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DPAC Test - Orion XT8 OTA

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#26 Jeff B

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Posted 29 January 2021 - 07:05 PM

Well, it certainly was not my intent to make anyone unhappy with their Orion Dob.  Just sharing what I found out concerning this particular sample, which correlates with the owner's feelings that something was not quite right.  Never be afraid of the data.  It's your friend.  

 

I've got it outside right now cooling off.  Like small Dobs, it's super easy to set up.  It's collimated and it seems to staying that way so far.  Star testing is on the agenda both at full and 7" apertures, maybe even some lunar viewing. 

 

Jeff  


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#27 Jon Isaacs

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Posted 29 January 2021 - 07:50 PM

Well, it certainly was not my intent to make anyone unhappy with their Orion Dob.  Just sharing what I found out concerning this particular sample, which correlates with the owner's feelings that something was not quite right.  Never be afraid of the data.  It's your friend.  

 

I've got it outside right now cooling off.  Like small Dobs, it's super easy to set up.  It's collimated and it seems to staying that way so far.  Star testing is on the agenda both at full and 7" apertures, maybe even some lunar viewing. 

 

Jeff  

Jeff:

 

I appreciate your testing these scopes and providing the DPAC images. Data is good.  I think anyone who owns an Synta or GSO scope knows that there is a significant sample to sample variation.  The fact that your friend felt that something was wrong and the DPAC showed that indeed there was validated your friend's intuition and experience.

 

I think it's worth noting that in this particular case, had the mirror been a good one, you probably wouldn't have tested it, the fact that it seemed somehow flawed was the reason it was tested.  This is a bias if one wants to think of this as a random sample of 8 inch Orion mirrors, it's not a random sample. 

 

Just as the owner of every XT-8 hopes theirs is not like this, every owner of a 10 inch GSO hopes their mirror is as good as the one you recently tested.. But they are what they are.. Some better than others.

 

Jon


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#28 Jeff B

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Posted 29 January 2021 - 11:30 PM

Good points Jon.  That 10" was a random sample.  I'm curious as to how he likes it as he just got the base for it.

 

Well, I just came inside after using the XT8Plus and the "XT7Plus" (wink.gif ).  There was one surprise, which I'll mention a little later, but no surprises really.  

 

The sky was clear and quite calm early on, which allowed for good star testing, which was done at 240X with a Clave 10mm and an older Celestron/Vixen Silver top 2X barlow.  Collimation was "spot on", pun intended.  I spent a lot of time on Aldebaran, some on Betelguese.  At the full aperture the turned edge was obvious in the inner/outer focus diffraction patterns being quite dissimilar.  Inside of focus the rings were washed out to a large extent, with the outside of focus patterns very distinct.  In focus, however, showed what some would say was a nice airy disk with a couple of uniform diffraction rings around it but some scattered junk too.  "Classic" turned edge behavior IME. 

 

At the 7" aperture, the inside/outside of focus rings were much more similar in appearance.  But not identical.  I could detect the outer zone on the inside of focus close to focus and the outside of focus pattern was a little more distinct.  But each side "behaved well", even if they appeared a little different than each other, with the close-in patterns falling evenly into focus and then back out the other side.  There was a nice "clean" airy disk at focus with one faint-ish ring, and notably less scatter/junk than at full aperture.  I spent some time on M42 and, especially the trap.  Here, aperture ruled for the most part, except at moderate to higher powers the trap, with the E&F stars visible, seemed "tighter", "cleaner" and to stand out from the nebula a bit better even though the view was a little dimmer at 7" than at full aperture.  Ditto with the finer, delicate structures and filaments at moderate power (~120x).  I've seen exactly this contrast thing before, once comparing one of my older pre-ED AP triplets to a very good C8.  The 6" gave the "crisper" image showing a wealth of fine structure that was just easier to see than in the C8, despite the C8's brighter view.   Ditto here but the extra light grasp of the 8" aperture was noticeable and also appreciated too.  

