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Setting up the GSO coma corrector

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#51 Jack Day

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Posted 18 June 2015 - 12:12 PM

Hi Don & John,

 

Thanks for the replies.  One other issue I also was concerned about was it would be somewhat difficult to attach a filter-slide with this setup. I really like the idea of the SIPS style setup, but in the end may just go the route of adding a 1" spacer between the barrel of the coma corrector and the top of the coma corrector to achieve the proper 75mm +/- spacing.  

 

So if I understand correctly:

 

1. Find the eyepiece which requires the most inward focus. In this case I plan to borrow a 31 Nagler as I hope I recall correctly they require a lot of in-focus compared to most eyepieces. 

 

2. Put coma corrector with 31 Nagler into the focuser.  Now optimize spacing between eyepiece and CC as needed. In the end make sure that there is still ~15 - 20 mm of additional in-focus available.

 

3. With the CC now optimized and focus set for the 31 Nagler, leave the focuser alone and find proper focus for each additional eyepiece by moving them into, or out from, the top of the corrector.  Use collars to set the now para-focalized eyepieces.

 

4. Grab beverage of choice and look through telescope.

 

Did I miss anything?

 

Thanks!

 

Jack



#52 Starman1

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Posted 18 June 2015 - 12:56 PM

Hi Don & John,

 

Thanks for the replies.  One other issue I also was concerned about was it would be somewhat difficult to attach a filter-slide with this setup. I really like the idea of the SIPS style setup, but in the end may just go the route of adding a 1" spacer between the barrel of the coma corrector and the top of the coma corrector to achieve the proper 75mm +/- spacing.  

 

So if I understand correctly:

 

1. Find the eyepiece which requires the most inward focus. In this case I plan to borrow a 31 Nagler as I hope I recall correctly they require a lot of in-focus compared to most eyepieces. 

 

2. Put coma corrector with 31 Nagler into the focuser.  Now optimize spacing between eyepiece and CC as needed. In the end make sure that there is still ~15 - 20 mm of additional in-focus available.

 

3. With the CC now optimized and focus set for the 31 Nagler, leave the focuser alone and find proper focus for each additional eyepiece by moving them into, or out from, the top of the corrector.  Use collars to set the now para-focalized eyepieces.

 

4. Grab beverage of choice and look through telescope.

 

Did I miss anything?

 

Thanks!

 

Jack

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#53 Oberon

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Posted 19 June 2015 - 12:11 AM

So what if....   

 

Let's pretend for a minute that we were building a new upper cage for a 17.5" f/4.5 scope.  What would prevent you from isolating the barrel (optics package) of the coma corrector from the focuser itself?

 

My thought was this: Suspend a 58 to 48mm filter adapter about 1/2 inch below the focuser board using some spacers mounted to the flange of the filter adapter.  To this you would screw on the optics package of the coma corrector just as if you were screwing it onto a 48mm filter.  The optics of the coma corrector are now fixed to the focuser board passing up through the board into the bottom of the focuser.  

 

Now the focuser can move up and down, as if it were a tunable top.  Add the 19mm barrel extension to the top of the coma corrector so it can be inserted into the focuser, this adds spacing needed to achieve the proper working spacing (73.5mm) between the eyepiece and coma corrector optics. 

This is more or less exactly what I am doing for my Sterope binoscope project. The decision is made easier because I cannot simply use a standard focuser arrangement anyway due to IPD constraints, so don't have to worry about how to slip the focuser barrel over the CC. However, like my Merope telescope, my truss system is simple to adjust and will provide a range of 100mm focus which is ideal for initial set up of correct focus for any CC, and even occasional tweaks (but not for routine focus changes as it also affects collimation).

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#54 scpanish

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Posted 24 August 2015 - 08:03 PM

Guys,

 

Thanks for the very, very informative thread.  I am considering a CC for my 10" F4.7 Skywatcher Dob because the coma drives me nuts at lower powers. I'm mostly using Celestron (Synta) 66 deg eyepieces.  The GSO is attractive price wise, and from the discussion, the setup algorithm is possible if annoying, although I'd likely never have figured it out on my own.  And one needs to find the additional focuser and spacers ($$$).

 

However, I noticed on Don's collimation essay on EyepiecesEtc.com that a CC makes collimation much more critical.  So, how practical are these things, really?  I use a collimation cap and (in the light) pretty much nail it before observing, but that won't work in the dark in the middle of a session.  Collimation on that collapsible Dob does go out when the tube is horizontal, otherwise, it seems to hold (still sharpest center of field).  If the CC exacerbates the variance, can the viewing be expected to degrade during a session?

