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Is This A Common Experience?

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#1 briansalomon1

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Posted 01 December 2021 - 08:16 PM

I have a brand new Obsession 15UC and have been reading Vic Menard's book on how to understand collimation, as well as some posts on collimation and after several weeks with the scope, I am fairly convinced I have the collimation very close to where it should be. The spider is centered on the Ostahowski mirror center mark viewed through the diagonal mounting hole, the diagonal is centered in my sight tube, the 1mm laser from my new Howie Glatter collimator is centered on the primary center mark and the center mark reflection is centered on the Barlow attachment reticule and agrees with my collimation cap.

 

I have another post regarding the secondary mirror on my telescope showing an odd "center marker". Now that I believe I have the collimation right, I can see the "center marker" is not only strange in appearance, it is not close to being centered and needs to be removed.

 

https://www.cloudyni...1201-163136115/

 

 

I believe I can remove the center marker with Acetone, but I'm very surprised that a brand new telescope from this manufacturer needs this done to it. Having owned only TeleVue telescopes for more than 30 years I'm not sure what I should have expected.

 

I realize demand for telescopes is very high, but I want to ask people who have owned Obsession telescopes for as long as I've owned TeleVue scopes - Has the brand changed? is my experience unusual?



#2 The Ardent

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Posted 01 December 2021 - 08:50 PM

https://www.cloudyni.../#entry10163454



#3 Markovich

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Posted 02 December 2021 - 10:01 AM

Couple of things- both the primary and secondary mirrors will likely be supplied and center marked by Ostahowski Optics. They probably arrived to you unopened from them or the coater.. The physical center of the secondary isnt necessarily the optical center ( see offset discussion) for correct collimation. Others here can be speak more authoritatively on this matter than I .


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#4 briansalomon1

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Posted 02 December 2021 - 10:17 AM

Couple of things- both the primary and secondary mirrors will likely be supplied and center marked by Ostahowski Optics. They probably arrived to you unopened from them or the coater.. The physical center of the secondary isnt necessarily the optical center ( see offset discussion) for correct collimation. Others here can be speak more authoritatively on this matter than I .

When I asked, Dave Kreige said he was the one who put the center mark on the secondary.

 

I've been reading Vic Menard's book on collimation and saw there are three models for aligning the diagonal. I'm one of those who like to understand things and find the explanations Interesting, I see that the classic model calls for an "vertical" offset of the diagonal but none of the three models calls for the center to be offset off to one side, where the center spot on my secondary is.



#5 Rob95060

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Posted 06 December 2021 - 06:44 PM

I am a newb, but when I read about collimation I left the primary center mark in place. I is it not the shadow of the secondary?

https://garyseronik....primary-mirror/

 

 



#6 MitchAlsup

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Posted 06 December 2021 - 08:07 PM

Yes, the mark on the center of the primary is entirely covered by the obstruction of the secondary.



#7 briansalomon1

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Posted 06 December 2021 - 08:08 PM

I am a newb, but when I read about collimation I left the primary center mark in place. I is it not the shadow of the secondary?

https://garyseronik....primary-mirror/

The center mark on the primary mirror was put there by Ostahowski and it looks fine.

 

The reflection of the odd shape is the center mark on the secondary mirror and is showing in the reflected light from the primary mirror. It isn't what I expected but as long as I can remove the mark I'll be OK.

 

I'm going to say here that after having gotten the scope well collimated and having used it now for a few weeks, I thinks it performs really well. I can see what people mean though when they mention a lack of stiffness in the UC design. When I put a Nagler 31mm eyepiece in the focuser it shifts the collimation.

 

This scope is very easy to transport and it sets up in less than 10 minutes, but the upper truss flexes so much with the UC design, it requires you to hang the eyepiece you're going to use from the focuser during collimation and to collimate almost every time you change the eyepiece.

 

For me, the Ostahowski optics, together with the tremendous ease of assembly and transport make the inconvenience of needing to re-collimate frequently with the eyepieces hanging from the focuser something I can live with.