 

I started the star testing with the 7" stop, waiting until I got a nice airy disk and decent in/outside of focus ring patterns before really looking carefully.  Then I removed the stop and OMG what a complete mess the patterns were with flaring and no distinct rings inside of focus and a fuzz ball at focus.  The 7" mask was doing exactly what my aperture masks are suppose to do with my big refractors during cooling, masking the over/under correction from the rapidly cooling outer edge.  This allows high power use during cooling but with reduced aperture.  Except it was way more obvious with this newt.  I had to wait a good while for the primary to stabilize more.  So, folks, even with a perfect mirror, it may make sense to make an aperture stop for you newt with conventional glass too, allowing it to operate well at higher powers earlier than normal during the cooling process.

 

At either full or 7" aperture, I saw no on-axis coma (good collimation!) or astigmatism, even at high power.

 

Now the surprise.  I also had a trash rescued Meade Polaris, 114mm F8 newt out on my Vixen SP mount.  I had cleaned the mirrors up, collimated it (actually a well put together OTA) and put it in DPAC.  It showed "jail bar" straight lines and served up just wonderful views tonight with a very nice star test. at 180X.  A complete surprise, though I have found several of these cheap, rescued from the dump, scopes lately that are really quite good.  

 

Jeff


Edited by Jeff B, 30 January 2021 - 07:41 AM.

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#29 Jeff B

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Posted 30 January 2021 - 06:18 PM

Based upon what I saw last night in the side-by-side visual comparisons, I retested the scope with a 7.5" aperture mask with a side-by-side-by-side image, outside of focus, showing, from the left side to the right side, full aperture (8"), 7.5", and 7.0" apertures (not bad, I used the word "side" seven...oops, I mean eight times in this sentence). 

 

As you can see, the 7.5" stop does cover the great majority of that turned edge, but not quite all of it.  Kinda splitting hairs here, but aperture stops are sooo easy to make and try out.

 

Jeff

Attached Thumbnails

  • XT8, Outside, 8.0, 7.5, 7.0 inch Apertures.jpg

Edited by Jeff B, 30 January 2021 - 06:19 PM.

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#30 Bonco2

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Posted 30 January 2021 - 06:48 PM

Jeff,

I really appreciate your revealing pictures. When I mentioned I was going to mask my XT8 some posted that I was going down a rabbit hole. I don't think so. Masking may not reveal the CAUSE of imperfection without a DPAC test but might provide an obvious performance improvement. And the cost is? Nothing but a piece of cardboard. Your pictures seem to support my thoughts. Recently had the best night with my XT8 stopped down to 7 inches. Best star images ever seen thru this telescope.

I might mask at 7.5 inches but I'm thinking I'm at the sweet spot now at 7 inch.

Bill


Edited by Bonco2, 31 January 2021 - 04:51 PM.

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#31 nirvanix

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Posted 31 January 2021 - 05:09 PM

From Agena Astro's website:

 

"GSO guarantees diffraction limited performance, but their mirrors typically have a mirror surface quality of 1/16 wave RMS at least, and often better. This very smooth mirror surface results in excellent optical performance with practically no light scatter."

 

So, if you've got a donkey GSO mirror when you were promised something better then please send it back. Not sure what to do about a Synta mirror. Perhaps the dealer can exchange it.

 

But first, always suspect seeing conditions, collimation, cooldown, and even proper mounting of optical components ahead of that. When I first got my dob the planetary views were like mushy peas, but after sorting out the mounting issues I've been able to push the scope to very high mags with tack sharp views.


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#32 Jon Isaacs

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Posted 31 January 2021 - 07:45 PM

Jeff:

 

How big is your flat?  How long does a test take?

 

Jon



#33 Jeff B

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Posted 31 January 2021 - 08:07 PM

Jeff:

 

How big is your flat?  How long does a test take?

 

Jon

Now in the UK, I'd say 3500 square feet grin.gif

 

It's 12 ".  Testing time depends on aperture and scope type.  The short answer is the basic test is 1-3 hours from placing the scope on the bench to finishing processing of the shots, depending on the scope.