 

So, I guess I'm asking for a value judgement on whether these CCs are worth the associated pain.

 

thanks,

 

Steve



#55 Starman1

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Posted 25 August 2015 - 12:39 AM

They are.  Star images in the outer 50-60% of the field begin to resemble those at the center.

 

Careful collimation probably already puts you within tolerance for use of a coma corrector.

The CC makes the primary collimation tolerance about 1/6 of what it is without a CC.

Figure, in your scope, about +/- 1mm.  That's not too hard to achieve with a decent cheshire or barlowed laser.

A collimation cap, followed by star collimation, might be OK.

I would seriously think about some better tools.

For centering the secondary under the focuser: a sight tube or combination tool

For aligning the focuser axis to the primary's center (adjusting secondary tilt): a sight tube or a collimated laser

For aligning a primary (what the collimation cap is used for): a cheshire or a barlowed laser or a combination tool or a Krupa collimator.

For eliminating residual errors and dialing in the focuser axis perfectly: an autocollimator.

 

You could get by with a 1.25" combination sight tube/cheshire tool and an aligning adapter like the Glatter Parallizer or one of the various collet styles.

Or you could go the whole hog and get a Catseye XLKP 3-piece tools set.

In between those extremes something like a Parallizer + 1.25" laser + barlow attachment.

There are many ways to collimate, but you do need something a little more advanced than a collimation cap since that tool isn't intended to help align the secondary.

.


Edited by Starman1, 25 August 2015 - 09:17 AM.

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#56 precaud

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Posted 25 August 2015 - 07:46 AM

Absolutely worth it. "Pain" is a bit of an exaggeration, as my fast dobs need little if any minor tweaking at each setup.

 

In between those extremes something like a Parallizer + 1.25" laser + barlow attachment.

 

Or Parallizer + 1.25" laser + a good 1.25" Cheshire, such as those made by Catseye or Farpoint. Also make sure your center spot is accurately placed, many if not most of them are misplaced by more than 1mm...


Edited by precaud, 25 August 2015 - 07:49 AM.


#57 pal77

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Posted 18 September 2015 - 04:13 PM

I purchased a GSO coma corrector and used the instructions in this thread to set it up with my 14" f/4.15 Dob. I am extremely happy with the results. Stars are sharp out to the extreme edges of my 20mm ES100° eyepiece. For my scope this eyepiece requires the most in travel and so I used it as the reference eyepiece. After testing I found that a 1" Blue Fireball extension provided the optimum amount of spacing for this eyepiece in my scope. However having just 2mm too much extension or too little extension makes the star images at the edge noticeably worse. For my other eyepieces I use parfocalizing rings and not having to refocus when changing eyepieces is very nice. I now leave it in my focuser all of the time, even for high magnification as it seems to improve planetary views as well.

 

For anyone looking for a lower cost alternative to the Paracorr, I think this is it. My total cost was $120 + $21 + $18 (CC + spacer + rings) = $159.


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#58 precaud

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Posted 18 September 2015 - 06:30 PM

However having just 2mm too much extension or too little extension makes the star images at the edge noticeably worse.

 

Congrats on your good results. My experience is the same as yours - the CC placement is not as forgiving as others have described. And when it's right - it's very good.



#59 JJack

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Posted 06 December 2015 - 02:53 AM

Just ordered mine; I'm woefully un-handy so we'll see if I can get it done.  Also got the spacer that was recommended; still not sure if that's all I need or if I need some type of adjustment for my eyepiece.



#60 Starman1

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Posted 06 December 2015 - 11:18 AM

Every eyepiece will need to be optimized.

You only have to optimize one eyepiece first, then all the others follow.

See post #40 for what to do to optimize all your eyepieces.



#61 JJack

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Posted 10 December 2015 - 01:04 PM

Okay, if I can do it, everyone can!

 

Got the CC yesterday.  I had a very brief time before the clouds set in, so I only had time to adjust one eyepiece, the 30mm 2-inch GSO that came with my scope. 

 

1. Entire process:

Unscrewed the corrector, screwed in the 19mm Blue Fireball extension ($16) and reassembled.  Put the EP into the corrector.

With the EP all the way it it already looked better.  

Fiddled with it a bit and finally settled on pulling the EP out maybe 5mm from the corrector.

Done.

 

 

Focusing on Plaiedes, there's still some irregularity around the fringe, but please understand that this EP normally has coma for half the field.

 

I will probably just mark the best position on the EP with a Sharpie rather than spend money on a parfocalizing ring.  It's locked in very tightly.

 

 

Total cost: $120

Thanks everyone.


