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Eyepiece building strategy

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#1 Dobs O Fun

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Posted 24 July 2021 - 01:23 AM

I started with an eyepiece plan for my 8" dob (1200mm). I have a post in this forum about my next step for an 8mm EP. Firstly here are some foundations.

Tonight was my first full moon and first Saturn. It was my second night seeing Jupiter.

1. I collimated my Dob. To test it, the planets look like perfect doughnuts when I unfocus.

2. Using my supplied 25mm Saturn and the rings were clear, even noticing the gap between them. Jupiter as well but only the one larger red band.

3. Using my supplied 10mm the clarity of the moon was retained as well Saturn and the rings. Jupiters one red band became a little more predominate. Same as the 25mm but larger in mag.

4. Using my newly acquired barlow (Agena 2x) with 25mm was as good as the 10mm by itself. Just the mag was a bit less.

5. Using the same barlow with the 10mm was the struggle for overall clarity. Jupiters bands could not be defined very well. Saturn was a bit fuzzy. As far as the outer edges I cannot say. I'm new at this. I could not get a clear image.

My goal? Planetary observation with higher mag.

To me it's hard to say that the clarity is a factor of atmosphere, quality of the 10mm or quality of the barlow, or a combo of these. I have reset my expectations but I'm the type to push my scope. If the 10mm alone looked great and I am not able to baseline it against another mfg, I can assume that it's a medium quality at best. The barlow seems to work well so far below 240x. With only two eyepieces it's hard to find out where it won't "shine" for me.

My original plan was to get an 8mm and barlow it (300x) but I dont think that will work based on tonight's performance.

My next EP I plan to get a wider view than 60° if I can.

Would I be nuts to skip the 8mm and go lower, maybe 6mm or smaller?
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#2 Mbinoc

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Posted 24 July 2021 - 02:09 AM

A 6mm works well in my 6" Celestron StarHopper Dob when used on planets, in fact its my favorite.

 

I also have a 3.2mm, and that sort of sucks with what I have available. At times it can be fun to play with, but never really gave me a clear view.

 

I would think a 6mm would work well in a larger 8" dob, but let others chime in. I do not have a lot of experience using eyepiece below 15mm, I'd however rather use my 6mm, then Barlow my 15mm.

 

My whole lot is on the budget side, and my 6mm is a redline svbony. I'm happy using it in my 6" Celestron StarHopper Dob, and also my 90mm Meade 390.

 

I would think a 6mm would even be better in your scope. I do not plan on trying to push my dob any further.

 

The first time I used the 6mm on Jupiter and Saturn, I was impressed, The jump up from other lower end 2x Barlow's plossl eyepieces was a improvement for me.


Edited by Mbinoc, 24 July 2021 - 02:19 AM.

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#3 Redbetter

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Posted 24 July 2021 - 02:33 AM

1. I collimated my Dob. To test it, the planets look like perfect doughnuts when I unfocus.

 

I have never heard of using a defocused planet to check collimation.   It is not clear to me if your collimation is okay or not.  The proper test of collimation is a star at high magnification, small exit pupil (1mm or preferably less.)  The 2x Barlow with your 10mm will provide 240x for ~0.85mm pupil which should be sufficient.  

 

120x should work decently in about any seeing with your 8", but as you approach 200x and beyond the seeing will need to be progressively steadier.  There is a big difference between seeing that supports 240x well, and seeing that supports 150 to 200x. Your range between 120 and 240x has a huge gap.  As a first guess, I would be looking at an eyepiece or 2x Barlow plus eyepiece that would yield 6 to 7mm effective focal length for ~170 to 200x.  8mm effective focal length only yields 150x, which is close to what you already have.

 

You can evaluate seeing at high power using Damian Peach's page on the Pickering Scale.  Link.  Seeing varies and is the ultimate arbiter of what detail can be seen with a given aperture, even if the scope is perfect.  At 8" aperture, the seeing generally caps the effective magnification for max detail well below what the scope can do.


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#4 Jon Isaacs

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Posted 24 July 2021 - 03:00 AM

The important thing to keep in mind is that when viewing the planets, the seeing (stability of the atmosphere) is all important.  Some nights it's rock solid and your scope will take 300-400x.  Some nights it highly turbulent and things wont be good at all.  Most nights are somewhere in the middle.  The strategy is to have a range of eyepieces to cover the variety of possible conditions. 

 

If your new Barlow is like this one from AgenaAstro, then you can unscrew the optics and screw them onto the eyepiece filter threads, this acts as a ~1.5 x Barlow, so you would have 120x, 180x and 240x.  

 

 https://agenaastro.c...arlow-lens.html

 

Also, making sure your scope is cooled down is important.. A fan cooling the mirror is the best way to ensure your scope has reached thermal equilibrium,

 

Jon


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#5 Supernova74

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Posted 24 July 2021 - 03:05 AM

Trying to collimate on a planet is a bad idea it will give you an inaccurate reading,even more so as most of the planets are still relatively low in the meridian the image be so mushy and full of tube currents due to the atmospheric turbulence even when the scope has relatively cool down time its always best to find a star high in the sky close to the zenith.