 

Now the long answer.

 

The biggest I've tested are my C11 and CZ based 11" F7.  If I can get it onto the bench, set up and a complete set of images in white light (inside, at and outside of focus) does not take long, maybe 30-45 minutes, depending on the scope, with multiple exposures at each focuser position, once I get going.  The CZ/Parallax OTA took a couple of hours as I had to leverage the OTA up on to horses, put the flat in front of it and adjust everything and I mean everything.  The xt8 Plus test took about an hour total for data collection at full and 7" apertures, not counting time spent collimating.  

 

Now that's to collect the images.  It actually takes longer to select, edit, crop and process them, especially with refractors (since I split the data into white, red, green and blue colors.)

 

Jeff


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#34 Jon Isaacs

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Posted 31 January 2021 - 08:47 PM

Jeff:

 

Thanks.. 

 

Interesting..   I'll have to lean more about it.

 

I could do an 80 mm as I have an extra 4 inch secondary mirror..

 

Jon


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#35 stargazer193857

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Posted 31 January 2021 - 10:08 PM

It would be nice to test some brand new GSO mirrors that are not already suspected of being bad and sent to testing for that reason.

I'm mostly curious about the 8" f6. At $205, it would be nice if the quality were good.

#36 stargazer193857

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Posted 31 January 2021 - 10:11 PM

Actually Jeff, that's an XT8-Plus, and the Optics are from GSO, not from Synta. All the Orion basic XT Dobs, with Springs and not Tension Handles, have Synta Optics. So do the Intelliscope models and GoTo models. The Plus versions and Skyline versions are from GSO.
And, my experience with these (I've had many dozens) mirrors your tests. Of the 40 or so Orion Synta Dobs I've owned, bought new OR used, the worse optics I've had were "above average". However, of the 30 or so GSO Dobs I've had (the above mentioned Orion models, Apertura models, Zhumell models, and GSO models) it was much more hit or miss ---- some quite excellent, some nearly unusable above 100X magnification.
Of course, all of my samples were perfectly collimated before EACH use, and all were placed outside hours before use, WITH fans running.


I think Orion sells 10" f4.7, whereas agenaastro (GSO) sells 10" f5.
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#37 Jeff B

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Posted 31 January 2021 - 10:24 PM

Jeff:

 

Thanks.. 

 

Interesting..   I'll have to lean more about it.

 

I could do an 80 mm as I have an extra 4 inch secondary mirror..

 

Jon

Sure Jon.  You can just gently place the scope, face down on to the secondary. No need for fancy fixtures, maybe just some paper shims to adjust alignments.  You can even simply place the Ronchi screen on the open focuser tube then hover the LED over it while looking and adjusting the focuser.  Quick and easy.....and revealing.

 

Jeff

Attached Thumbnails

  • C5 in DPAC.jpg
  • TEC 7, DPAC, Screen Rightside Up.jpg

Edited by Jeff B, 31 January 2021 - 10:24 PM.

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#38 stargazer193857

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Posted 31 January 2021 - 10:35 PM

NewMoonTekescopes offers the option of GSO optics in his expensive structures. Do you think he would do that if they were really bad? He does not seem to upcharge for the optics. He tests them under the stars. I wonder if they send him their best.

#39 Jeff B

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Posted 25 February 2021 - 06:49 PM

A little bit of an update.

 

At the suggestion of Orion to the owner, I removed the mirror and cell assembly from the OTA (super easy) to check the tightness of the mirror clips.  They were indeed a bit tight IMO, so I adjusted them to where the rubber retainer pads just rested nicely on the mirror with a tiny bit of wiggle and I could rotate the mirror with moderated effort.  I also applied three small dabs of silly cone caulk in the gap between the cell and mirror walls to keep the mirror from shifting at all (I've done this before and it works really well).  After a three day wait to let the glass snap back if it was under stress and to let the caulk dry, I assembled everything, recollimated it and stuck it back in DPAC.  

 

Nothing changed.  It still has the bum edge.