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#62 precaud

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Posted 10 December 2015 - 01:34 PM

That sounds way too easy, you must have done something right  :)

 

Chances are pretty good that you can find a 2" I.D. x 5mm thick flat washer in the plumbing department of a local store... Ace keeps a good selection on hand.


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#63 Starman1

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Posted 10 December 2015 - 02:03 PM

Okay, if I can do it, everyone can!

 

Got the CC yesterday.  I had a very brief time before the clouds set in, so I only had time to adjust one eyepiece, the 30mm 2-inch GSO that came with my scope. 

 

1. Entire process:

Unscrewed the corrector, screwed in the 19mm Blue Fireball extension ($16) and reassembled.  Put the EP into the corrector.

With the EP all the way it it already looked better.  

Fiddled with it a bit and finally settled on pulling the EP out maybe 5mm from the corrector.

Done.

 

 

Focusing on Plaiedes, there's still some irregularity around the fringe, but please understand that this EP normally has coma for half the field.

 

I will probably just mark the best position on the EP with a Sharpie rather than spend money on a parfocalizing ring.  It's locked in very tightly.

 

 

Total cost: $120

Thanks everyone.

Now that you have optimized that eyepiece, all your other eyepieces will follow.  Without moving the focuser, when your 30mm is in focus in its optimum position, simply replace it with one of your other eyepieces and pull it out of the corrector until it's in focus.  When each eyepiece is at its optimum position, it will be parfocal with the 30mm--even if a 1.25" adapter is used.


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#64 jtsenghas

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Posted 11 December 2015 - 10:25 AM

A compromise between relying on a Sharpie mark and buying parfocalizing rings would be to put some O rings on the eyepieces to approximately parfocalize them.

An O ring with an inside diameter of just under 2" (or 1 ¼") will fit tightly on an eyepiece, and a stack of several could reside permanently on each. Although these may be pricey bought individually from the drawers at a local hardware store, packets of larger counts can be ordered and keep you permanently in stock.

Some advantages of this method is that they won't ever scratch your barrels, they can be added and removed without tools, and they don't interfere at all with storage in an eyepiece case.

The only downside I see is that you are limited to increments of the ring thickness, but this could be little over a millimeter, and tiny focus adjustments should be no problem for coma corrector distances.

Edited by jtsenghas, 11 December 2015 - 10:29 AM.

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#65 jtsenghas

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Posted 11 December 2015 - 12:07 PM

On a related note, I'm considering experimenting with homemade parfocalizing rings parted off from PVC pipe on my wood lather to make using various eyepieces with my Paracorr easier.  I'm thinking of slicing some 1 1/4" PVC pipe to the precise lengths I've determined I want for all of my 1 1/4" eyepieces, then slit about 1/4" out of each of them, binding them tight and heating them so that they are a couple of millimeters undersized in the relaxed configuration. 

 

This way they will grip the barrels enough to avoid slipping off without requiring set screws and can be dropped right into the focuser drawtube.  Since all will register against the shoulder of the eyepiece and the top of the focuser they needn't be attached tightly.  My supposedly parfocal Celestron  X-Cel LX eyepieces actually vary more than a millimeter for focus, so this should clean that variance up. My other eyepieces all require more out-focus, so they'll get thicker rings.


Edited by jtsenghas, 11 December 2015 - 12:14 PM.


#66 scpanish

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Posted 08 January 2016 - 12:46 PM

Hi Guys,

 

Back to this thread.  I bought the GSO CC and Blue Fireball spacer.  (And with a star alignment, my collimation looks really good...)

 

But....on my SkyWatcher collapsible 10" Dob, the stock drawtube/nosepiece/2" eyepiece holder chain does not allow the CC to enter the drawtube,  so I cannot achieve focus.  The drawtube is internally threaded, the nosepiece screws inside the drawtube, and this threaded region is resultingly internally constricted, blocking the CC (or any 2" eyepiece) from actually slipping inside the drawtube.  The drawtube itself is internally just over the dimension of the CC and would accept it if there was a different arrangement for attaching the nosepiece (my use of this term may be incorrect).  I removed the nosepiece and inserted the CC directly into the drawtube and achieved focus - and the image looked good.  And, yeah, I'm not willing to just hold the CC/eyepiece in while I observe....it's kind of heavy....

 

What would work  would be an alternative nosepiece that slipped onto the outside of the drawtube (OD=2.206") and tightened with setscrews, leaving an unobstructed inner diameter that would pass the CC.

 

Can't find one....do you know of any such?  Or am I just doing something stupid?  I can send pictures if the description is inadequate.

 

Any other alternatives? I guess I could also likely find a replacement focuser for less cost than having the alternative nosepiece machined.

