Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Focal Length Advice

  • Please log in to reply
8 replies to this topic

#1 writerguy

writerguy

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Feb 2020

Posted 15 July 2020 - 01:03 PM

Hi everyone,

 

I could use some advice on focal lengths.

 

I have an AT102ED (102 mm aperture, 714mm focal length, f/7).

 

I also have a 32mm GSO plossl, a 25mm Astro-Tech Value Line plossl, an 8mm Paradigm Dual ED, and a 2x barlow. And I plan to pick up a 12mm Paradigm when those are back in stock -- hopefully in a week or two.

 

I started off with just the 25mm and the 8mm, barlowing them to extend my range. I recently acquired the 32mm, and it has replaced the 25mm as my finder eyepiece. Once I get the 12mm Paradigm, I will no longer need to barlow the 25mm either. (In fact, I decided to order the 12mm because I spent so much time barlowing the 25mm, but loved the view through the 8mm Paradigm.)

 

So...I'm thinking the 25mm will soon be redundant, and I should probably retire it to make room in my eyepiece tray.

 

But I was recently observing M81 and M82, and actually spent the most time with the 25mm, which allowed me to get both objects in the field of view at once with better contrast than the 32mm.

 

As a result, I am now confused. (Well, I'm usually confused; maybe *more* confused would be more accurate.)

 

Should I sit tight and hang onto the 25mm for those occasions when it comes in handy? Or should I replace it with something like a 20mm plossl, which will fit better within my range of focal lengths, especially with the barlow thrown in? (I figure I could barlow a 20mm to fit neatly between the 12mm and 8mm Paradigms. I also figure it would work better than an 18mm eyepiece, which would barlow pretty close to the 8mm.) 

 

Since I already plan to buy the 12mm, I'm looking to keep my costs down; so I'm really just considering another decent, inexpensive plossl (e.g., $30 or so) as a potential replacement for the 25mm. And I think I'll shy away from one of the inexpensive wide-angle eyepieces at this point, since I'd prefer to have a nice sharp image all the way to the edge.

 

Any advice would be most appreciated!

 

Thanks,

 

AG



#2 CrazyPanda

CrazyPanda

    Vanguard

  • *****
  • Posts: 2,297
  • Joined: 30 Sep 2012

Posted 15 July 2020 - 01:15 PM

If you want a bargain 20mm, consider the 20mm "Gold Line" (aka generic Orion Expanse).

 

In your scope, it will have about the same true field of view as a 25mm Plossl, but give you higher magnification and a more immersive 66 degree AFOV. It's also right at your budget of $30:

 

https://www.amazon.c...ding=UTF8&psc=1

 

If you look around on Ebay or AliExpress, you can get these for around $20, but you have to wait 6 weeks for it to arrive from China.

 

Then you can retire your 25mm Plossl to simplify your kit.

 

At F/7, the edges of the field will be OK in your scope. Not great, but reasonable enough for $30.


Edited by CrazyPanda, 15 July 2020 - 01:17 PM.


#3 sg6

sg6

    Cosmos

  • *****
  • Posts: 8,368
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 15 July 2020 - 01:32 PM

You have a 32mm GSO plossl, a 25mm plossl, an 8mm Paradigm Dual ED, and a 2x barlow, and you plan to pick up a 12mm Paradigm.

That will only make 4 eyepieces, and I cannot see any real point in losing the 25mm plossl.

 

Myself I would keep the 32, 25, 8 then as you want add the 12mm and then I would go search out a reasonable 6mm. Ending up with 6, 8, 12, 25 and 32. Which for an f/7 scope would do well.

 

In a way selling a 25mm value plossl is not going to bank you a lot of $$ and it seems useful. So keep it and use it. It will be of use at times. Otherwise the jump is from 12mm to 32mm and that seems a big jump.


  • areyoukiddingme likes this

#4 writerguy

writerguy

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Feb 2020

Posted 15 July 2020 - 02:16 PM

Thanks, CrazyPanda! I'd considered the Agena EWA 20mm version of the Orion Expanse, but got the impression folks thought it worked better in slower scopes (e.g., f/8 and slower), with the edges going soft at f/7 or below -- hence the preference for a plossl. (Even if the FOV is narrower, I'd like it to be sharp to the edges; I already suffer from age-related astigmatism, so every bright star I see *anywhere* in the field at low magnification looks as if it's stretching out its arms to hug me...)

 

sg6, that makes sense. I'd actually be happy to barlow the 12mm Paradigm when I get it to plug the 6mm hole; the 8mm Paradigm barlows really nicely. I was considering a 20mm because I thought it might give me an even bigger jump in contrast than the 25mm over the 32mm, while offering the equivalent of a 10mm with the barlow. But I have a tendency to overthink what ought to be simple decisions, and I suspect I'm doing  that right now. tongue2.gif


Edited by writerguy, 15 July 2020 - 02:17 PM.


