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Question about 8" Dobsonian and higher magnification

beginner dob eyepieces
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#1 brimacx

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Posted 18 July 2019 - 11:20 AM

Hi all, 

 

I am relatively new to this forum (and new to astronomy) and have gleaned a lot of great advice and info. I have an 8" Apertura Dobsonian that came with the 9mm and 30mm eyepieces. I bought a highly recommended 2" GSO 2X Barlow that has an adapter to fit the 1.25" lens.

 

I've been able to view the moon up close with the 2x Barlow and the 9mm installed and they seemed pretty sharp. I am able to view Jupiter with the 2X barlow and the 30mm and everything appears to be sharp. Things don't look so great when I remove the 2" 30mm EP and and put the 1.25" 9mm into the barlow (with adapter). I can't get it to focus as sharp. I am pretty certain it's because I'm not doing something right. Maybe there's too much focal length in the barlow and EP for it to be able to achieve the focal sweet spot? Do I need to use the 35mm extender tube at some point to make it focus on further objects? I tried adjusting the lengths of the barlow tube and EP position.

 

I found M13 last night after sweeping the skies for a good 15-20 minutes. with the 2X barlow and 30mm lens it looked like a small grey cloud. I attempted to get closer by putting in the 9mm EP into the barlow but I could not get it to focus and see any small pin point stars. Just a bigger out of focus grey cloud...

 

Am I missing something with regards to the 9mm and 2x Barlow combination?

 

I was thinking of maybe getting a 6mm Goldline EP that everyone recommends - can someone give me a link to one of those? I've been searching but not sure if my results are the correct EP...

 

Thanks for any advice! 

 

 

 

 

 

 

 

 

 

 



#2 SeattleScott

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Posted 18 July 2019 - 11:34 AM

A barlowed 9mm is a bit much magnification for Jupiter. Typically I would use 5-7mm for Jupiter with your scope. It is possible collimation is affecting your high power viewing but you say the Moon looks good. Keep in mind Jupiter is low in the sky this year for northern hemisphere viewers, so that is likely the biggest issue.

One option would be to get a 12mm so you could do medium power DSO viewing without a barlow, and then barlow it for Jupiter.

Scott
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#3 sg6

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Posted 18 July 2019 - 12:04 PM

May be several things:

Scope says it came with a collimator - check the state of yours. May need to check the collimator first however.

 

Barlow makes 2400mm so 2400/9 = 266. Might be a bit high, and atmosphere and height of Jupiter may make it worse but would have said Jupiter is high enough. Does depend on where you are. Alaska means it will be very low, Australia better.

 

Scope comes with a 35mm extension.

You may need to use that with the barlow. A camera usually needs to go inwards, so I assume thje extension is for visualage. Not 100% sure where it would actually be fitted. Guessing Focuser-Extension-Barlow-Eyepiece. Although that would be awful for changing the barlow in and out. Try Focuser-Barlow-Extension-Eyepiece.

 

Says 9mm Plossl, on f/6(ish) should be OK No description of the 30mm but may be more basic. They are not going to include an 30mm ES 2" eyepiece. Also the barlow may not be great either.

 

M13 is a cloud of stars, be a little careful of what you expect. Ignore all images you have seen, images are different.

 

I never use a barlow. Tend to keep the amount of glass down and barlows are not the greatest precision item. The barlow supplied was almost certainly not specifically designed for the scope. Means the scope has to work with the barlow and the barlow has to then work with the eyepiece. If one combination is weak the result is a poor view.

 

6mm eyepiece, little idea. I have a WO 6mm planetary copy/clone/rebrand. There is a 6.5mm Meade HD. A 6mm plossl I would avoid.

 

1200 at 9mm is 133x. That should do Jupiter, M13, Saturn and many others happily. If the 9mm works forget the barlow. Buy a 12mm or 15mm for mid power and later consider a 6mm of some variety for the higher aspect.


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#4 SeattleScott

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Posted 18 July 2019 - 12:59 PM

If M13 was an unresolved grey cloud with an 8” scope, you have collimation issues, or considerable light pollution. Even in light pollution you should be able to resolve some stars using averted vision.