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#6 Dobs O Fun

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Posted 24 July 2021 - 05:13 PM

After pontificating on these answers I agree that I have a "gap". The Barlowed 25mm works great, the Barlowed 10mm on planets did not do well. On the moon it was ok, just that the contrast or lack of makes it harder to define.

Instead of an 8mm I just ordered a 6mm for now. Before I purchased the barlow the highest mag I had was 120x.

Once I get the 6mm in use I can determine what's next. I just was not sure why I could not get a clear shot at Saturn with a 10mm + barlow.

#7 Thomas_M44

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Posted 24 July 2021 - 07:33 PM

I started with an eyepiece plan for my 8" dob (1200mm). I have a post in this forum about my next step for an 8mm EP. Firstly here are some foundations.

Tonight was my first full moon and first Saturn. It was my second night seeing Jupiter.

1. I collimated my Dob. To test it, the planets look like perfect doughnuts when I unfocus.

2. Using my supplied 25mm Saturn and the rings were clear, even noticing the gap between them. Jupiter as well but only the one larger red band.

3. Using my supplied 10mm the clarity of the moon was retained as well Saturn and the rings. Jupiters one red band became a little more predominate. Same as the 25mm but larger in mag.

4. Using my newly acquired barlow (Agena 2x) with 25mm was as good as the 10mm by itself. Just the mag was a bit less.

5. Using the same barlow with the 10mm was the struggle for overall clarity. Jupiters bands could not be defined very well. Saturn was a bit fuzzy. As far as the outer edges I cannot say. I'm new at this. I could not get a clear image.

My goal? Planetary observation with higher mag.

To me it's hard to say that the clarity is a factor of atmosphere, quality of the 10mm or quality of the barlow, or a combo of these. I have reset my expectations but I'm the type to push my scope. If the 10mm alone looked great and I am not able to baseline it against another mfg, I can assume that it's a medium quality at best. The barlow seems to work well so far below 240x. With only two eyepieces it's hard to find out where it won't "shine" for me.

My original plan was to get an 8mm and barlow it (300x) but I dont think that will work based on tonight's performance.

My next EP I plan to get a wider view than 60° if I can.

Would I be nuts to skip the 8mm and go lower, maybe 6mm or smaller?

Atmospheric conditions and/or mirror collimating and/or insufficient mirror cool down are most likely your limiting factors presently as based upon your view description.

 

With an 8-inch, even at 100X, you should be seeing multiple bands on Jupiter with even average/ fair seeing. 

 

I can regularly see see multiple bands on Jupiter with my 6-inch Newtonian, despite the poor thermal environment here at our home.

 

Make sure everything else is good before bothering with anything much above 150X.

 

If Jupiter is high in the sky and your scope is temperature equalized and well collinated, you should see lots more detail even well below 150X.

 

300X may rarely be useable for you even with good conditions.

 

Also: 60-degrees AFOV is plenty wide for hand-guided planetary observing up to above 200X.

 

It will cost a lot more to get a much wider AFOV eyepiece (70 degrees or more) that will be as sharp and high contrast as a better quality eyepiece in the 60-Degree AFOV class.


Edited by Thomas_M44, 24 July 2021 - 07:38 PM.

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#8 teashea

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Posted 24 July 2021 - 07:58 PM

I started with an eyepiece plan for my 8" dob (1200mm). I have a post in this forum about my next step for an 8mm EP. Firstly here are some foundations.

Tonight was my first full moon and first Saturn. It was my second night seeing Jupiter.

1. I collimated my Dob. To test it, the planets look like perfect doughnuts when I unfocus.

2. Using my supplied 25mm Saturn and the rings were clear, even noticing the gap between them. Jupiter as well but only the one larger red band.

3. Using my supplied 10mm the clarity of the moon was retained as well Saturn and the rings. Jupiters one red band became a little more predominate. Same as the 25mm but larger in mag.

4. Using my newly acquired barlow (Agena 2x) with 25mm was as good as the 10mm by itself. Just the mag was a bit less.

5. Using the same barlow with the 10mm was the struggle for overall clarity. Jupiters bands could not be defined very well. Saturn was a bit fuzzy. As far as the outer edges I cannot say. I'm new at this. I could not get a clear image.

My goal? Planetary observation with higher mag.

To me it's hard to say that the clarity is a factor of atmosphere, quality of the 10mm or quality of the barlow, or a combo of these. I have reset my expectations but I'm the type to push my scope. If the 10mm alone looked great and I am not able to baseline it against another mfg, I can assume that it's a medium quality at best. The barlow seems to work well so far below 240x. With only two eyepieces it's hard to find out where it won't "shine" for me.

My original plan was to get an 8mm and barlow it (300x) but I dont think that will work based on tonight's performance.

My next EP I plan to get a wider view than 60° if I can.

Would I be nuts to skip the 8mm and go lower, maybe 6mm or smaller?

You need to use a star, not a planet for the test.


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#9 zakry3323

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Posted 24 July 2021 - 08:16 PM

With my skies, it's a rare night indeed when I can use a focal length shorter than 6mm with my 8" F/6


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