 

I've spent my entire career aligning optical systems and it's something I do habitually. I can definitely see how the flexible upper truss on this design would be an issue for many people.



#8 The Ardent

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Posted 06 December 2021 - 08:22 PM

There is a flexure solution. 

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#9 Starman1

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Posted 07 December 2021 - 08:08 PM

OK, an explanation.

There are two kinds of offset in the position of the secondary mirror, though both result in full offset at the focal plane.

1) Bidirectional offset:  This is where the secondary is dropped toward the primary to center it under the focuser and positioned away from the focuser

so the optical axis coincides with the center line of the tube.  The optical axis will not hit the geometric center of the secondary.  The reflection is exactly 90°.

2) Unidirectional Offset: This is where the secondary is dropped toward the primary but centered between the walls of the OTA. The optical axis is slightly tipped toward the focuser

and the reflection at the secondary is slightly greater than 90°.

 

In both cases, the offset of the optical axis on the secondary is the same.  And in neither case does the optical axis hit the center of the secondary, so there is no reason to centerdot  the mirror.


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#10 tubehead999

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Posted 07 December 2021 - 09:23 PM

Actually, the center dotted secondary is helpful with initial setup. Upon arrival, the UTA is just bolted together..the spider hasn't been set properly nor has the secondary. The UTA is the "elephant in the room " with any UC discussion. I own the 18UC.

If I was younger and had a more intensive viewing schedule, the UTA and associated (sans Feathertouch) would be replaced. I still may replace it......IF!... it continues to tick me off. (smile)

The spider vanes are too thick, the secondary holder won't easily stay locked in place (finally used a pair of locking pliers and an associate to hold it while I cranked down) (ridiculous) and the UTA ring (cheap laminated board with flexure and not enough real estate to mount accessories) would be replaced with a more solid solution.

All that being said: The Ostahowski mirror and the LTA are the heros of the scope. I have no issues with either.
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#11 The Ardent

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Posted 12 December 2021 - 06:54 PM

New vs used $$$
https://www.cloudyni...5" UC Dobsonian
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#12 RLK1

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Posted 12 December 2021 - 07:22 PM

Actually, the center dotted secondary is helpful with initial setup. Upon arrival, the UTA is just bolted together..the spider hasn't been set properly nor has the secondary. The UTA is the "elephant in the room " with any UC discussion. I own the 18UC.

If I was younger and had a more intensive viewing schedule, the UTA and associated (sans Feathertouch) would be replaced. I still may replace it......IF!... it continues to tick me off. (smile)

The spider vanes are too thick, the secondary holder won't easily stay locked in place (finally used a pair of locking pliers and an associate to hold it while I cranked down) (ridiculous) and the UTA ring (cheap laminated board with flexure and not enough real estate to mount accessories) would be replaced with a more solid solution.

All that being said: The Ostahowski mirror and the LTA are the heros of the scope. I have no issues with either.

"Actually, the center dotted secondary is helpful with initial setup."

My Astrosystems manual for my 16" recommended doing the same...



#13 Starman1

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Posted 12 December 2021 - 07:49 PM

If you center the secondary under the focuser, the center dot on the secondary mirror won't be useful to line up the focuser axis or the primary's optical axis.

It would only be useful if it had the necessary offset toward the upper end of the secondary mirror.

That doesn't seem to be the case when secondaries are "center" dotted.

Hence, the center dots do nothing useful and should be left off secondary mirrors.

Center dots on secondaries came from the f/10-f/12 era of newtonians and have no place in current market offerings.


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#14 dnrmilspec

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Posted 12 December 2021 - 08:09 PM





 

This scope is very easy to transport and it sets up in less than 10 minutes, but the upper truss flexes so much with the UC design, it requires you to hang the eyepiece you're going to use from the focuser during collimation and to collimate almost every time you change the eyepiece.