Scott
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#5 brimacx

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Posted 18 July 2019 - 01:18 PM

Thanks guys for the great info - really appreciate it. I'm in Southern California Bortle 6 skies. Last night didn't see moon out so that was good for darkness. From what I understand the collimation should be ok since when it's out of focus and looking at jupiter I see a perfect donut shape - which my understanding is correct... I'll check it with the laser collimator provided to be sure.  What I don't get is using the 2x barlow with the 9mm EP showed the moon as pretty sharp and close up. It's when looking at further objects like Jupiter and M13 with same set-up that I couldn't get good focus on. If the optics of the barlow are bad wouldn't it not work on the moon? The optics are great on the barlow when using it with the 30mm EP - just when I attach the 9mm EP is when things start getting bad. I'll just have to monkey around more with the different combinations to see if anything works as expected...

 

Thanks again!



#6 Volvonium

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Posted 18 July 2019 - 02:14 PM

Hey brimacx, I'm also in SoCal, but under more severe Bortle 8 ... M13 is a grey blob with my 8" F6, and 10" F5 dobs on most nights.  Even though it looks like a blob, it's still a very cool target to even be able to see, considering how far away it is.  

 

With the 10" I can resolve stars in M13 without too much difficulty using averted vision, and with the 8" I need to be dark adapted and have to make an effort with averted vision.  Light pollution strongly diminishes the ability to resolve DSOs, even brighter ones like M13.   Getting out to a darker area or conversely having a larger mirror to collect more light will have the biggest effects for targets like globular clusters. One other huge factor is that you use an eyepiece/magnification that produces an ideal exit pupil. 

 

Using a 2X barlow with the 9mm produces too small of an exit pupil for DSO viewing-- DSOs put out very little light and going under 1mm exit pupil will generally diminish your view far too much to be practical.  With your setup, a 2x barlowed 9mm in an 8" F6 will have an exit pupil of about 0.75mm which is likely too small to resolve any additional details.  On something like the moon, a 0.75mm exit pupil is perfectly fine, as the moon is just overflowing with reflected photons...whereas a DSO does not have that benefit.  With DSO's your telescope is literally scrounging for photons and you have to work harder to detect them with your eye. 

 

I love using this website to assist me with exit pupil and FOV between eyepieces:

https://astronomy.to.../field_of_view/

 

Your 9mm by itself should provide the best views of M13 under common viewing conditions, providing a 1.5mm exit pupil.  I generally use 1mm as my practical limit; getting any additional details from anything smaller than that requires excellent optics, collimation, and seeing conditions.  


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#7 NorthernlatAK

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Posted 18 July 2019 - 02:34 PM

I am guessing that seeing is not supporting that mag especially on Jupiter if you are in the north. It is low for northerners for several years so don't expect a clear view. Perhaps try a Target that is high in the sky at that magnification. I have a similar scope (8" f5.9), and use a 9mm and Barlow on the moon when high enough and the view is jaw dropping when seeing supports it. I do not get a clear view of Jupiter at all in any eyepiece at my 60° north latitude, it's only 12° high at max. Collimation is important at high magnification as well.
Btw, m13 is stunning in my 9mm t1 nagler. Chains of stars and the propeller is visible.

Edited by NorthernlatAK, 18 July 2019 - 02:39 PM.

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#8 vdog

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Posted 18 July 2019 - 02:35 PM

Your 9mm by itself should provide the best views of M13 under common viewing conditions, providing a 1.5mm exit pupil.  I generally use 1mm as my practical limit; getting any additional details from anything smaller than that requires excellent optics, collimation, and seeing conditions.  

waytogo.gif

 

For planets, something around 6mm might not be a bad idea, but it would be worth it to invest in a really good eyepiece if you can.  I had a 6mm plossl once, and looking at planets through one is possible but the tight eye relief and narrow field of view take a lot of the fun out of it.  I also went through some mini-upgrades before finally settling in on a Meade 5.5mm as my go-to eyepiece for Jupiter. 


Edited by vdog, 18 July 2019 - 02:36 PM.

