Which 5mm eyepiece and 2x Barlow lens for my Gskyer AZ70700 Refractor Telescope

[erseavetir]

Hello all! I am a total noob and I have an eyepiece and a Barlow lens question: 

 

I have the Gskyer AZ70700 Refractor Telescope that I bought from amazon for my father and we have been using it for a week. It came with the 10 and 25mm eyepieces and a 3x Barlow lens. 

 

My father's old "BRESSER TRAVEL 20-60X60 SPOTTING SCOPE" that our cat destroyed had a zooming option that my father loved so after some research, I've also ordered the "Celestron 8-24mm Zoom Eyepiece (1.25")" from amazon and I hope it will work well with the telescope when it arrives in a week. I also assumed it would be a better quality eyepiece than the eyepieces that came with the telescope.

 

Now my question is: Do you think a 5mm eyepiece would work well with my telescope? If so, what is the best 5mm eyepiece that I can buy below $30. Another question I have is that the 3x Barlow Lens that I have is making the view blurry and not clear like when I don't use the Barlow lens. Do you think a 2x Barlow lens would be better for our telescope? If so, what is the best 2x Barlow lens that I can buy below $30. I saw 2 Barlow lenses that were made by SVBONY and Solomark that are around $16 but I don't know if they are the right ones for our telescope and I wouldn't know which one to buy. I assume maybe I won't need to buy the 5mm eyepiece for better viewing if the 2x Barlow lense will work with my 10mm eyepiece or with the Celestron 8-24mm Zoom Eyepiece. What do you guys think? I would really appreciate any answers.

[Ricochet]

I suspect there are two issues with the Barlow lens: 

1. The eyepieces and barlows included with this class of telescope tend to be of low quality. 
2. The useable magnification is limited by diffraction which increases exponentially as you decrease the size of the exit pupil. I suggest a minimum exit pupil of 0.85-1mm to keep a reasonably sharp image. As your scope is f10 this means your highest power eyepiece should be in the 8.5-10mm range. Using the 10mm (which is already my limit) and then adding a Barlow creates a tiny exit pupil, massive diffraction and therefore blurriness. 

Given the above I suggest that you do not buy any Barlow for this telescope, it simply isn't needed. The Celestron zoom should cover all your high power needs by itself. The only eyepiece you might want to buy is in fact a low power eyepiece to give you as wide a field of view as possible. A 32mm Plossl would be a good choice for this and at a reasonable price. 

[erseavetir]

Thank you so much for responding. However, I am a little confused now because someone else on another forum told me this: 

 

"With the limited aperture of that scope, 70mm or just under 3 inches, the amount of magnification you can reasonably use ll be considerably less than the 400+ listed on the box. So, with your 10mm eyepiece and the 3X barlow, that puts you at around 210X magnification, which in my opinion is pushing this apertureto its limits. A good rule of thumb would be 50X your aperture in inches... so lets call your aperture 3 inches... 50 X 3 = 150X, a much more reasonable magnification. So now we have 700mm scope focal length divided by 10mm eyepiece focal length = 70X times a 2X barlow to equal 140X magnification. With good seeing, this will give you decent views of Jupiter and Saturn and the moon. And some other stuff..." 

 

My telescope's aperture is 2.8 inches and times that by 50 gives us 140 clear magnification power (according to him as far as I understood). If this is true, a 2x Barlow lens would work great with the 10mm eyepiece on my telescope as that would be equal to exactly 140 magnification power. If this is all true, and I would get the same clear views that I am getting with 10 mm eyepiece on 2x Barlow lens with twice closer visuals and with same clarity then I believe it would be worth it to get a 2x Barlow lens. And I've searched Amazon a little bit and for my budget, this seems to be the best Barlow lens for me: 

https://www.amazon.com/SVBONY-Broadband-Standard-Telescope-Eyepiece/dp/B00WW0TC32/

 

Would you agree with that? Or do you think I would waste my money if I bought this Barlow lens because I would still get blurry visuals it?

[John]

The barlow lens that your link leads to seems to be a 5x one. Far too much magnification for your scope.

140x magnification would be the absolute limit with your scope I think and that would assume very good observing conditions and quality optics plus the right targets eg: Saturn, the Moon or double stars.

For many other objects such as star clusters and the bright galaxies and nebulae you will not need to use high magnifications so the 10mm and 25mm eyepieces will be fine. The Celestron 8-24 zoom lens will also be useful for such targets.

