about quality of seeing. This is what many people tend to forget about. Again, test results obtained during poor seeing conditions (and poor seeing can be even during coudless, seemingly clear night). I know that all experienced observers know all this, but I feel that it is important to remind these facts to everyone. Especially when people start posting reports obtained during the cooling/lens shifting time.
best regards,
Ales
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gord
Full Member
Posts: 82
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Post by gord on Apr 1, 2013 20:14:38 GMT -5
Hi Ales,
I think you are making a bit to much of a blanket statement here about environmental factors, testing, and what you can and can't do. Certainly your point about testing to examine the full potential of an optic is correct in terms of taking these factors into serious consideration.
However, the point that was made is that test ing during the cool down period in and of itself is a test, and a useful one. It helps understand the optics behaviour and how it is impacted by these conditions and lays the ground work for being able to determine how it is performing (or not performing...) at other times. If it shows under correction while cooling, but perfect once stable, that tells us something during normal use: if you are seeing under correction, then the scope hasn't reached perfect equilibrium.
It is quite possible to determine information from the star test even in less than optimal conditions (environment or scope). The only limit as you state, is that the most critical examination of the patterns is not going to be possible.
Clear skies,
-Gord
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