 

best,

 

Steve



#67 Starman1

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Posted 08 January 2016 - 01:01 PM

Scopestuff.com sells a replacement top collar for the focuser which is 2" in I.D.

 

However, this focuser is designed to be used with the 2" adapter inserted.

If you just replace the collar, this may move everything in too far to get to focus.

 

The easiest answer is to enlarge the hole in the threaded collar to 2" so that a 2" accessory inserted into the 2" adapter then bypasses the collar.

The I.D. of the collar is 1-15/16", so not much has to be removed.  You can use sand paper or emory paper to sand out the interior to a 2" diameter.

It doesn't even have to be round as long as the 2" accessory passes through the collar.

 

The company I used to work for lobbied to have SkyWatcher fix this several years ago.  I'm sorry to hear they still haven't fixed it.

There have been many threads on all the forum sites about this for several years.

This "defect" affects the use of 2" barlows as well.



#68 scpanish

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Posted 08 January 2016 - 09:39 PM

Thanks Don.  You know your stuff!  I was worried that reducing the ID in  the threaded area of the collar would result in too  little integrity to support what could be a couple pounds of CC + adapters+eyepiece, but I'll revisit it, and look at the ScopeStuff site to see how their collar works.  

 

BTW, Skywatcher apparently has fixed the problem.  I called their USA branch and the new 2" focusers do not have the issue.  However, the tech I spoke to was unfamiliar with the older stock and didn't know if there was part compatibility between new and old units, such as replacing the old drawtube with the new design.  I purchased my scope used last spring - not sure how old it is.  Wish I could see a new one, but there are no dealers nearby.

 

I'll post my results when done.

 

best,

 

Steve



#69 JJack

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Posted 15 January 2016 - 02:50 PM

Well I've now apparently gotten used to my GSO CC; I took it out of the focuser last night and said "yikes" when I saw the view without it.


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#70 scpanish

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Posted 17 January 2016 - 11:20 PM

Well, wish I could say I felt the same way!   I'll report on the installation with my (older) SkyWatcher 10" Dob, this may be useful to others with the same scope.  This is somewhat preliminary information because it has been miserably cloudy here in New Hampshire for a couple weeks, and I've only been able to use the setup once.

 

The SkyWatcher uses an inexpensive single-speed Crayford focuser, looks just like the cheapest Orion, likely made by Synta.  A collar screws onto the  internally-threaded downtube.  The scope is supplied with 1.25" and 2" eyepiece holders that fit the collar.  The collar adds 1/2" to the drawtube, the eyepiece holders add another 1 3/8".

 

The GSO CC, with 3/4" extension tube, initially could not be fully inserted into the 2" eyepiece holder.  It was blocked by the collar ID, which in these older scopes had an internal diameter of 1 15/16".  Apparently the new SkyWatchers don't have this problem (no response from SkyWatcher Canada as to how resolve this issue on older scopes like mine).  I followed Don's advice to open up the collar ID as needed.  I used a Dremel with sanding drums for the bulk removal and finished with 220, 440 and 1200 grit emery paper.  The CC now goes in all the way, but it is a tight fit to the inside of the drawtube, and scrapes slightly on insertion.  

 

The CC's eyepiece holder is now fitted with a standard 2" to 1.25" adapter to hold the eyepieces.  The adapter has a compression ring but isn't a Glatter Parallelizer or other fancy centering adapter.  With the CC in place, using a collimation cap, the collimation appears to still be good, with the spot dead center in the mirror's centering ring.

 

However, the CC just barely has enough in-focus with this arrangement.  Using my 20 and 25mm eyepieces, focus is achieved only with the focuser bottomed out.  No focus margin at all.  My 12.5mm illuminated crosshair eyepiece, which needs lots of in-focus, can't achieve focus at all through the CC.  My 15mm eyepiece is in the correct focus range.  An additional complication is that the weight of the CC, adapters, and eyepiece is too much for the focuser.  When bottomed, the focus knobs won't move the drawtube.  You have to help it along by pulling up on the CC.  I can likely adjust the focuser to resolve this, but haven't looked into how to do that yet.  Once the drawtube is up a bit, the focuser works again.

 

Don pointed out that ScopeStuff sells a screw on 2" eyepiece holder for this focuser ($39), which replaces the collar.  That may be a better solution when using the CC exclusively.  It would provide more in-focus than the above arrangement, which would be useful.  However, it might have too much in-focus when not using the CC, and switching between the two setups would be awkward, especially at 15 or 20 deg. F.  Those threads are pretty fine.