For a 2x barlow lens at a low cost but reasonable quality I would suggest this one:

https://www.amazon.co.uk/Celestron-Telescope-Camera-Adapter-Barlow-x/dp/B00009X3UV/ref=sr_1_2?s=electronics&ie=UTF8&qid=1501121852&sr=1-2&keywords=barlow+lens

It is not perfect but it, or one of the many clones available under different branding, will do a reasonably good job of giving you higher magnifications when used with your zoom and other eyepieces.

 

[Ricochet]

4 hours ago, erseavetir said:

Thank you so much for responding. However, I am a little confused now because someone else on another forum told me this: 

 

"With the limited aperture of that scope, 70mm or just under 3 inches, the amount of magnification you can reasonably use ll be considerably less than the 400+ listed on the box. So, with your 10mm eyepiece and the 3X barlow, that puts you at around 210X magnification, which in my opinion is pushing this apertureto its limits. A good rule of thumb would be 50X your aperture in inches... so lets call your aperture 3 inches... 50 X 3 = 150X, a much more reasonable magnification. So now we have 700mm scope focal length divided by 10mm eyepiece focal length = 70X times a 2X barlow to equal 140X magnification. With good seeing, this will give you decent views of Jupiter and Saturn and the moon. And some other stuff..." 

 

My telescope's aperture is 2.8 inches and times that by 50 gives us 140 clear magnification power (according to him as far as I understood). If this is true, a 2x Barlow lens would work great with the 10mm eyepiece on my telescope as that would be equal to exactly 140 magnification power. If this is all true, and I would get the same clear views that I am getting with 10 mm eyepiece on 2x Barlow lens with twice closer visuals and with same clarity then I believe it would be worth it to get a 2x Barlow lens. And I've searched Amazon a little bit and for my budget, this seems to be the best Barlow lens for me: 

https://www.amazon.com/SVBONY-Broadband-Standard-Telescope-Eyepiece/dp/B00WW0TC32/

 

Would you agree with that? Or do you think I would waste my money if I bought this Barlow lens because I would still get blurry visuals it?

As far as I am concerned the 50-60x limit is the limit for technically being able to separate double stars when diffraction causes each star to appear as a series of concentric rings (known as the airy disk). To get nice sharp views of planets you have to drop down to the 25-30x region, perhaps being able to go a little higher on the moon. Given that you've already ordered the zoom eyepiece you might as well wait for that to arrive and then test how well the image holds up. If you get to the 8mm setting and feel that there is more useful magnification to get out of the telescope then you can order the barlow and give it a go. The one you link to appears under a few different brand names and is of surprisingly good optical quality for the price. The only downside is that the barlow is not threaded for filters as you will find on more expensive options.

Looking at pictures of your telescope I notice that it has a 45° diagonal. This usually indicates that it is an erecting prism type (the image is the right way up and correct left-right). At this price point it is likely to be a weak link and so you may want to consider swapping it for a 90° star diagonal at some point. This will also make looking at objects around the zenith more comfortable.

[Stu]

 Very good advice so far.

I would certainly wait for the zoom to appear so you can try it out and see the image size and quality you get.

Don't forget that you will be able to get to the same magnification using the 3x Barlow with the zoom set to 15mm as with the 10mm and a new 2x Barlow so you will be able to test this out before buying. If this works to your satisfaction then you could try to pick up a better quality 2x and the results could be improved.

[domstar]

Absolutely try the zoom first. You might find that your views are so improved that you'll never want to use your old eyepieces again (that's what happened with me though I didn't buy a zoom)

[Cosmic Geoff]

I used to own a 70mm 700mm fl budget scope. It had a 4mm eyepiece which I almost never used as it gave too much magnification. The scope just wasn't up to it and the tripod was too wobbly at that magnification.  Adequate Plossl eyepieces that will work in a f10 scope are cheap and  you could acquire one or two to plug weaknesses in your eyepiece set instead of relying on Barlows.

Rather than relying on formulas you should go with what works.

I still have a vintage long-focus 70mm brass telescope whose performance with astronomical eyepieces blew the budget scope out of the water. So telescopes of this aperture can perform to their theoretical limits, but I'd expect modern budget instruments of this class to perform poorly until they prove otherwise.  That terrestrial 45 deg prism won't do it any favours either. You should get a budget 90 deg. star diagonal.