 

 

Stopped to 7" aperture, the figure is overall pretty good as a system.  It's decently smooth and mimics about 1/7 to 1/8 wave over corrected mainly from a mild donut zone from about 60% span out to the edge.  I could detect no astigmatism or coma, when properly collimated.  Stopped to 7.5" the turned edge is still just visible over about a 60 degree segment of the perimeter.  I suspect a 7.25" stop would hide the bum edge completely.

 

Visually, I spent a nice evening with the scope, well collimated and cooled, looking at the first quarter moon getting up to 200X at times, but more typically seeing limited me to around 160X.  The image was very sharp at 7" aperture.  No complaints at all with a nice "snap" to the focus.  At 7.5" aperture, it still was rather sharp but I had to hunt for focus, it being a bit ambiguous compared to 7" aperture.  Ditching that stop and looking at full aperture, I had to subtly refocus but focus was now rather ambiguous, being notably harder to pin down than at the reduced apertures, especially at 7" aperture.  Crater definition was less sharp with what I would call a slight "double vision" look.  More importantly, the entire FOV was covered with a thin but very noticeable haze or "fog".  It was not subtle to me and I suspect it's from the bum edge as I did not see any extra or increased reflections off of the tube inner wall under the diagonal when I removed the masks.  

 

In the mean time the owner harassed Orion enough to get a return authorization for the mirror.

 

So here's the thing.  I'm concerned that if the mirror is returned, we may end up with a different one of unknown quality, and, perhaps, inferior in either SA correction and or smoothness.  So it's the age old dilemma, do we put known very good (7" aperture) at risk for the possibility of "better" at full aperture?

 

Jeff


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#40 scotsman328i

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Posted 25 February 2021 - 06:55 PM

A little bit of an update.

 

At the suggestion of Orion to the owner, I removed the mirror and cell assembly from the OTA (super easy) to check the tightness of the mirror clips.  They were indeed a bit tight IMO, so I adjusted them to where the rubber retainer pads just rested nicely on the mirror with a tiny bit of wiggle and I could rotate the mirror with moderated effort.  I also applied three small dabs of silly cone caulk in the gap between the cell and mirror walls to keep the mirror from shifting at all (I've done this before and it works really well).  After a three day wait to let the glass snap back if it was under stress and to let the caulk dry, I assembled everything, recollimated it and stuck it back in DPAC.  

 

Nothing changed.  It still has the bum edge.

 

 

Stopped to 7" aperture, the figure is overall pretty good as a system.  It's decently smooth and mimics about 1/7 to 1/8 wave over corrected mainly from a mild donut zone from about 60% span out to the edge.  I could detect no astigmatism or coma, when properly collimated.  Stopped to 7.5" the turned edge is still just visible over about a 60 degree segment of the perimeter.  I suspect a 7.25" stop would hide the bum edge completely.

 

Visually, I spent a nice evening with the scope, well collimated and cooled, looking at the first quarter moon getting up to 200X at times, but more typically seeing limited me to around 160X.  The image was very sharp at 7" aperture.  No complaints at all with a nice "snap" to the focus.  At 7.5" aperture, it still was rather sharp but I had to hunt for focus, it being a bit ambiguous compared to 7" aperture.  Ditching that stop and looking at full aperture, I had to subtly refocus but focus was now rather ambiguous, being notably harder to pin down than at the reduced apertures, especially at 7" aperture.  Crater definition was less sharp with what I would call a slight "double vision" look.  More importantly, the entire FOV was covered with a thin but very noticeable haze or "fog".  It was not subtle to me and I suspect it's from the bum edge as I did not see any extra or increased reflections off of the tube inner wall under the diagonal when I removed the masks.  

 

In the mean time the owner harassed Orion enough to get a return authorization for the mirror.

 

So here's the thing.  I'm concerned that if the mirror is returned, we may end up with a different one of unknown quality, and, perhaps, inferior in either SA correction and or smoothness.  So it's the age old dilemma, do we put known very good (7" aperture) at risk for the possibility of "better" at full aperture?