 

So, how is the viewing?  Well, not so good, but that may well be a collimation issue, or due to my inexpensive eyepieces.  Stars that are dead center have some distortion or astigmatism with the CC, but are pinpoint without.   Stars in the periphery are indeed sharper with the CC, but still not pinpoint, and still seem to show either coma or astigmatism.   But - I'm not blaming the CC, since many users report excellent results with it.   As Don pointed out, I really should use some better collimation tools, and I have not checked to ensure that the collimation donut on the mirror is precisely centered.  That said, using the collimation cap and looking through the CC, collimation still looks perfect.

 

I haven't decided what to do next.  A new focuser would help, tools would help, and a really good eyepiece would provide a sanity check.  I'm thinking about it and hoping for clear skies.

 

best,

 

Steve



#71 precaud

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Posted 17 January 2016 - 11:49 PM

Steve, it's pretty clear your problems are not so much with the coma corrector or collimation, but with the focuser. If stars still show coma, you've not yet found the proper CC placement.



#72 Starman1

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Posted 18 January 2016 - 01:48 AM

If the simple new collar from Scopestuff would solve the CC dilemma, I'd do it.

if you're worried about having the necessary out focus without the adapter, there are 1.25" adapters that are taller than most (e.g. TeleVue Hi-Hat and others)

that could give enough out travel for 1.25" eyepieces.  And, if not, there are extension tubes to insert in the focuser that add about as much out focus as the original adapter, only they are 2" in I.D. so would still need the 1.25" adapter.

So, with a normal 1.25" adapter, an extension tube, and the new threaded-on ring, you're set.

 

But, if the Sky Watcher is the typical collapsible type, all of this is somewhat moot.

Because all you have to do for more in-travel is to lower the poles (by equal amounts of course), and tighten the scope down, then recollimate, and you have no issues with in-travel on the focuser.

 

You do need to learn how to adjust it, of course.  You want it to be able to lift the necessary weight.



#73 precaud

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Posted 18 January 2016 - 09:21 AM

if you're worried about having the necessary out focus without the adapter, there are 1.25" adapters that are taller than most (e.g. TeleVue Hi-Hat and others)

that could give enough out travel for 1.25" eyepieces.  And, if not, there are extension tubes to insert in the focuser that add about as much out focus as the original adapter, only they are 2" in I.D. so would still need the 1.25" adapter.

 

Or, you could just use one of the 2" to 1.25" helical focus adapters, such as the Orion unit mentioned earlier in this thread, which gives added height *and* adjustability...

An alternate to the Orion unit is a generic 2" to T2 male adapter coupled with one of the Baader #2458125 T2 to helical microfocuser adapters. This arrangement has a taller minimum height and smaller range of adjustment (6mm) but smoother feel, FWIW... I have both and prefer the Orion's larger range.



#74 jtsenghas

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Posted 18 January 2016 - 10:01 AM

Might you be able to gain a few precious millimeters by adjusting your primary mirror upwards a little, even if this means buying a few longer screws of matching pitch? You might just be able to bring all of your eyepieces into focus.



#75 scpanish

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Posted 18 January 2016 - 11:19 AM

 Thanks guys.  Currently there are too many variables.  First, I want to look at the Ceragioli posts to better understand how the CC works and is optimally positioned with respect to the primary's focal plane.   Then I can lock the CC  position with the focuser and pull the 15mm eyepiece in and out to experiment with the CC-to-eyepiece spacing.  If I can get good performance with that eyepiece I can hopefully figure out what the problem is.  I do need more in-focus, but I don't think that is causing the apparent aberrations, since with that 15mm eyepiece at the 75mm distance from the CC top, and optimal focus, I still don't get acceptable image quality, and the center image is degraded compared to w/o the CC.  I am still not convinced my collimation is adequate, since **if** the primary's center circle is off, so is my collimation.  I need a positioning template to check that.

 

This is a collapsible Dob and, as Don says, I can partially collapse it to get more in-focus.  I normally leave it extended, so I could (perhaps) improvise some lock/positioning devices to clamp onto the tubes to make the in-focus position repeatable.  That spares me the cost of all the adapters, extenders, etc.   A better focuser would be more of a priority, and I do intend to get a wider field, high quality eyepiece (something significantly better than the 66 deg Celestron, and with better eye relief) once I can lick the coma/aberration issues.

 

Then I just need some clear skies.  Truly a poopy winter so far - not enough snow (skiing), not enough sun (solar house), and hardly any good viewing nights.  There is a cell tower I can target that is about 30 miles away, but it doesn't show the aberrations that show up clearly when viewing a field of stars.

 

best,

 

Steve




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