 

For me, the Ostahowski optics, together with the tremendous ease of assembly and transport make the inconvenience of needing to re-collimate frequently with the eyepieces hanging from the focuser something I can live with.

 

 

 

 

 

Is this common?  Is the design so flexible that one has to collimate frequently when changing EP's? 

 

This would be a deal breaker for me.  I am glad to know it now as I have been considering an Obsession.  I see the solution but it seems that a scope with Obsession's reputation would have fixed so obvious a problem out of the gate.



#15 RLK1

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Posted 12 December 2021 - 08:24 PM

Don't have my Astrosystems manual handy as it's buried in a box somewhere in the garage but I vaguely recall something about center spotting the secondary as being helpful in either the offset or the mechanical adjustment of the spider at its attachment points to the cage, which are adjustable in the Telekit units and perhaps in the 18UC...



#16 briansalomon1

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Posted 12 December 2021 - 08:52 PM

 

 

Is this common?  Is the design so flexible that one has to collimate frequently when changing EP's? 

 

This would be a deal breaker for me.  I am glad to know it now as I have been considering an Obsession.  I see the solution but it seems that a scope with Obsession's reputation would have fixed so obvious a problem out of the gate.

 

I noticed it right away, but I have no experience with dobsonians so don't know what's normal.

 

I put a laser collimator in, centered it and hung a 2.5 lb weight on the focuser as a way of comparing our telescopes here on the forum. With a dial caliper, the inner ring of my center marker is 5mm, so I get ~2.5mm of deflection with 2.5lbs.

 

https://www.cloudyni...1211-045326505/

https://www.cloudyni...1211-042258408/

https://www.cloudyni...1211-045357359/

 

I think it's a lot of flex. What do your scopes do?



#17 briansalomon1

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Posted 12 December 2021 - 09:12 PM

Don't have my Astrosystems manual handy as it's buried in a box somewhere in the garage but I vaguely recall something about center spotting the secondary as being helpful in either the offset or the mechanical adjustment of the spider at its attachment points to the cage, which are adjustable in the Telekit units and perhaps in the 18UC...

I'm verifying the focuser positioning as per Vic's instructions in "Perspectives" and am using a sight tube to center the secondary. I have the 1.25" sight tube which has a narrow enough tube to get the outline of the diagonal exactly matched to the tube. The center spot will come off with some acetone and it isn't a major issue.

 

Interestingly, after I had bought the 1.25" sight tube I realized the 2" (f4) sight tube was more appropriate for my scope and was thinking I'd return the 1.25" model, but after comparing them found the small sight tube is definitely more precise for centering the secondary and it's a keeper.

 

(not to be overly precise), but I am verifying the focuser alignment using a flexible metal scale. I'm using the laser collimator and a combination square on a flat surface to reference the focuser center.


Edited by briansalomon1, 12 December 2021 - 11:28 PM.


#18 Tom Stock

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Posted 12 December 2021 - 09:34 PM

I would ignore and (me) remove the spot on the secondary. Not sure why this is done, but it's completely unnecessary imo and only adds to confusion.


Edited by Tom Stock, 12 December 2021 - 09:35 PM.

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#19 Starman1

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Posted 13 December 2021 - 12:52 AM

Don't have my Astrosystems manual handy as it's buried in a box somewhere in the garage but I vaguely recall something about center spotting the secondary as being helpful in either the offset or the mechanical adjustment of the spider at its attachment points to the cage, which are adjustable in the Telekit units and perhaps in the 18UC...

As I said in post #13, if the "center" spot is put on the secondary with the correct offset, it isn't a problem.

It's unnecessary, but at least it's not a real problem.

If, on the other hand, it's put on the secondary dead center, it cannot be used for collimation or it will result in misalignment in the system.

It's just better if it were not there, and I have no clue why it would be put on the secondary in the first place.

It reveals a misunderstanding of collimation.



#20 briansalomon1

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Posted 13 December 2021 - 10:54 AM

There is a flexure solution. 