 

Jeff

You’re right, Jeff...six of one or half a dozen of the other. It’s a gamble any way you look at it.



#41 peleuba

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Posted 26 February 2021 - 11:30 AM

NewMoonTekescopes offers the option of GSO optics in his expensive structures. Do you think he would do that if they were really bad? He does not seem to upcharge for the optics. He tests them under the stars. I wonder if they send him their best.

 

 

Its been my experience that most dob makers are rather poor at testing optics - though I can think of two exceptions to this rule.  For the most part the "structure" maker relies on the reputation of the optician and is more concerned with the build-out of the telescope.  This includes "star-testing" the scope at low power to insure the focus point is within the travel of the focuser.  If its not, the truss poles are shortened (or lengthened).  During this test, maybe grossly visible optical aberrations are picked up, but I am not so sure.

 

I test a lot of telescopes and attend some really large gatherings/starparties throughout the year.  Most of the optics I see are not collimated and not all that good.  The owners' seem to be blissfully unaware of this.  A few are terrific, but these are usually owned by longtime amateurs who understand the nuances between fair/good/great and the path to get there. 

 

I think Ryan probably offers the option of GSO optics for two reasons:  (1) Price point.   (2) Availability.  He can build a structure faster then the boutique opticians can make a mirror.   This takes care of the "instant gratification" crowd - which, admittedly, I can be part of.   lol.gif

 

Over the last 20+ years I been fortunate to have a good friend who has tested many more scopes then I, some we've done together.  The one thing I've gleaned from his tutelage is the best thing one can do before buying a dob (with any type of mirror) is to understand how to test it.  


Edited by peleuba, 26 February 2021 - 11:40 AM.

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#42 gwlee

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Posted 26 February 2021 - 02:42 PM

I think the XT-8 Plus is a Synta scope. 

 

I have been following the Orion XT series and GSO Dobs since they were first introduced in 2000.  The original XT-6, XT-8 and XT-10 were manufactured by GSO. In a year or two, Orion switched to Synta. I think I have pretty good eye for identifying Orion Dobs made by Synta and Orion Dobs made by GSO and to my eye, everything about the current XT-8 series points to Synta/Skywatcher as the manufacturer.  Here are some of the obvious reasons:

 

- The XT-8, the XT-8 Plus, the XT-8i and the XT-8G use mirrors made from "low thermal expansion Borosilicate glass".  GSO Dob mirrors are made from BK-7.  This is very telling. 

 

- The XT-8 Plus does not have a GSO focuser.  It has the upgraded two speed that comes with the XT-8G and other Synta scopes.  Only the Skyline has a GSO focuser.  

 

- The XT-8, the XT-8 Plus, the XT-8i and the XT-8G OTA's weigh 20.3lbs-20.8 lbs, the Skyline 23.8 lbs. This has always been a good way to tell the difference. 

 

- The XT-8 Plus does not use the GSO altitude bearing design, it uses one very similar to the XT-8i.  

 

I could go on but I think it's clear that the scope Jeff tested was manufactured by Synta/Skywatcher.  

 

We all have our favorites and our own experiences but the beauty of the DPAC is that the pictures tell the story and this one is a clunker.  

 

Look at the photos, compare the specs:

 

Orion XT-8 Classic 

 

Orion XT-8 Plus

 

Orion 8 inch Skyline

 

Orion XT-8i

 

Orion XT-8G

 

Jon

I also believe the Plus is a Synta scope. 



#43 helpwanted

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Posted 26 February 2021 - 02:58 PM

loosing one inch of aperture is not much at all



#44 NYJohn S

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Posted 27 February 2021 - 04:04 PM

I have 2 XT8’s. An older intelliscope and a newer XT8 plus. I never had the mirrors tested but the star tests look good on both. Maybe I got lucky?

#45 Jeff B

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Posted 27 February 2021 - 05:31 PM

I have 2 XT8’s. An older intelliscope and a newer XT8 plus. I never had the mirrors tested but the star tests look good on both. Maybe I got lucky?

