?

Thank you for your help.? The previous owner contacted me after I had put the scope to bed last night.? He had the same problem, but he said he moved the mirror up via the collimation bolts such that his eyepieces (ETHOS) would come to focus.? I moved the mirror up with the collimation bolts to the point I could only see three threads of travel left on each.? I then re-collimated first with the Howie Glatter and then dialed it in with a visual Cat's Eye tool set.? This allowed my Nagler 17mm to pass through focus, allowing me to focus that eyepiece.? The seeing and transparency was horrible last night, but the view of Jupiter was very well defined with the Nagler 17.? Stars are well formed, a little comet shaped much closer to the edge.? The mirror combination is exponentially better than the Ostahowski mirror in my UC 18.

I was able to bring a few of the other eyepieces to focus, mostly those above 17mm.? The range of 7-12mm were not able to pass through focus inward, some were close but no banana. ? NONE of the eyepieces would come to focus with my adjustable Paracor.

My favorite eyepieces, Pentax XW series (5mm, 7mm,10mm, and 14mm) will not come to focus at all.

This issue must be corrected.? It is apparent to me that the strut poles are too long.? All of the poles measure the same length plus or minus a 1/32".? The depth of my blocks measures? 2 3/4" each, +/- 1/8".

I am inclined to cut my poles by a half of an inch each.? My non-existent? ATM skills/experience would tell me that this should solve my inward focus issue if I place the mirror in about the middle of the collimation bolt's travel.? What I don't know is what effect the Secondary mirror has on focal length calculations, if any at all.?

I am inclined to believe that either the 45 degree angle and/or just the reflection off of the secondary mirror has an effect on the focal length somehow???

Lets say I get my 17mm Nagler to come to focus with the mirror positioned (and collimated) near the midpoint of travel of the collimation bolts, if I then were to reduce the length of the poles by a 1/2", will this result in an extra 1/2" of inward focus travel at the focuser?? Or is this somehow increased or decreased due to the effect of the Secondary mirror??

v/r
Chuck J.

?

I bought my Obsession 18” UC several years ago after the owner had contracted a serious medical condition while he was waiting to receive the scope from Obsession.? He sold me the scope and had never even been able to look through it.? It was essentially brand new.? I previously owned briefly another brand of “compact” Dobsonian with a 20” mirror.? The optics were excellent on the 20” but the packaging (mount) was worse than a dime store refractor’s tripod.? That is another very long story, but suffice it to say I returned the 20” and about a week later acquired the 18” UC.

?

At this point in my Dobsonian stable I have the new to me 20” Obsession Classic, an 18” Obsession UC, an Orion 12” truss tube goto (which I believe, and many others have conceded to as well, that it has the best 12” mirror they have ever looked through), a 10” Orion Intelliscope, and a fun little 6” Dob.? I have observed with many different dobs at many star parties, one of my observing buddies includes a very accomplished ATM fellow who has recently won the highest ATM award for Newtonian (Dob style) reflectors at Stellafane – something he has accomplished more than once.? I have learned from some very seasoned and experienced observers, and I have looked through some amazing scopes and some not so amazing scopes.? And I will stand by my statement that my Obsession 18” UC Ostahowski mirror is not even very good, I dare say that my 10” Intelliscope has a better view.? The dismal performance of the UC’s optics coupled with its inability of the UC design to remain collimated through a heavy night of observing and/or public outreach led me down the path to a Classic Obsession.? I tried to buy a NEW one from Dave, but they have abandoned the 20” Classic design in favor of the UC design.

?

?

?

The only way your Ostahowski mirror is exponentially worse is if you have it mounted upside down.

?

The diagonal is flat, so there isn’t a multiplying/fudging factor.? As you move a secondary closer to a primary the light cone it intercepts gets bigger.? That’s why you have “planet killer” Newtonians:? with really slow focal ratios you can mount a small secondary and get better contrast.? ?” won’t move you to another secondary size.

?

If it were me, I’d cut ?” off, try the eyepieces and paracorr.? Then I’d make another round cutting, if necessary.? Only because I’ve overcut more than once.?

?

-jeff

?

?

?

Thank you for your help.? The previous owner contacted me after I had put the scope to bed last night.? He had the same problem, but he said he moved the mirror up via the collimation bolts such that his eyepieces (ETHOS) would come to focus.? I moved the mirror up with the collimation bolts to the point I could only see three threads of travel left on each.? I then re-collimated first with the Howie Glatter and then dialed it in with a visual Cat's Eye tool set.? This allowed my Nagler 17mm to pass through focus, allowing me to focus that eyepiece.? The seeing and transparency was horrible last night, but the view of Jupiter was very well defined with the Nagler 17.? Stars are well formed, a little comet shaped much closer to the edge.? The mirror combination is exponentially better than the Ostahowski mirror in my UC 18.

I was able to bring a few of the other eyepieces to focus, mostly those above 17mm.? The range of 7-12mm were not able to pass through focus inward, some were close but no banana. ? NONE of the eyepieces would come to focus with my adjustable Paracor.

My favorite eyepieces, Pentax XW series (5mm, 7mm,10mm, and 14mm) will not come to focus at all.