If anyone has resolved this issue, I'm all ears.



#21 Starman1

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Posted 13 December 2021 - 12:52 PM

If anyone has resolved this issue, I'm all ears.

I think I remember reading here on CN (there have been many threads) about doing the following:

--replacing poles with a larger diameter

--sandwiching the top ring between two other rings to make the top ring stiffer

--Adding a brace to the focuser board to reduce flexure

I vaguely remember other modifications, but CRS prevents me from remembering all the details.grin.gif


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#22 dustyc

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Posted 13 December 2021 - 02:59 PM

I noticed it right away, but I have no experience with dobsonians so don't know what's normal.

 

I put a laser collimator in, centered it and hung a 2.5 lb weight on the focuser as a way of comparing our telescopes here on the forum. With a dial caliper, the inner ring of my center marker is 5mm, so I get ~2.5mm of deflection with 2.5lbs.

 

https://www.cloudyni...1211-045326505/

https://www.cloudyni...1211-042258408/

https://www.cloudyni...1211-045357359/

 

I think it's a lot of flex. What do your scopes do?

I have the same scope and my laser spot moves around but stays within the center ring from horizon to zenith. I don't have a 31mm Nag but with my 41mm Pan I can't see much issue with the image being degraded on stars. A defocused image shows the slight change in the diffraction rings around the center point but at focus the stars are points with no flare. 

Now, maybe at 600 power on Jupiter might show up something, but , not with the seeing conditions I usually get. 

 

Spend enough time on this site and you'll fine that every design has some trade offs. Both due to design and build. 

 

Try this some night. Align the scope the best you can. Observe. Take it apart. Assemble it again a few nights later but don't collimate. Observe. See any difference? At powers you would typically use? In focus? (not with a defocused star image)


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#23 briansalomon1

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Posted 13 December 2021 - 05:59 PM

I have the same scope and my laser spot moves around but stays within the center ring from horizon to zenith. I don't have a 31mm Nag but with my 41mm Pan I can't see much issue with the image being degraded on stars. A defocused image shows the slight change in the diffraction rings around the center point but at focus the stars are points with no flare. 

Now, maybe at 600 power on Jupiter might show up something, but , not with the seeing conditions I usually get. 

 

Spend enough time on this site and you'll fine that every design has some trade offs. Both due to design and build. 

 

Try this some night. Align the scope the best you can. Observe. Take it apart. Assemble it again a few nights later but don't collimate. Observe. See any difference? At powers you would typically use? In focus? (not with a defocused star image)

I will regularly observe at 400X and will definitely want to go higher.  With the mirrors in this scope, (and the price I paid for it) I can't find a reasonable explanation for why it would seem acceptable for the focuser axis to visibly shift position when  one of the recommended eyepieces is used.

 

If it were a Chinese knockoff, I wouldn't mention it, but I bought an Obsession because of the reputation, and I'm genuinely surprised at what I received.



#24 dustyc

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Posted 13 December 2021 - 07:48 PM

The thing that I puzzle over is that the focuser mounting plate is parallel (or close to) to the force of gravity as the scope is rotated in altitude. If it was perpendicular I could see it bending as the scope was tilted more towards the horizon. The ring and 3 vanes are pretty stout. I think the secondary holder and the single pivot bolt might be the issue. The holder, mirror, and bolt are cantilevered off the spider. Perhaps it sags as the front is tilted down to the horizon.

 



#25 Starman1

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Posted 13 December 2021 - 07:49 PM

The thing that I puzzle over is that the focuser mounting plate is parallel (or close to) to the force of gravity as the scope is rotated in altitude. If it was perpendicular I could see it bending as the scope was tilted more towards the horizon. The ring and 3 vanes are pretty stout. I think the secondary holder and the single pivot bolt might be the issue. The holder, mirror, and bolt are cantilevered off the spider. Perhaps it sags as the front is tilted down to the horizon.

Yes, that occurs with those scopes.




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