Well, I certainly hope you got good ones.

 

A turned edge, however, can be a little difficult to see in the star test.  The inner and outer diffraction rings are not all that dissimilar except in contrast and subtle brightness differences.  You can even get what seems like a nice Airy disk at focus too. 

 

And, indeed, that's exactly what I saw with this mirror with the star test.  Without DPAC, it would have been easy to pronounce the primary mirror a good one based upon the star test alone.  But then, what's up with that haze, difficulty getting sharp focus, slightly soft images, and the muted contrast on Jupiter?  Those were the real visual consequences for this mirror.

 

Even using a single pass Ronchi eyepiece and a star should reveal a moderate turned edge at high power.  With DPAC, it sticks right out...or up...or down.

 

Jeff


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#46 NYJohn S

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Posted 28 February 2021 - 09:17 AM

Well, I certainly hope you got good ones.

 

A turned edge, however, can be a little difficult to see in the star test.  The inner and outer diffraction rings are not all that dissimilar except in contrast and subtle brightness differences.  You can even get what seems like a nice Airy disk at focus too. 

 

And, indeed, that's exactly what I saw with this mirror with the star test.  Without DPAC, it would have been easy to pronounce the primary mirror a good one based upon the star test alone.  But then, what's up with that haze, difficulty getting sharp focus, slightly soft images, and the muted contrast on Jupiter?  Those were the real visual consequences for this mirror.

 

Even using a single pass Ronchi eyepiece and a star should reveal a moderate turned edge at high power.  With DPAC, it sticks right out...or up...or down.

 

Jeff

Thanks Jeff. I'm no expert but  I'm not seeing what you described with the inside/outside of focus diffraction patterns. I don't have any difficulty focusing at high power either. I've had the XT8i for almost 4 years now and compared it side by side with quite a few production scopes. The views are alway more similar than different. Side by side with an Aperture 12" it's about what you would expect. A brighter image in the 12 with a little more detail but similar contrast and sharpness. The XT8 Plus I just purchased from a friend and have looked though it many times so I knew it had a good mirror. I will say the scopes I have compared it to most often are all production scopes so maybe my expectations are not that high. I still think from your description of the hazy, hard to focus views it sounds like that mirror should be sent back. 

 

John



#47 starman876

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Posted 28 February 2021 - 09:46 AM

Sure Jon.  You can just gently place the scope, face down on to the secondary. No need for fancy fixtures, maybe just some paper shims to adjust alignments.  You can even simply place the Ronchi screen on the open focuser tube then hover the LED over it while looking and adjusting the focuser.  Quick and easy.....and revealing.

 

Jeff

are you not worried about scratching the flat by placing the scope on top of it like that?  I know I would.


Edited by starman876, 28 February 2021 - 11:27 AM.


#48 starman876

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Posted 28 February 2021 - 09:48 AM

Talking about ronchi screens.  Anyone know who still sells them for a reasonable price?  I tried to print some on clear plasic with my laser printer, but you need a pretty good printer to do that.  



#49 CHASLX200

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Posted 28 February 2021 - 10:19 AM

I have never been a happy pappy with these mass made scopes. But i push them to crazy powers with my good seeing and they just never give me that WOW a good Zambuto does or other top notch made mirror does.  There really is a big diff in the view with the best mirrors in super steady seeing.



#50 starman876

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Posted 28 February 2021 - 11:29 AM

Based upon what I saw last night in the side-by-side visual comparisons, I retested the scope with a 7.5" aperture mask with a side-by-side-by-side image, outside of focus, showing, from the left side to the right side, full aperture (8"), 7.5", and 7.0" apertures (not bad, I used the word "side" seven...oops, I mean eight times in this sentence). 

 

As you can see, the 7.5" stop does cover the great majority of that turned edge, but not quite all of it.  Kinda splitting hairs here, but aperture stops are sooo easy to make and try out.

 

Jeff

Has anyone see a chart that helps one determine what the DPAC test is revealing.  I have seen a few, but they are pretty simple.  




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