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Diagonal Compatibility

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#1 flyer92

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Posted 11 January 2020 - 12:59 PM

Greetings all...newbie to this site and fairly new to telescopes in general, and have been enjoying the hobby thus far.  All has been pretty straightforward from an equipment perspective, but I recently ran into an interesting issue that has me a little confused.  For Christmas, I was gifted an ES Firstlight 102mm refractor, and attempted to replace the included diagonal with my Orion model 8880 1.25 inch dielectric star diagonal.  There is no issue fitting the Orion diagonal into the 1.25mm drawtube, but no matter what EPs I use with it, I cannot resolve any images.  Even with the drawtube all the way in or all the way out, everything is a blur.  When I use the diagonal that came with the telescope, I can resolve everything just fine, no matter which EPs I use.  Strangely, the Orion diagonal works fine in other 102mm telescopes, and I haven't had any issues with it until I plugged it into the Firstlight 102mm.  Given this info, can anyone explain why this telescope/diagonal combo is wonky?  No need to explain that the Firstlight is a low end refractor...I am very much aware of this, but it still doesn't seem to be the cause because other diagonals work just fine with it. 

 

While we are on this subject, I am also seeking an affordable 2x barlow for this telescope, and would appreciate any recommendations. 

 

Thanks in advance for your assistance, and glad to be part of this great community!  

 



#2 TOMDEY

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Posted 11 January 2020 - 01:08 PM

Your telescope came with a 2" diagonal, the 1.25" that you are trying to use requires more back focus. Get an extension tube and you should then be able to achieve focus.    Tom


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#3 msl615

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Posted 11 January 2020 - 01:10 PM

First, welcome to the forums on Cloudy Nights and I hope you are enjoying your new telescope and having fun learning about the sky.  You are sure to get many comments and ideas about your focus issue, but let me ask a few initial questions to help future readers.

 

I looked up the scope on the web, and it included the following in the specs:

 

Adapter 2.5" Hexagonal Focuser; 2" 90° Diagonal

 

Can you send us some photos or other information about that connection?

What size is the draw tube by itself? It sounds like it is 2 inches.

What is this 2.5" adapter?

How are you connecting the 1.25 inch diagonal? You wrote that it fit in easily, but into what?

 

There is the possibility that something is weird about the way that the 1.25 Orion is connecting to the scope. Some pictures would be great to help debug this.  Tom;s note above is most likely correct.

 

Mike


Edited by msl615, 11 January 2020 - 01:11 PM.


#4 vtornado

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Posted 11 January 2020 - 01:30 PM

I think Tom is right, the 2 inch diagonal has a long light travel path compared to the 1.25 inch diagonal.

Turn the focuser all the way out.  Don't lock the eyepiece into the diagonal and slowly draw it out with your hand,

while looking through it.  See when the image comes to focus.  You can even keep moving the eyepiece farther away, out of the diagonal.

Once you see the image focus you know how long of an extension tube you will need.

 

VT.


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#5 flyer92

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Posted 11 January 2020 - 11:45 PM

Thanks for the great replies!  To clarify, the 102mm telescope I have is model # FL-AR1021000EQ3, which has a plastic 1.25' draw tube, 1.25" diagonal, and no adapter.  Model # FL-AR1021000MAZ01 is essentially the same telescope, but with a metal 2" draw tube, 2" diagonal, and apparently includes an adapter for 1.25" components.  Here's a link to both for comparison's sake:

 

1.25":  https://explorescien...fl-ar1021000eq3

2.00":    https://explorescien...12f5234be&_ss=r

 

The 1.25" Orion diagonal was inserted in the draw tube exactly like the cheapo stock diagonal that came with the telescope.  Unfortunately, I can't send any photos of this because I just returned the telescope back to ES, and will use the $$ for something else.  What's more important, is to understand why this happened, because I'd like to use the same diagonal in other 1.25" telescopes, and don't want to run into this problem again.  From a practical perspective, I agree that an extension tube would solve the problem, but don't understand how a telescope with a 1.25" draw tube, designed for 1.25" components, is not compatible with a pretty standard 1.25" star diagonal.  Thoughts?  



#6 Nakedgun

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Posted 12 January 2020 - 01:40 AM

Not all diagonals have the same length light path, and mirror vs. prism diagonals have different lengths, as well.


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#7 Jon Isaacs

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Posted 12 January 2020 - 04:54 AM

From a practical perspective, I agree that an extension tube would solve the problem, but don't understand how a telescope with a 1.25" draw tube, designed for 1.25" components, is not compatible with a pretty standard 1.25" star diagonal.  Thoughts?

 

 

I have no ideas.  It's true that various diagonals can have somewhat different optical path lengths but in my experience, it's never enough that it's not with the range of the focuser.  

 

The one thing that occurs to me is that maybe the First light diagonal has a built in barlow lens or some other optics, this could change the range of focus substantially.

 

Jon



#8 Rock22

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Posted 12 January 2020 - 08:41 PM

I’m not sure what was going on with your scope and diagonal combo.

I have that scope, and haven’t had any problems with any of my diagonals when I had the stock focuser or after I changed to a 2” Crayford. I also have the same Orion 1.25” diagonal and didn’t have any problems with it in the scope. The stock focuser was not bad, actually.

I don’t consider it a low-end scope at all. It’s a good mid-level scope in build and function. I’m sure the accessories won’t be good enough for those more experienced, but I think it’s a good long, 4” achromat. I have a lot of fun with it.

Probably better that you returned it, but I think it’s a good bargain.

As for a good 2x Barlow for a good price, the GSOs get good reviews. I have the 2” and it does perform well IMO.

#9 Bean614

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Posted 12 January 2020 - 10:27 PM

Perhaps one diagonal was a prism and the other a mirror.  Different light path lengths. 


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#10 flyer92

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Posted 12 January 2020 - 11:50 PM

Thanks again for all the great feedback and support for this newbie.  I actually agree with Rock about the Firstlight 102mm and didn't intend to disparage it at all.  Just seems that when discussing telescopes and other gear of this caliber, all I hear are rants from the "aficionados" (aka gear snobs) about how I shouldn't have even purchased such garbage in the first place, etc. etc...I'm sure you all have seen this before.  In my opinion, every piece of gear at every level has it's place in one's journey from novice to expert, but apparently, not everyone shares this perspective.  Just refreshing that I haven't experienced this sort of negative feedback in my first week here, which says a lot about this group.

 

Still a mystery why the diagonal didn't work, but appreciate everyone's time and attention....and thanks to Rock for the Barlow recommendation!     




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