6 replies to this topic

[CraftyCat]

Right, probably a rookie question, but I am a rookie so 😁

Is there a way to actually calculate what length extender you need to reach focus? The current pieces I'm trying to put together are a Skywatcher 200PDS, a 2x Barlow, and a 5mm eyepiece. In order to attach all of this i need the 2" to 1.25" adapter, and one came supplied with the scope. It consists of two pieces, one being the adapter ring that goes into the focuser (i think this is a T2 ring?), and a 2" extension tube.

Since I can reach focus with just the ring, extender and an eyepiece, it wasn't that surprising that putting a Barlow into the mix would bring everything out way too far.
I suspect that I need a much smaller extender, that essentially just has the bit that allows you to attach an eyepiece.

Which brings me to my original question. Is there a way to calculate this? I figure the distance needed will vary depending on the combination of items you want to put together, rather than a one time fixed solution, so I figure it'd be good to know!

🙏

[eblanken]

Hello (aka CraftyCat),

 

First, welcome to Cloudy Nights (CN).

 

Second, please confirm that your telescope is:

 

"Sky-Watcher Explorer-200PDS 200mm (8"") f/1000 Parabolic Dual-Speed Newtonian Reflector Optical Tube assembly".

 

Another way of saying this is "200mm x 1,000mm, f/5 Newtonian Reflector"

 

Third, there is a way to find the focal plane of your scope: Point it at the Moon and put a piece of tape on the focuser and adjust so that the real image of the moon is in focus on the tape. Here's a photo provided by CN member TOMDEY.

 

Fourth, then add the Barlow and repeat.

 

Ed

Attached Thumbnails

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[eblanken]

Hi Again,

 

So the short answer is "yes, you can calculate . . . ", but it might be more instructive to measure and learn by doing . . . 

 

The "prime focus" should be 1,000mm from the primary mirror . . . but this is the total optical distance from 

the primary mirror to the secondary mirror plus the distance from the secondary mirror to "prime focus"

 

The Moon's image should be 9mm diameter . . . in your situation . . . 

 

Best,

 

Ed

 

P.S. Let's dialogue first about these basics before diving into the operation of the 2x Barlow . . . ok ?

 

P.P.S. It would be helpful to know what 5mm eyepiece you own and what 2x Barlow you own . . .

          Pictures would be helpful . . . key question: Does the Barlow have a lens cell that unscrews from the Tube body ?

          I also see that the Moon is trending toward new, so there might be a terrestrial object you might be able to use instead . . . 

Attached Thumbnails

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Edited by eblanken, 22 June 2025 - 12:22 PM.

[triplemon]

Which brings me to my original question. Is there a way to calculate this? I

 

Absolutely, the math can be found here https://astunit.com/...hp?topic=barlow or here https://agenaastro.c...o-barlow-lenses

 

You will have to measure the mechanical dimensions of your barlow and its housing, from that you can obtain all the other parameters, like the barlows focal length and then the distance d1. And yes, barlow housings (blue in the AgenaAstro drawings, green on astunit) are often built so that the barlows barrel length is just a bit shorter than d1, so those barlows require the focuser to have some extra infocus left to reach focus with the barlow in place. The Agena drawing is a bit msleading, they show a barlow housing with a barlow barrel length that is much longer than the extra barlow infocus d1. I have never encountered such an unconventional barlow housing in real life.

 

 

Another approach would be to determine the amount of infocus you are missing by using the barlow and focus on some nearby object - where you hopefully can still reach focus. Then you can use the lensmakers formula 1/f = 1/f1 + 1/f2 to determine how much further you would have to move the entire barlow/eyepiece assembly inwards to reach focus at infinity.

 

Example; telescope focal length f=1m, object distance f1=30 meters (100 feet) , actual image distance f2 = 1 / (1/f - 1/f1) = 1.034m. So focusing this close moves you focal plane 34mm out relative to where it is at infinity focus. Which is in your case likely more than infocus left with the long 2" extension tube left in place.

Edited by triplemon, 22 June 2025 - 05:05 PM.

[eblanken]

Hi again,

 

(aka triplemon) is correct and gives two good links. His recommendation for an object at 100 feet (30 m) is also good. That 34mm of "out-travel" seems reasonable to me also. A photo of your focuser and the extension tube and 2 inch to 1.25 inch adapter might help us help you. If you could place a well-lit ruler at 100 feet (30m) with a dark background and look in the eyepiece at the numbers on the rule, then put the 2x Barlow and get additional numbers, you can compute the Barlow's magnification: (a2-a1)/(b2-b1). This assumes you can get focus for both without (a2-a1) and with (b2-b1) numbers on the ruler.

 

I do question your plan to use the 2x Barlow with a 5mm eyepiece: That would imply 400x in your scope, which would be more power than my seeing would support, so unless your seeing is better than mine would possibly not be too good. Do you have a longer focal length eyepiece ? I also was asking about the Barlow and the possibility of the removable lens cell, because the cell alone might screw onto the eyepiece directly and give you about 1.5x instead of the normal 2x at the normal Barlow spacing. That might work better with the 5mm eyepiece for about 300x instead 200x or 400x.

 

Best,

 

Ed

Edited by eblanken, 22 June 2025 - 08:46 PM.

[CraftyCat]

Good morning,
 
Wow, thank you both so much for your help! I have a feeling that my math skills need a little upgrade, but I think what I will attempt first is to measure a long distance on earth and see where I can reach focus. Honestly seems like the easiest way, only downside is having to wait for the rain to clear up! That being said, I will definitely read up a bit on the links provided, because I prefer having an understanding of what I'm doing, if only vaguely.
 
Just wanted to clarify some things. It's indeed the Skywatcher Explorer 200PDS 8" f/1000 that I have. I'm aware that going up to 400x magnification is probably less than ideal and will give me some really crappy views, however it was just the two pieces I have at home atm, I don't have patience for the Swedish postal system I only figured that a similar problem will present itself regardless of the eyepiece, since the only way I have to attach anything 1.25" to my focuser is through a 2" long extender tube. I have a ring that sits in the focuser, but it doesn't have any attachment screws. I clearly need to get a 2" to 1.25" reducer that isn't a long tube to reach focus. I simply figured that it would in general be good to know how to calculate how much less of an extension (if any at all) I would need to attach a given eyepiece and barlow together, since that would probably vary slightly depending on what eyepiece I get.
 
I've attached a photo of the bits I've got right now below.

 
 
I have realised that I CAN in fact unscrew the barlow lens from the tube (cheers eblanken!), and it can be screwed into the filter thread of the eyepiece. I think I might be able to reach focus with it attached like that, but we shall see.
Hopefully I'll get a day of clear skies soon so I can do some experimenting with long distance focus and the moon. I'm very far north so we have daylight all night now, so those two and the sun are probably the only objects I'll be able to see for the foreseeable future

 

Thank you again for your kind explanations, I clearly have some reading up to do!

[triplemon]

I think what you got is what is called a "high-hat" 2"-1.25 inch adapter:


These are poorly suited for many eyepieces, as they reduce infocus by a lot. What you want here is an ordinary 2/1.25" adapter that would leave you with almost an full inch more infocus:


https://astronomics....82b649545&_ss=r
 

Edited by triplemon, 23 June 2025 - 06:19 PM.