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Bresser or GSO?


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Hi! After gathering more knowledge about this topic I'm stuck between 2 8"dobs:

https://astromagazin.ro/telescoape/3261-telescop-bresser-dobson-8.html with a red dot finder, 25 SP and most important, after me, 2.5 hexafoc focuser single-speed (because it gives no coma);

https://www.astroshop.eu/telescoape/gso-telescop-dobson-n-200-1200-dob-deluxe-version/p,14189 with 8x50 scope, Erfle 30mm 2", SP 9mm 1,25", a 2" Crayford with 1:10 micro-reduction, a cooling fan

For any of them I want to buy an Celestron X-Cel eyepiece for planetary( I don't know what to choose between 5mm,7mm or 9mm) and a 2x Barlow or a 1:10 gear if I go with the Bresser.

I want firstly to go on planetary observing but after some time and on DSO.

Hope that someone can give me an advice.

Clear skies!

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Good day,

for planetary observation, focal length is the main consideration. Designs such as SCT, Maksutov and Classic Cassegrain have a clear advantage.

Maybe you want a 3x Barlow in order to gain enough magnification.

Check with astronomy tools "Field of view calculator" (I used a generic 8" Dobsonian as a base and tried various eypieces and Barlow combinations)

https://astronomy.tools/calculators/field_of_view/

Select Jupiter as an example case, and experiment with various Barlow-EP combinations to see which offers a good enough magnification (I guess that a zoom EP like the SVBONY 7-21mm and a 3x Barlow would be a flexible combination)

N.F.

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11 hours ago, Andreiiorg said:

2.5 hexafoc focuser single-speed (because it gives no coma);

I’m not sure if you are using the correct terms here, the focuser has no effect on coma, you need a coma corrector for that. Did you mean something else?

I do find dual speed focusers very useful for getting focus exact for planetary and lunar observing.

8 hours ago, nfotis said:

I guess that a zoom EP like the SVBONY 7-21mm and a 3x Barlow would be a flexible combination

In a 1200mm focal length scope, I don’t see any need for a x3 barlow. With the zoom it would give x514 as a max which is not useful. A x2 maybe useful, with the zoom you would have a total range of x57 up to x342.

For Jupiter currently x150 to x180 is often a good power to use and Saturn normally takes more say x200 to x220 is often good, although it is getting very low now. Mars, when at opposition and well placed can take x250 or more to show up the detail if conditions are good.

So, it’s good to have a range of eyepieces at the x150 to x200 level to give options depending on the seeing. A decent zoom can fit this bill too. This one is pretty good, and barlows well.

https://www.firstlightoptics.com/ovl-eyepieces/hyperflex-72mm-215mm-eyepiece.html

 

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Based on reviews and reports I've read either of these scopes will give you excellent planetary views and support a growth into DSO observing. I would have a slight preference for the Bresser simply because I've had a play with the ten inch version and the build quality was excellent, but I don't think you can go wrong with either choice.

On the eyepiece subject I have the X-Cel 7mm and it works very well in my SW 8 inch (similar to both of these) giving 170x magnification, so well inline with @Stu's guidance. 

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Thank you all! I have played a bit with astronomy.tools and I found out that the best combination for me is 9mm + 2x barlow. About coma I m still a little bit confused, but I ve read that the 2,5 hexafoc focuser gives no coma at 2” eyepieces, not sure I understood right. Now if anyone can help me to choose  one of the two telescopes.

Clear skies!

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1 hour ago, Andreiiorg said:

About coma I m still a little bit confused, but I ve read that the 2,5 hexafoc focuser gives no coma at 2”

I think there must be a misunderstanding somewhere. Coma is an effect of the primary mirror, and as said before, needs a coma corrector to get rid of it.

https://en.m.wikipedia.org/wiki/Coma_(optics)

No focuser can change the level of coma in a scope, they are not an optical component, they just allow you to adjust the distance between the eyepiece and the mirror to reach focus.

Can you link to what you read?

Either way, rule this factor out of your decision making process.

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18 hours ago, Andreiiorg said:

I want firstly to go on planetary observing but after some time and on DSO.