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#9 Volvonium

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Posted 18 July 2019 - 03:19 PM

waytogo.gif

 

For planets, something around 6mm might not be a bad idea, but it would be worth it to invest in a really good eyepiece if you can.  I had a 6mm plossl once, and looking at planets through one is possible but the tight eye relief and narrow field of view take a lot of the fun out of it.  I also went through some mini-upgrades before finally settling in on a Meade 5.5mm as my go-to eyepiece for Jupiter. 

Yeah, I think a quality multi-element ep around 6mm would be the best for planetary.  I have the same scope as OP and I found that my best planetary ep so far was my ES82 6.7mm-- with a 1.17mm exit pupil, I got a very sharp, contrasty, consistent planetary image and could see fine details, albeit the planet was visually smaller.   I could push 5.5mm or even a barlowed 8mm->4mm, but the image started getting a bit fuzzy and there was more fighting/waiting for moments of clarity amidst the atmospheric conditions.  

 

I'm still very much a beginner and I still have to intentionally stop myself from trying to push the heck out of the magnification, hoping to get a view of the planet that fills the eyepiece... and just appreciate how much better contrast and sharpness I get when I dial it back.  Jupiter is a very tricky target that has a lot of low contrast.  At a lower power you can see darker banding that might make you think you can push it harder, but when you do, it can get too washed out.


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#10 brimacx

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Posted 18 July 2019 - 03:23 PM

Hey brimacx, I'm also in SoCal, but under more severe Bortle 8 ... M13 is a grey blob with my 8" F6, and 10" F5 dobs on most nights.  Even though it looks like a blob, it's still a very cool target to even be able to see, considering how far away it is.  

 

With the 10" I can resolve stars in M13 without too much difficulty using averted vision, and with the 8" I need to be dark adapted and have to make an effort with averted vision.  Light pollution strongly diminishes the ability to resolve DSOs, even brighter ones like M13.   Getting out to a darker area or conversely having a larger mirror to collect more light will have the biggest effects for targets like globular clusters. One other huge factor is that you use an eyepiece/magnification that produces an ideal exit pupil. 

 

Using a 2X barlow with the 9mm produces too small of an exit pupil for DSO viewing-- DSOs put out very little light and going under 1mm exit pupil will generally diminish your view far too much to be practical.  With your setup, a 2x barlowed 9mm in an 8" F6 will have an exit pupil of about 0.75mm which is likely too small to resolve any additional details.  On something like the moon, a 0.75mm exit pupil is perfectly fine, as the moon is just overflowing with reflected photons...whereas a DSO does not have that benefit.  With DSO's your telescope is literally scrounging for photons and you have to work harder to detect them with your eye. 

 

I love using this website to assist me with exit pupil and FOV between eyepieces:

https://astronomy.to.../field_of_view/

 

Your 9mm by itself should provide the best views of M13 under common viewing conditions, providing a 1.5mm exit pupil.  I generally use 1mm as my practical limit; getting any additional details from anything smaller than that requires excellent optics, collimation, and seeing conditions.  

Thanks so much for the great info! That really explains my issue and will help me avoid throwing money at eyepieces that probably won't help my issue. I'll definitely be referring back to this post



#11 brimacx

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Posted 18 July 2019 - 03:44 PM

Thanks for the insight everyone - I'll look into the eyepieces recommended. Ideally I'd like to view M13 as something other than a grey cloud. If I could see just a few twinkling stars inside the cluster that would be awesome!



#12 Volvonium

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Posted 18 July 2019 - 03:44 PM

Thanks so much for the great info! That really explains my issue and will help me avoid throwing money at eyepieces that probably won't help my issue. I'll definitely be referring back to this post

Yeah if you're not too far, can probably bring the scopes out some time and try out a few different eyepieces.  I threw a bit of money at random eyepieces when I started early this year, before really optimizing my collection for my telescopes/viewing conditions.

 

I still need to reach out to OC Astronomers and join that astronomy club, but it would be good to meet other amateur astronomers and try different combinations.  I'm in Long beach/North OC



#13 brimacx

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Posted 18 July 2019 - 04:11 PM

Yeah if you're not too far, can probably bring the scopes out some time and try out a few different eyepieces.  I threw a bit of money at random eyepieces when I started early this year, before really optimizing my collection for my telescopes/viewing conditions.

