6 vs 7 mm eyepiece for 130/920 Reflector telescope?

[michelbechara]

Hello!

I have a 130/920 Reflector telescope and want to buy either this 6mm eyepiece  or 7 mm eyepiece. 

I know that the maximum magnification for that telescope is x260.

The 6 mm will give me x153 magnification - with 2x Barlow x306 - with 1.5x Barlow x230
The 7 mm will give me x131 magnification - with 2x Barlow x260 - with 1.5x Barlow x197

The objective is to observe the planets and gets the most useful closeup image, mainly for Saturn and Jupiter. 

By looking at these, do you think I should go with the Planetary 6 mm or Cornus 7 mm to achieve the best viewing of the planets with the 1.5 Barlow lense?

The planetary 6mm seems to achieve also higher contrast and clearer images compared to the 7mm and can have a good magnification with the 1.5 Barlow.

What do you think or am I having a wrong approach to the topic? Any help/guidance is much appreciated!

 

[John]

The maximimum theoretical magnification is rarely useful because of the seeing conditions we have to endure.

I think a more practical range of high magnifications for your scope would be 140x - 200x.

 

[Dantooine]

Split the difference and a wider field - Explore Scientific 82º 6.7mm 👍

[Louis D]

Or get a 7.2-21.5mm zoom with a Baader Q-Turret 2.25x Barlow as @John has often suggested.  That way, you'll have all the focal lengths between 9.6mm and 3.2mm.

 

[mikemarotta]

Welcome aboard! You should take all opinions with a grain of salt but we are all here to help each other and, ultimately, we are all beginners one way or another.

Myself, because you have the Barlows, I would go with the 7mm because it has a better field of view and eye relief. Most seeing is done with the brain. The eye - and its instrumentation - just provide raw data. The well-known case of the Full Moon on the Horizon should be proof enough of that.  To me, it is not so much what the telescope reveals - though that is important - as how much time I actually spend looking into it, studying a target, thinking about what I am seeing. I speak to myself often out loud about what I see. 

To me,  that is the benefit in sketching and keeping a notebook generally. It is not so much that I can produce a realistic picture (nice though that might be) as that I have put into my brain what my eyes delivered, even if what my hand produces on paper is not at all similar. It is like taking notes in class. The point is not to get your professor's brogue into your notes, but to get the concepts into your head.

So, from my point of view, the 7mm is a shade better for all of the above.

Anyway, it seems to me that you have a win-win, whatever you decide. I mean, having recommended the 7mm, if the 6 mm showed up on my doorstep, I would give it a home.

Best Regards,

Mike M.

 

[Don Pensack]

On 11/01/2021 at 13:40, michelbechara said:

Hello!

I have a 130/920 Reflector telescope and want to buy either this 6mm eyepiece  or 7 mm eyepiece. 

I know that the maximum magnification for that telescope is x260.

The 6 mm will give me x153 magnification - with 2x Barlow x306 - with 1.5x Barlow x230
The 7 mm will give me x131 magnification - with 2x Barlow x260 - with 1.5x Barlow x197

The objective is to observe the planets and gets the most useful closeup image, mainly for Saturn and Jupiter. 

By looking at these, do you think I should go with the Planetary 6 mm or Cronus 7 mm to achieve the best viewing of the planets with the 1.5 Barlow lens?

The planetary 6mm seems to achieve also higher contrast and clearer images compared to the 7mm and can have a good magnification with the 1.5 Barlow.

What do you think or am I having a wrong approach to the topic? Any help/guidance is much appreciated!

 

I assume you mean later in the year when Jupiter and Saturn are higher in the sky.  Right now, they're down in the muck and even low power sees a bad image.

The planet "season" is over.

With a 130mm reflector, you'll be lucky to break 200x and get a clear image even when they are high in the sky.

As a general rule, magnifications over 30x/inch (154x in your scope) get questionable and require pretty darned good seeing conditions, rare when the planets are high in the sky, but

impossible below 20°, where they are now.

I would suggest you make sure your lower powers are filled in.

For example, a good collection of eyepieces for you might be 28mm, 20mm, 14mm, 10mm, 7mm

You have a good way of looking at it--achieve ultra-high powers with a Barlow--but the number of nights you'll be able to use any magnification over 200x will be few

and the Barlow will receive very little use.  

I'd get the 7mm--it'll receive a lot more use.

But also look to filling in your lower powers if you don't already have them.

Edited January 12, 2021 by Don Pensack