Jupiter and Saturn are now getting lower in the sky so I think you would be better off considering more DSO observation over the winter with planetary coming back into play next summer. Your choice of Celestron X-Cel LX eyepieces is a good choice, so long as you stick to the 12mm and shorter focal lengths. The 18 and 25mm will have quite noticeable aberrations in an f6 scope and you can make better use of your money elsewhere. I have the Bresser 8" and in my opinion no 1.25" eyepiece has a wide enough for your lowest power eyepiece. My lowest power eyepiece is the 28mm Nirvana and I think this eyepiece fills the role very well. You can buy the current version of this eyepiece from Telescope Express, and possibly some other brands that I am not aware of, for a very reasonable price: TS-Optics 28mm Ultra Wide.

Going back to the Celestron X-Cel LX range, you definitely want the 12mm as this will give you an exit pupil of 2mm, which is ideal for extended objects  (12 / f6 = 2) and for DSO observations you could stick with just these two eyepieces. The 9 and to a lesser extent the 7 will also be useful for DSOs (where they fit into the field of view), and for planetary I always found the 7mm to be a goldilocks focal length under UK skies, although the 5mm should be the optimum choice when sky conditions allow it.

Of the two scopes under consideration the altitude bearings, mirror cell and focuser are reasons why I would buy the Bresser again if I was making the choice again. I purchased the 1:10 dual speed upgrade for the focuser (as well as 1.25" and 2" Baader clicklocks) and bought a 12V computer fan and battery quite cheaply to cool the primary mirror.

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Firstly, the link about the thing with coma: https://www.astroshop.eu/telescopes/bresser-dobson-telescope-n-200-1200-messier-hexafoc-dob/p,47842 The excellent large Hexafoc focuser has an aperture of 64mm making it impossible to notice any shading from the focuser tube, even with large widefield eyepieces.

15 hours ago, Ricochet said:

 

Of the two scopes under consideration the altitude bearings, mirror cell and focuser are reasons why I would buy the Bresser again if I was making the choice again. I purchased the 1:10 dual speed upgrade for the focuser (as well as 1.25" and 2" Baader clicklocks) and bought a 12V computer fan and battery quite cheaply to cool the primary mirror.

The 2,5 hexafoc focuser is better than the 2 crayford? Noticeable in focusing or other way?

Clear skies!

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14 hours ago, Andreiiorg said:

The excellent large Hexafoc focuser has an aperture of 64mm making it impossible to notice any shading from the focuser tube, even with large widefield eyepieces.

"Shading" is vignetting, not coma. I'm not sure if you would notice any vignetting visually even with a "normal" 2" focuser when used visually as you need to lose half the light to notice.

14 hours ago, Andreiiorg said:

The 2,5 hexafoc focuser is better than the 2 crayford? Noticeable in focusing or other way?

It is a very strong focuser, with no flex at all at any extension so the weight of the eyepiece will never affect the collimation. The Stellalyra is probably a bit smoother to turn because it is a Crayford but I don't know how it handles heavy loads. If you want the best of both worlds there is a Feathertouch replacement available for the Bresser but it costs more than the scope. Another thing I like about the Hexafoc is that you can rotate it 45° so that the focuser wheels are always in a convenient position.

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21 minutes ago, Andreiiorg said:

Hi! Thanks! @Ricochet  can you tell me the acclimation time for the bresser 8" telescope with and without the fan? And maybe a link to that computer fan you use?

To be honest, I'm not sure. It's been years since I used it without the fan and I tend to get it out in advance to cool anyway. Low power views are fine straight away but high power planetary views are maybe half an hour with the fan, an hour without. If the air temperature drops rapidly a fan helps the mirror keep up, otherwise the mirror may never actually acclimatise. 

The battery I use is like the one linked below, but I think it is a lower mAh, cheaper version. 12v battery

The fan is an 80mm coolink fan. I also fitted a generic pc speed controller into the telescope base but it isn't needed, running the fan on full speed is fine if it is sufficiently isolated to prevent vibrations being passed to the scope. You will need to do some diy to change connectors so that everything ends up being compatible.

I replaced the screws that hold the mirror cell to the tube with longer ones so that I could attach the fan via hair ties. There is also a baffle to ensure air passing through the fan enters the scope. The baffle has a length of p-strip around the circumference which presses against a second length of p-strip around the opening in the rear of the scope. The hair bands and p-strip are both used for vibration suppression. 

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An 8" dob might be smaller than you think. Especially when considering storage. 

Here's my Bresser 8" tucked away in a cool corner of the kitchen. My grab-and-go ST80/AZ5/Manfrotto next to it takes up more space!

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