This issue must be corrected.? It is apparent to me that the strut poles are too long.? All of the poles measure the same length plus or minus a 1/32".? The depth of my blocks measures? 2 3/4" each, +/- 1/8".

I am inclined to cut my poles by a half of an inch each.? My non-existent? ATM skills/experience would tell me that this should solve my inward focus issue if I place the mirror in about the middle of the collimation bolt's travel.? What I don't know is what effect the Secondary mirror has on focal length calculations, if any at all.?

I am inclined to believe that either the 45 degree angle and/or just the reflection off of the secondary mirror has an effect on the focal length somehow???

Lets say I get my 17mm Nagler to come to focus with the mirror positioned (and collimated) near the midpoint of travel of the collimation bolts, if I then were to reduce the length of the poles by a 1/2", will this result in an extra 1/2" of inward focus travel at the focuser?? Or is this somehow increased or decreased due to the effect of the Secondary mirror??

v/r
Chuck J.

?

I bought my Obsession 18” UC several years ago after the owner had contracted a serious medical condition while he was waiting to receive the scope from Obsession.? He sold me the scope and had never even been able to look through it.? It was essentially brand new.? I previously owned briefly another brand of “compact” Dobsonian with a 20” mirror.? The optics were excellent on the 20” but the packaging (mount) was worse than a dime store refractor’s tripod.? That is another very long story, but suffice it to say I returned the 20” and about a week later acquired the 18” UC.

?

At this point in my Dobsonian stable I have the new to me 20” Obsession Classic, an 18” Obsession UC, an Orion 12” truss tube goto (which I believe, and many others have conceded to as well, that it has the best 12” mirror they have ever looked through), a 10” Orion Intelliscope, and a fun little 6” Dob.? I have observed with many different dobs at many star parties, one of my observing buddies includes a very accomplished ATM fellow who has recently won the highest ATM award for Newtonian (Dob style) reflectors at Stellafane – something he has accomplished more than once.? I have learned from some very seasoned and experienced observers, and I have looked through some amazing scopes and some not so amazing scopes.? And I will stand by my statement that my Obsession 18” UC Ostahowski mirror is not even very good, I dare say that my 10” Intelliscope has a better view.? The dismal performance of the UC’s optics coupled with its inability of the UC design to remain collimated through a heavy night of observing and/or public outreach led me down the path to a Classic Obsession.? I tried to buy a NEW one from Dave, but they have abandoned the 20” Classic design in favor of the UC design.

?

?

?

The only way your Ostahowski mirror is exponentially worse is if you have it mounted upside down.

?

The diagonal is flat, so there isn’t a multiplying/fudging factor.? As you move a secondary closer to a primary the light cone it intercepts gets bigger.? That’s why you have “planet killer” Newtonians:? with really slow focal ratios you can mount a small secondary and get better contrast.? ?” won’t move you to another secondary size.

?

If it were me, I’d cut ?” off, try the eyepieces and paracorr.? Then I’d make another round cutting, if necessary.? Only because I’ve overcut more than once.?

?

-jeff

?

?

?

Thank you for your help.? The previous owner contacted me after I had put the scope to bed last night.? He had the same problem, but he said he moved the mirror up via the collimation bolts such that his eyepieces (ETHOS) would come to focus.? I moved the mirror up with the collimation bolts to the point I could only see three threads of travel left on each.? I then re-collimated first with the Howie Glatter and then dialed it in with a visual Cat's Eye tool set.? This allowed my Nagler 17mm to pass through focus, allowing me to focus that eyepiece.? The seeing and transparency was horrible last night, but the view of Jupiter was very well defined with the Nagler 17.? Stars are well formed, a little comet shaped much closer to the edge.? The mirror combination is exponentially better than the Ostahowski mirror in my UC 18.

I was able to bring a few of the other eyepieces to focus, mostly those above 17mm.? The range of 7-12mm were not able to pass through focus inward, some were close but no banana. ? NONE of the eyepieces would come to focus with my adjustable Paracor.

My favorite eyepieces, Pentax XW series (5mm, 7mm,10mm, and 14mm) will not come to focus at all.

This issue must be corrected.? It is apparent to me that the strut poles are too long.? All of the poles measure the same length plus or minus a 1/32".? The depth of my blocks measures? 2 3/4" each, +/- 1/8".

I am inclined to cut my poles by a half of an inch each.? My non-existent? ATM skills/experience would tell me that this should solve my inward focus issue if I place the mirror in about the middle of the collimation bolt's travel.? What I don't know is what effect the Secondary mirror has on focal length calculations, if any at all.?

I am inclined to believe that either the 45 degree angle and/or just the reflection off of the secondary mirror has an effect on the focal length somehow???

Lets say I get my 17mm Nagler to come to focus with the mirror positioned (and collimated) near the midpoint of travel of the collimation bolts, if I then were to reduce the length of the poles by a 1/2", will this result in an extra 1/2" of inward focus travel at the focuser?? Or is this somehow increased or decreased due to the effect of the Secondary mirror??

v/r
Chuck J.