[erseavetir]

Thank you so much for all of the replies, it was really helpful for me. Also before I went to sleep yesterday i've realized I've posted the wrong link (the 5x Barlow instead of 2x) and then I've edited it for the 2x Barlow but I guess I failed. This was the link I wanted to post initially:

https://www.amazon.com/SVBONY-Broadband-Standard-Telescope-Eyepiece/dp/B00WW0TC32/

And this is the second option that I am considering but can't decide on which one to buy:

https://www.amazon.co.uk/gp/product/B00009X3UV/ref=ox_sc_act_title_1?smid=A3G3C9BZOBVXZR

 

As a final verdict though, I guess I should wait for the zoom eyepiece and see how that will work with my telescope. I am convinced that the 5mm eyepiece would be too much for my telescope and probably not get used but the reason I wanted to experiment with a better quality Barlow that is 2x instead of 3x that is combined with the Celestron 8-24mm Zoom Eyepiece, is this: 

 

The first time we saw the Saturn with my father 2 days ago with the 10mm eyepiece, we got the chills and felt amazing. It was so clear and 3D! We didn't even know we could see it (or where it was) so we didn't look at it for 5 days before that and when my father first discovered it by mistake and saw it's rings, he was in awe (I was in awe 5 seconds later as well)  And the first time we saw the moon with this telescope yesterday, we were amazed that it gave us way better views than the "BRESSER Travel 20-60x60 Spotting Scope" that we had before.

 

However, when we put the 3x Barlow on, (I am also confused if it makes a difference to to put the Barlow on top of diagonal or before the diagonal) we can barely catch the saturn and when we do, it's not clear and it runs away from us quickly (even though that was so cool to find out as well). And for the moon, it made things blurry as well but it made us feel like we are on the moon. So I thought a better quality Barlow lens with 2x option instead of 3x would work perfectly for our telescope when it comes to catching things and seeing them more clearly. Of course the question is, would that be true?  I shall experiment with the 8mm first to find out.

 

Another confusing thing for me is this: Since you guys believe 5mm eyepiece is too much for my telescope, then buying the 2x Barlow lens and using it with my 10mm eyepiece would be same as using a 5mm eyepiece (as far as I understand) and this is why you guys are saying experiment with the 8mm first right?

 

Lastly, which budget 90 deg. star diagonal would you guys recommend for me and would it really make a lot of difference compared to my terrestrial 45 deg prism?

[Ricochet]

Those two barlows are the same, just with different names stamped on them. Buy the cheaper one if you do end up getting one. 

Yes, you should experiment with the zoom first as it will give an indication of whether you can push the magnification or not. 

I'll leave diagonal recommendations to those who use them but the general advice seems to be that any mirror based diagonal will give reasonable results. 

[Stu]

3 hours ago, erseavetir said:

(I am also confused if it makes a difference to to put the Barlow on top of diagonal or before the diagonal)

Definitely put it after the diagonal. Increasing the distance between the Barlow and the eyepiece increases the multiplying factor so you will be multiplying by well over x3 making things worse.

Yes, the reason to wait for the zoom is that you can try it out at 8mm and see how you get on. Often less is more i.e. higher power can give you blurry images either because the scope is not up to it, or the atmosphere, plus using a less that solid mount can leave you prone to big vibrations and make tracking your target tricky.

[erseavetir]

OK I got a final question. I've searched for 90-Degree Diagonal for my telescope and for my budget. I made a list of them below. I would like to pick one of them as the winner. Would you guys please help me decide on which one to buy?

 

1) Celestron 90° Star Diagonal (1.25")

https://www.bhphotovideo.com/c/product/201981-REG/Celestron_94115_A_Star_Diagonal_1_25.html

 

2) GSO 1.25" 90-deg Mirror Star Diagonal for Refractors # GSST

http://agenaastro.com/gso-1-25-90-refractor-mirror-star-diagonal.html

 

3) GSO 0.965"/1.25" 90-deg Hybrid Mirror Star Diagonal

http://agenaastro.com/gso-0-965-1-25-90-hybrid-mirror-star-diagonal.html#product_tabs_power_reviews

 

4) GSO 1.25" 90-deg Erect Image Amici Prism Diagonal # PD90

http://agenaastro.com/gso-1-25-90-erect-image-prism-diagonal.html

 

5) Meade 1.25" #918A Diagonal Prism # 07202

http://agenaastro.com/meade-1-25-918a-diagonal-prism.html

 

6) Meade #918A 90-Degree Prism Diagonal (1.25")

https://www.bhphotovideo.com/c/product/202609-REG/Meade_07202_918A_90_Degree_Prism_Diagonal.html

 

Also I don't know if mirror works better than prism. If you guys know the answer then please let me know so that I can eliminate a good chunk out of my list. Lastly, will the views on mirror ones be reversed views or inverted "left to right"? Because I don't really like that. Please let me know if any of these options has a fully erecting / corrected image output.

 

EDIT: I've bought the first option (Celestron) and I hope it will work well while looking at moon and Saturn.

[erseavetir]

Hello all! It's been almost a year since this post and I am now back with a new post and I need you guys' opinion on this please if you may: