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Newbie learning the finer points

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#1 Mordgeld

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Posted 25 May 2019 - 04:40 AM

Yes, newbs gonna newb. Experienced people are going to say "duh". But for the edification of others, I thought I'd continue to relate my journey. Tonight I had some clear skies starting at about 10pm. I was able to catch Jupiter and Saturn again with the 90mm but not with quite the seeing as last time. Here are some of the things (and some truisms you can find easily by searching the forum) I've discovered for myself as true...

 

  • While shorter eyepieces have worse eye relief, they generally yield a sharper view than a barlowed longer eyepiece. (Amongst similar quality EP)
  • Decent eyepieces barlowed have better views than shorter EPs that are cheap.
  • Cheap EP look okay until you barlow or get into shorter ones. The first decent EP you buy will make this obvious per the above point. Using a refractor rather than a Dob seemed to play a part in this too.
  • My cheap 2x barlow makes everything look horrible. I need to buy a different one. My 3x Astromania is good, tho.
  • My $0.99 5x barlow is not bad at all when I put a quality EP in it. Sometimes you get lucky.
  • I'm surprised at the amount of chromatic abberation I get at F10.1 on bright objects. Perhaps I am sensitive to it? Stopping down the aperture or backing off the magnification does mitigate this.

The moon is coming up now. Perhaps I will learn something new.


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#2 pregulla

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Posted 25 May 2019 - 04:55 AM

It all depends on eyepieces and barlow in question. Decent barlow won't noticeably degrade the view and may even improve performance of some eyepieces that struggle at faster focal ratios.

Poor quality barlow will ruin the views through any eyepiece.


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#3 Mordgeld

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Posted 25 May 2019 - 05:21 AM

Agreed. I think upgrading my 2x would be a good first step. I like the 25mm EP I got from ES with my newt. It doesn't have a brand name on it and doesn't resemble the other EP on their site. It just says Super Plossel FMC. I'd buy more like it if I could.



#4 SeattleScott

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Posted 25 May 2019 - 09:36 AM

The chromatic abberation you see may be from the eyepiece instead of the scope. Many newbies mistake lateral color for CA. A 4” F10 refractor shouldn’t show much CA.

A good test is, if you are seeing CA in the Dob, it isn’t really CA.

Scott

Edited by SeattleScott, 25 May 2019 - 10:12 AM.


#5 penguinx64

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Posted 26 May 2019 - 10:36 AM

I use a #8 light yellow filter to help reduce CA.  The #8 doesn't reduce CA as well as a Baader Contrast Booster filter, but it costs $80 less.

 

I had very good results using an Orion Trimag 3x barlow and longer focal length eyepieces to get more eye relief.  It's a long barlow that doesn't fit refractor diagonals very well.  The shorty barlows don't seem to work as well as the longer ones.  



#6 tony_spina

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Posted 26 May 2019 - 11:12 AM

The chromatic abberation you see may be from the eyepiece instead of the scope. Many newbies mistake lateral color for CA. A 4” F10 refractor shouldn’t show much CA.

A good test is, if you are seeing CA in the Dob, it isn’t really CA.

Scott

Scott is correct.   The other thing you should do a star test.  Do you get nice concentric rings when you are slightly under focused or over focused?   If not misaligned lenses can also add color to your views



#7 Mordgeld

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Posted 26 May 2019 - 12:58 PM

I've got a #8 filter on order, but I don't have it yet because an item in my order is backordered. I've used the same cheap EPs in a dob and they looked great but I haven't been able to try the better ones in it yet. I agree that 90mm at f/10 should not show too much CA. The purple smear isn't apparent until I push the mag up. Stopping down the aperture definitly helps. I'm planning to get some more decent EP for the higher magnifications. I hope that will improve my view. I'll try to do a proper star test next time I can see the sky. Is a pre-owned refractor so maybe it needs some care.

#8 aeajr

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Posted 09 June 2019 - 01:08 PM

When evaluating eyepieces you need to be specific about what eyepiece you are using and in what scope.  Making broad generalizations when your experience is limited is not a good idea.

 

While cost will usually track to eyepiece value/quality it may not always produce the improvement you expect compared to the price increase.

 

CA, chromatic aberration, is something you should expect in achromatic refractors, especially those below F10.   

 

CA is not something you should expect to see in a Newtonian scope such as the one on your Dobsonian mount.   If you see CA, something else is wrong.

 

Remember that expensive and cheap have no meaning.   They are relative terms.   A $100 eyepiece is expensive compared to a $10 eyepiece but it is cheap compared to a $300 eyepiece.  Yet, its performance doesn't change.

 

Cheap and expensive are more a statement about your attitude and your budget.  So try to be specific when you are comparing things.    My Explore Scientific 82 degree eyepieces are expensive compared to my Meade Plossls but they are cheap compared to the comparable Tele Vue eyepieces.

 

The best way to judge a Barlow is to compare it to the view in an eyepiece alone.

 

A barlow does not affect the actual focal ratio of your scope.  As such an F5 refractor with a 2X barlow will not have the same CA characteristics as an F10 refractor.  Barlows do not reduce CA.

 

20 mm + 2X Barlow vs. 10 mm eyepiece of the same brand and type is a valid comparison.   If the barlow is good, the view should be about the same in both. The amount of light lost to the Barlow should be so small as to be virtually invisible to the eye.  The barlow should not cut off the field of view, vignette, nor should it add distortion around the edge of the view.  It should disappear, so to speak.  


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#9 Mordgeld

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Posted 10 June 2019 - 01:48 PM

Has been quite iffy seeing anything lately for star testing and such. I used the collimator with obj lens cap on with the 90mm refractor to see the diffraction rings. Was able to tighten up the focuser alignment. So let me clarify a bit on brand as opposed to cheap/'spensive with respect to purchased-as-new eyepieces only. I got a 25mm and a 9.7mm plossels from ES as part of a telescope package. The 25mm looks kinda like the Bresser EP on their site. The 9.7 is fully plastic. I have a 32mm Celestron Omni. I also have 6/12.5/20mm plossels + 2x barlow from Gosky. Additional barlows are 3x AstroAgeena and 5x Datyson. I do most of my viewing with the longer EPs and the AstroAgeena barlow. This is clearer than the Gosky EPs by themselves. (but not clearer than the ES 9.7mm) The view is still sharper with the Datyson 5x and longer EP but with some internal reflections. The Gosky EPs by themselves are clearer than any EP through the 2x Gosky barlow. The blue smear that looks like CA in the Gosky stuff wouldn't be CA at f/10.1, if I'm not mistaken.

 

I was thinking that I would like to go with more of the Celeston Omni EPs and a barlow to replace the Gosky stuff but my reading here has me considering the Paradigm/Starguider line.

 

Oh, and ES now has an update/cable that will allow me to control my EXOS2-gt mount with a computer! I don't remember this being available before but I'm pretty happy to have this capability available.



#10 aeajr

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Posted 10 June 2019 - 03:02 PM

Yes, I would recommend the Paradigm over the Celestron Omni.  Wider AFOV and longer eye relief at the shorter focal lengths. 



#11 mjulihn

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Posted 10 June 2019 - 04:04 PM

I am quite happy with my StarGuider EPs in 15mm and 6mm. Both are wide view with extended eye relief.



#12 Mordgeld

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Posted 12 June 2019 - 04:45 PM

I gave up on the counterweight I had on backorder to balance my light refractor. It has been over a month. Anyway, the manufacturer (Ioptron) has changed their description from "fits 21mm" to "fits 20.5mm". I let Astronomics know this as they have it as 21mm center hole. On the up-side, my light blue and light yellow filters will ship now. Tonight, I may get the firmware loaded to my Exos2-gt for computer control. I have my 4.5lb 90mm refractor on it as an easy setup for learning and quick viewing. It does not quite balance with just the empty cw bar. I was wondering if this is actually light enough to run sans cw. I'm going to have look around the house, though. It can't need very much cw. 1-3lbs maybe?



#13 Mordgeld

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Posted 17 June 2019 - 04:55 PM

I found a counterweight for my 90mm. I used a GoPro clamp to clamp an AR-15 barrel wrench to the counterweight bar. It balanced pretty well. I got my new ES EXOS2 data cable in and updated the handbox firmware for my EXOS2-gt to v2.3. The instructions were confusing but once I figured it out, it was actually a pretty simple process and no handboxes were harmed. After I got the ASCOM driver installed on my computer I was able to start the POTH (plain old telescope handset) program. Some configuration later, I was slewing using computer control. I'm not going into details here in the newbie threads. I'll start a new one in the appropriate area after I figure out how to leverage Stellarium with this. I'd also be happy to communicate with other EXOS2-gt owners if they are interested in what I'm doing with this.



#14 gnowellsct

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Posted 17 June 2019 - 10:29 PM

It all depends on eyepieces and barlow in question. Decent barlow won't noticeably degrade the view and may even improve performance of some eyepieces that struggle at faster focal ratios.

Poor quality barlow will ruin the views through any eyepiece.

This is true.  It is to be noted that shorter focal length eyepieces in some lines (such as XWs, Delos) maintain the same eye relief.  This is one of the reasons why people drop a bundle on them.   But let's say you've drop $80 or $100 on one of those eyepiece kits that Celestron and Meade sell, those little 5 mm eyepieces with a .5 mm piece of glass to look through are going to have you wondering why people do this king of thing.

 

Astro-physics barlows (are they called barcons?) and Televue power mates do not noticeably degrade the performance of the eyepiece.  It is to be noted that higher magnification almost always shaves some sharpness from the view, given average seeing conditions most places most times.

 

Greg N



#15 Mordgeld

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Posted 10 July 2019 - 12:19 PM

Finally got a regular weight to balance my 90GT OTA. Is a 4lb weight. The EXOS2 takes 21mm ID. Thinking this thread might be useful for others that might modify their Celestron 90GT OTA or use one a regular EQ mount. So now that I have 2" focuser/diagonal and some weightier eyepieces, the 90mm would no longer balance front to rear. I didn't have enough dovetail to move it forward enough. So I screwed the front of the dovetail into the rear set of screw holes and tested the balance. It was good. Used the rear holes as a guide to drill 2 slightly undersized holes. Then I carefully eased the screws back in by increments which had the effect of thread tapping the holes in the softer aluminum tube. Set back up in time to try my new Starguider EPs and my lighter colored filters on Saturn right at opposition. Now that was a great view. Definite coloration visible on the planet and the split in the rings was observable. Is a bit light polluted where I'm at but I think I was able to make out its moon Titan.




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