#5 SeattleScott

SeattleScott

    Cosmos

  • *****
  • Posts: 8,415
  • Joined: 14 Oct 2011

Posted 17 July 2020 - 10:45 AM

The 20mm is not a good performer in fast scopes.

Yes a 20mm Plossl would do a better job filling the gap between 32 and 12. You would be pretty well set at that point.

If you want something nicer, a 22 olivon (or one of the rebrands) is said to be quite good and not overly expensive, although more expensive than your other eyepieces. Another possibility would be a 25 Celestron Xcel LX to replace your 32mm. Over 90% of the field of view with more magnification/darker background sky. And very well corrected. Then you might find the jump from 25 to 12 small enough that you don’t need anything in-between.

Another option to consider would be the AT 18 UFF. A little pricey again but not overly so.

Scott

Edited by SeattleScott, 17 July 2020 - 10:47 AM.


#6 MitchAlsup

MitchAlsup

    Skylab

  • -----
  • Posts: 4,467
  • Joined: 31 Aug 2009

Posted 17 July 2020 - 10:55 AM

I also have a 32mm GSO plossl, a 25mm Astro-Tech Value Line plossl, an 8mm Paradigm Dual ED, and a 2x barlow. And I plan to pick up a 12mm Paradigm when those are back in stock -- hopefully in a week or two.

With Barlow you already have 32mm, 25mm, 16mm, 12.5mm, 8mm, and 4mm.

 

If the 25 were closer to 23 and the 32 was closer to 35, you would have an essentially perfect progression.

{Assuming you like those EPs}.

 

I, personally {And if you like THOSE EPs} I would skip buying the 12mm (redundant) but pick up something like a 6mm which would give you the progression::

 

32mm, 25mm, 16mm, 12.5mm, 8mm, 6mm, 4mm, 3mm



#7 writerguy

writerguy

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Feb 2020

Posted 17 July 2020 - 11:09 AM

Thanks, Scott! I'll definitely check those out.

 

Mitch, I do like the 32mm and 25mm plossls -- I was without a telescope for a long time, and my previous experience was primarily with plossls and other simple designs with what are now considered narrow fields of view, so in a sense I don't know what I'm missing lol.gif  -- but I also find the eye relief to be a bit long when barlowing them.

 

In the case of the 32mm, I tend to just avoid doing so unless something really benefits from that particular magnification; but since I spend a lot of time on clusters and galaxies, I also spend a lot of time barlowing the 25mm and using the 8mm Paradigm. The difference between the two (i.e., the barlowed 25mm plossl and the 8mm Paradigm) was what got me interested in a 12mm Paradigm: it's just a really comfortable eyepiece, and I figured it would make a nice upgrade.

 

I also figured the barlowing the 12mm would give a nice 6mm equivalent while preserving the eye relief of the 12mm, along with its wide (for me) FOV.

 

But I can see the advantage of going straight to a 6mm!


Edited by writerguy, 17 July 2020 - 11:10 AM.


#8 SeattleScott

SeattleScott

    Cosmos

  • *****
  • Posts: 8,415
  • Joined: 14 Oct 2011

Posted 17 July 2020 - 11:14 AM

With Barlow you already have 32mm, 25mm, 16mm, 12.5mm, 8mm, and 4mm.

If the 25 were closer to 23 and the 32 was closer to 35, you would have an essentially perfect progression.
{Assuming you like those EPs}.

I, personally {And if you like THOSE EPs} I would skip buying the 12mm (redundant) but pick up something like a 6mm which would give you the progression::

32mm, 25mm, 16mm, 12.5mm, 8mm, 6mm, 4mm, 3mm

The 32 will no doubt vignette with a barlow so still a benefit to have the 12 and maybe something around 20. The 32 is only 49 AFOV as it is. With vignetting it might be narrower than an ortho.

Scott

#9 writerguy

writerguy

    Sputnik

  • -----
  • topic starter
  • Posts: 31
  • Joined: 24 Feb 2020

Posted 17 July 2020 - 11:20 AM

I've actually been using the 32mm as a finder EP with no real finder: No finderscope, no RDF, no nothing.

 

I'm amazed at what I've been able to find without anything else, but I attribute that to the dark skies I've been enjoying and the quality and characteristics of the AT102ED. The 32mm gives me approximately 2.3 degrees of TFOV in that telescope, and in a place where I can see almost every guide star I need either naked eye or with my old 7x50 binoculars, I can pick out most Messier objects without any trouble!

 

Put the barlow in, though, and it's like looking through a keyhole...




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics