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Question about 1.25" or 2" accessories...

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#1 Pcbessa


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Posted 11 February 2019 - 07:52 AM



I have a question regarding 2" vs 1.25" fittings.


My newly bought XT10i Orion telescope comes with a 2" Crayford focuser and two 1.25" Plossl eyepieces, and a removable 1.25" adapter for those eyepieces. In the future I will be buying good quality eyepieces, both 2" and 1.25".


I want to buy a ES UHC Nebula filter and a ES 2x Barlow. They both come in 2" and 1.25" formats.


I do not understand anything about how to fit accessories yet, so please apologize for my question!


  • Can I fit the 2" UHC filter into the 2" focuser, and then use, depending on my choice, either a 2" ES low power eyepiece, or with a 1.25" eyepiece (if I use the adapter)? Or is this not doable? (I will probably use the filter more with 2" eyepieces in the future, but for the moment I only have a 1.25" 25mm Plossl) I would assume choosing a 2" filter over a 1.25" would allow me this choice, whilst a 1.25" filter would restrict me only to 1.25" eyepieces, which is not a good idea for nebulas.
  • How about for the Barlow? I will be using probably only with 1.25" ES eyepieces. Shall I buy a 1.25" Barlow or can I buy a 2" and do the same playing around?

Sorry for my question but I am mostly a beginner with the XT10i, having only had a 60mm refractor on the past.

With many thanks!

#2 SloMoe



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Posted 11 February 2019 - 08:29 AM

Morning, look on the bottom of your 1.25" adapter, if there are threads there they are for filters, just like in the bottom of your eyepiece.


Filters screw onto the bottom of eyepieces and adapters, not the focuser.


Your best viewing of nebulae and other DSO targets are best viewed at 100x mag or less down to about 40X, 


The eyepieces that will yield that for you best will be 2", 

So in answer to your first question, I would recommend a 2" filter and thread it onto the bottom of your 1.25" to 2" adapter for now.


The high power eyepiece just doesn't gather as much light and there are other factors like exit pupil to consider, the higher the mag the smaller the exit pupil and so the DSO will be very faint.


The high power would be used for elementary and lunar viewing.


As Barlow's go, some use them a lot, some don't use them at all.


If I under stand the ad description both of your eyepieces are 1.25", and an inexpensive Barlow, like the Orion will let you use your 25mm as a 12.5mm and your 10mm as a 5mm.


The Barlow basically doubles the magnification capa bility of an eyepiece in your scope.


At this point in your learning curve I would recommend a 1.25" Barlow

Edited by Mike W., 11 February 2019 - 08:32 AM.

#3 Jond105


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Posted 11 February 2019 - 08:32 AM

I would second the 1.25" Barlow over the 2". If you're already looking at premium better corrected eyepieces, the weight would be gruesome in my opinion with a 2" Barlow.

#4 mrowlands



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Posted 11 February 2019 - 08:45 AM

I second the 2 inch filters for reasons described above.  If your 2 to 1.25 inch adapter doesn't have threads on the bottom, you should get one that does.  Just be aware that a standard 1.25 inch barlow may have a long enough barrel to contact the filter.  A shorty barlow shouldn't have that problem.


Having said that, as for eyepieces, many of us end up with what we need so that no barlow is necessary.


Mike R.

#5 SloMoe



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Posted 11 February 2019 - 08:47 AM

I'm not sure if you're aware  yet or not one of the first things we learn about our telescopes is how to figure magnification capability's of our eyepieces in them.


You divide the telescopes focal length, yours is 1200mm, by the focal length of the eyepiece.


The focal length is the 10mm & 25mm printed on the side of the eyepieces


Simple math, 1200 / 25 = 48X    and 1200 / 10 = 120X


then Barlow'd  1200 / 12.5 = 96X and 1200 / 5 = 240X


There's also the fact that as you magnify you're magnifying everything between your scope and the target, so everything floating around in our atmosphere and space will also get magnified to the point of interfering with detail in the view, 

We call this "seeing conditions", average seeing conditions will let you get up around 225X, give or take 30X for high power viewing.


Low power there's generally no issue except for clouds and such.

Edited by Mike W., 11 February 2019 - 08:47 AM.

#6 SloMoe



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Posted 11 February 2019 - 09:03 AM

Now, about filter's, a good quality and proven filter should be the purchase, these things are small and expensive.


So do your research before buying one, they last forever,,,, well almost,,,,, anyway the tow types of filters most commonly used now are narrow band filters and line filters


Examples are for the narrow band filters DGM NPB, and some UHC filters, UHC was a type that Lumicon developed and sold a number of years ago but many manufactures have used the UHC description for their filters but in fact are very substandard performer's.


O-III filters are line filters, a majority of the DSO's emit light in the Oxygen wave length and these filters only permit that light to pass thru, now that dim's the view considerably because they don't emit a bunch of light so all you're seeing is their light. 


In this forum a few topics down is a very good discussion about these filters,


#7 Pcbessa


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Posted 11 February 2019 - 09:35 AM

Thanks. I am quite informed about eyepieces, barlows and the different types of nebula filters (just spent the last few days reading on them), and I am also very aware of exit pupil and magnifications work, because I had a 60mm refractor for decades.


My only question is about fitting nebula filters into the eyepieces that I may use with the XT10i.


At the moment I have two 1.25" Plossl but I might buy some ES68 or ES82 2" eyepieces for a low power wide angle. Then I want to use a UHC filter to increase the brightness of nebulas (or a OIII to increase contrast). I am guessing I may want to use the filter both with low power and sometimes with higher power (but these eyepieces are only 1.25"). So how do I do? Do I need both 2" and 1.25" versions of the same filter?


Likewise, the other question is if is possible to fit a 1.25" Barlow into the 2" focuser (with the 2" to 1.25" adapter), to be used with 1.25" eyepieces.


Users of XT Orion Dobsonian telescopes, please feel free to support me on these two questions.



Edited by Pcbessa, 11 February 2019 - 09:37 AM.

#8 astro744


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Posted 11 February 2019 - 12:38 PM

UHC = Ultra High Contrast. Both this filter and the O-III increase contrast and neither filter increases brightness.


It is best to get a 2" filter for 2" eyepieces and a 1.25" filter for 1.25" eyepieces.  The filter screws to the bottom of the eyepiece you're using it with.  You can get a 2"-1.25" adapter that is threaded and will take a 2" filter.  You then use this for your 1.25" eyepieces only.  However many recommend this option as a way of saving money by only having to buy a 2" filter.  I do not recommend this as the risk of smashing your 2" filter with a long barrel 1.25" eyepiece or Barlow or an eyepiece with filter attached that you forgot to take off is too great.  You will note Tele Vue adapters are not threaded to eliminate this option.





#9 brentknight



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Posted 11 February 2019 - 12:46 PM



If your 2"/1.25" adapter will take standard 2" filter threads, don't waste money on 1.25" filters.  Also, I learned that you can attach something like the Baader Finetuning ring to the filter to give you more "grip" when using it and to also give a bit more depth so that longer barlows will fit in the adapter.  I use the 14mm ones below and just leave them permanently attached to each of my filters.




I would also stick with 1.25" barlows.  I think it would be rare that you would ever want to put a 2" eyepiece into a barlow.  Most of the time you will be using the barlow to get much higher magnifications.  These higher magnifications might not be needed as much and you can save some cash by using a good barlow on your shorter FL 1.25" eyepieces.  The Tele Vue 3x is very good for this...

#10 Wesker


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Posted 11 February 2019 - 01:09 PM

I would suggest the Lumicon UHC filter. Seems to get the most general praise of the 3 or 4 really nice ones out there. They have a 2" for $200 and a 1.25" that I believe is around $125.

As far as which size to get, I would see how your 1.25 eypeieces fit into your adapter on the scope. You dont want a long 1.25 to pierce your 2" filter, but you also dont want to waste money necessarily either. Ive heard of folks who bought both and ended up never using the 1.25 filter.

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