 

I still need to reach out to OC Astronomers and join that astronomy club, but it would be good to meet other amateur astronomers and try different combinations.  I'm in Long beach/North OC

Sounds good and yeah I've been meaning to look into some clubs down here too but just got into it so still getting a feel for everything so I know what questions to ask and really what to look for, etc.  



#14 SteveG

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Posted 18 July 2019 - 04:14 PM

From what I understand the collimation should be ok since when it's out of focus and looking at jupiter I see a perfect donut shape - which my understanding is correct... I'll check it with the laser collimator provided to be sure. 

 

Thanks again!

That is not an indication of good collimation. You can use a star, but it must be centered, and only enough outside or inside of focus to show diffraction rings.

 

Collimation is a 3 step process, that typically will hold well in a solid tube 8” reflector:

1. Make sure the secondary is centered under the focuser.

2. Make sure the secondary axis is aligned to the center of the primary.

3. Make sure the primary is aligned to the focus axis.

 

Step #1 can be not perfect but close.

Step #2 is critical, and can be done with a site tube, combo cheshire tube, or well aligned laser.

Step #3 is also critical, and can be done with a combo tube, barlowed laser, or a simple collimation cap.



#15 vdog

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Posted 18 July 2019 - 04:23 PM

I found that my best planetary ep so far was my ES82 6.7mm-- with a 1.17mm exit pupil, I got a very sharp, contrasty, consistent planetary image and could see fine details, albeit the planet was visually smaller.  

That's a good EP, too.  It's a bit more pricey but they come up in the classifieds all the time.

 

As far as optimal magnification goes, that seems a very personal thing, although certainly affected by conditions at your location. But I push the envelope on magnification all the time.  I don't mind if the seeing is just OK; I guess I have a higher tolerance for this than some.

 

Anyway, might as well see what your gear can do before you decide what combination of it provides your best view.



#16 brimacx

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Posted 18 July 2019 - 05:10 PM

That is not an indication of good collimation. You can use a star, but it must be centered, and only enough outside or inside of focus to show diffraction rings.

 

Collimation is a 3 step process, that typically will hold well in a solid tube 8” reflector:

1. Make sure the secondary is centered under the focuser.

2. Make sure the secondary axis is aligned to the center of the primary.

3. Make sure the primary is aligned to the focus axis.

 

Step #1 can be not perfect but close.

Step #2 is critical, and can be done with a site tube, combo cheshire tube, or well aligned laser.

Step #3 is also critical, and can be done with a combo tube, barlowed laser, or a simple collimation cap.

ok, thanks... would the laser collimator that came with my scope be all that I need to properly collimate?  Would I even need to do collimation if stars look perfectly sharp in my scope?



#17 SirLoyne

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Posted 18 July 2019 - 05:26 PM

I've got an 8" Dob. I had one of those 6mm goldline eyepieces. I sold it and got one of these. It's a much better eyepiece, and it was the best view of Jupiter I've ever got. That's without my Barlow.

 

Meade HD-60 Eyepiece 6.5mm - $75

https://agenaastro.c...0-eyepiece.html


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#18 mjulihn

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Posted 18 July 2019 - 07:00 PM

ok, thanks... would the laser collimator that came with my scope be all that I need to properly collimate?  Would I even need to do collimation if stars look perfectly sharp in my scope?

Maybe, but it depends on whether your laser is itself centered/collimated. Often they are not, and require some work to be accurate. There are plenty of posts here on CN how to do this.


Edited by mjulihn, 18 July 2019 - 07:01 PM.

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#19 brimacx

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Posted 18 July 2019 - 08:26 PM

I've got an 8" Dob. I had one of those 6mm goldline eyepieces. I sold it and got one of these. It's a much better eyepiece, and it was the best view of Jupiter I've ever got. That's without my Barlow.

 

Meade HD-60 Eyepiece 6.5mm - $75

https://agenaastro.c...0-eyepiece.html

Thanks - I've heard good things about these Meade 5000 eyepieces. 



#20 SeattleScott

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Posted 18 July 2019 - 08:56 PM

I just use a laser. You can roll it on a countertop and watch the beam against a wall across the room to verify the alignment of the laser.

Reflectors lose collimation so you might as well get used to it, I dialed in a buddies Dob but a month later it needed collimation again.

Scott

#21 SteveG

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Posted 19 July 2019 - 03:03 PM

ok, thanks... would the laser collimator that came with my scope be all that I need to properly collimate?  Would I even need to do collimation if stars look perfectly sharp in my scope?

Place the laser in your focuser, then spin it around. Does the beam stay in one spot on your primary mirror?

Also, does the laser point to the same spot every time you place it into the focuser?

 

An uncollimated scope can still show sharp stars at low powers, but can suffer as you increase the magnification.



#22 brimacx

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Posted 19 July 2019 - 04:36 PM

Place the laser in your focuser, then spin it around. Does the beam stay in one spot on your primary mirror?

Also, does the laser point to the same spot every time you place it into the focuser?

 

An uncollimated scope can still show sharp stars at low powers, but can suffer as you increase the magnification.

Cool - good to know... I will check it out tonight - Thanks!



#23 taylordesign

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Posted 19 July 2019 - 07:03 PM

I'm in Southern California Bortle 6 skies...What I don't get is using the 2x barlow with the 9mm EP showed the moon as pretty sharp and close up. It's when looking at further objects like Jupiter and M13 with same set-up that I couldn't get good focus on.

As a point of reference I just moved to a Bortle 5 area with an 8" SCT. The moon holds up fairly well IMHO regardless of seeing conditions. It's a big target so even with wind at high magnification it's easy to focus and it comes off sharp with atmospheric waves in front of it (if that makes sense).

 

Jupiter and Saturn, however, are greatly affected by conditions. Those 'waves' wipe out the detail. My 40mm (50x) and 30mm (68x) eyepieces will do fine, but my 11mm (185x) can vary from fuzzy hard-to-focus ball to fantastic easy-to-focus planet based on the wind. I recall the first night I had the scope out...with some wind...I kept feeling like something was wrong with the 11mm ep, like I couldn't get things in focus with the exception of the moon. And then on the first steady night focusing was a snap and the views were incredible.

 

So while you will certainly want to check collimation with all the great tips here, it's possible you just haven't had a night of steady skies yet.

 

I'll also note that I can see individual stars in M13 pretty easily, but again that's a target that can feel 'out of focus' depending on the seeing conditions. First night out in the Mojave I was enthralled with M13, but I kept refocusing it feeling like it wasn't as sharp as it could be. Which it wasn't due to the conditions.



#24 Pelayo

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Posted 21 July 2019 - 03:29 AM

I've got an 8" Dob. I had one of those 6mm goldline eyepieces. I sold it and got one of these. It's a much better eyepiece, and it was the best view of Jupiter I've ever got. That's without my Barlow.

 

Meade HD-60 Eyepiece 6.5mm - $75

https://agenaastro.c...0-eyepiece.html

 

Thanks - I've heard good things about these Meade 5000 eyepieces. 

It's a good advice what you have received here. I had a 8'' dobsonian and that eyepiece. It's comfortable, the lunar and planetary views were very good and the apparent field of view (60º) it's enough to keep the planet in place for some time. 

 

Just in case it is for any help, these are the eyepieces I used:

  • GSO Erfle 30 mm. Then I changed it by a Explore 24 mm 82º. BIG galaxies, open clusters and nebulaes.
  • Hyperion 13 mm 68º. An ES 14 mm came afterwards. Galaxies, open clusters and nebulaes.
  • Celestron X-Cel LX 9 mm. Globular clusters such as M13. Lunar and planetary. An 8 mm would be also great.
  • Meade HD 6,5 mm. Lunar and planetary. 

You could consider also an economic 5 mm planetary eyepiece. Manual tracking becomes harder and seeing will not tolerate very often that magnification (depending on your observación place, of course). I had one, but the truth is that it stayed in the suitcase almost every nights. No Barlow in my case, I find them useful under different conditions than observing with a dob.


Edited by Pelayo, 21 July 2019 - 03:33 AM.


#25 sec4aa

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Posted 21 July 2019 - 10:32 AM

I have a similar telescope and I use the 8mm LVW, 7mm Nagler and 5.5mm Meade on planets.

See my signature...I keep these ep's because they work.




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