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On the Care and Feeding of a Large Dob (Advice for new 18"?)

Dob DSO Reflector Equipment
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#1 Dobserver

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Posted 26 February 2021 - 01:47 AM

Hello all. I recently purchased this 18" "Obsession Clone" on Cloudy Nights. It's my dream scope, and a big upgrade from my 12" (had to drive to Phoenix from LA to get it). I bought it from the second owner, who said the original owner constructed it in the 90s. I have Bortle 9 skies, so it's mostly been lunar viewing from my home. I've gotten to take it to a Bortle 2 and a 3 site since I've had it (2.5 and 3.5 hour drives). The views of DSOs out there are phenomenal, but I won't get into that, since this is not an observing forum.

 

I would like to ask the community for any advice you might have, things to watch out for, mistakes I might make, and recommendations you might have. For instance, I caught it slowly dipping downward at my dark site when I had a heavy eyepiece in it and was pointing it low (~25° above the horizon). I think I narrowly avoided disaster on that one! Simply put, this is a lot of scope, and it makes me feel like I could make a wrong move and cause severe damage. I already damaged the rocker box by trying to load it into a van by rolling it up a wheelchair ramp, when "something got caught on something" and somehow damaged the wood. 

 

As for equipment/accesories, what do you suggest I add? I have a 35mm Panoptic, a 38mm Q70, a 24mm ES 82°, a 14mm ES 82°, and a 7mm Delite, plus a 2" Barlow. I got a Glatter collimator with purchase of the scope (love it!).

 

I have a few filters (variable polarizing moon filter, 2" Baader Contrast Booster (love it), 2" Orion Ultrablock (don't care for it!), and just got a DGM NPB filter (haven't used it yet). At f/4.5, I do notice some coma, especially at lower powers, but it's not too bad. Do you recommend a coma corrector, and if you do which one do you use/like? I haven't been able to get the NGC MAX to work, as the encoders seem to have trouble working... That's something I need to work on. A light shroud is coming this week.

 

Thank you in advance to anyone who has advice. I haven't posted much on CN, but I am an avid reader of your forums!

20210127_110439_Fotor.jpg


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#2 havasman

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Posted 26 February 2021 - 02:36 AM

Nice looking scope.

It's your forum too!

At f4.5 with all that aperture, yeah a coma corrector is a major asset. The Astro-Tech (GSO) coma corrector is optimized for f4.5 and works. The Paracorr 2 is more precise and adaptable to different eyepieces.

I had a buddy back in the day that was a guitar teacher and all these rich kids with their new Martin D35's came in and had cloths they'd prop their guitars on against the wall. He's hand 'em a 5' cord, tell 'em to tie it around the neck and drag it around a day or two to work the precious out of them. Of course they didn't but a hack or 2 in the wood's not going to hurt your scope.

Glatter collimation tools -  waytogo.gif 

DGM NPB filter is a good one.

Looks like you're going to have some FUN!


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#3 ngc7319_20

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Posted 26 February 2021 - 02:56 AM

Hello all. I recently purchased this 18" "Obsession Clone" on Cloudy Nights. It's my dream scope, and a big upgrade from my 12" (had to drive to Phoenix from LA to get it). I bought it from the second owner, who said the original owner constructed it in the 90s. I have Bortle 9 skies, so it's mostly been lunar viewing from my home. I've gotten to take it to a Bortle 2 and a 3 site since I've had it (2.5 and 3.5 hour drives). The views of DSOs out there are phenomenal, but I won't get into that, since this is not an observing forum.

 

I would like to ask the community for any advice you might have, things to watch out for, mistakes I might make, and recommendations you might have. For instance, I caught it slowly dipping downward at my dark site when I had a heavy eyepiece in it and was pointing it low (~25° above the horizon). I think I narrowly avoided disaster on that one! Simply put, this is a lot of scope, and it makes me feel like I could make a wrong move and cause severe damage. I already damaged the rocker box by trying to load it into a van by rolling it up a wheelchair ramp, when "something got caught on something" and somehow damaged the wood. 

 

As for equipment/accesories, what do you suggest I add? I have a 35mm Panoptic, a 38mm Q70, a 24mm ES 82°, a 14mm ES 82°, and a 7mm Delite, plus a 2" Barlow. I got a Glatter collimator with purchase of the scope (love it!).

 

I have a few filters (variable polarizing moon filter, 2" Baader Contrast Booster (love it), 2" Orion Ultrablock (don't care for it!), and just got a DGM NPB filter (haven't used it yet). At f/4.5, I do notice some coma, especially at lower powers, but it's not too bad. Do you recommend a coma corrector, and if you do which one do you use/like? I haven't been able to get the NGC MAX to work, as the encoders seem to have trouble working... That's something I need to work on. A light shroud is coming this week.

 

Thank you in advance to anyone who has advice. I haven't posted much on CN, but I am an avid reader of your forums!

attachicon.gif20210127_110439_Fotor.jpg

Beautiful scope!!!

 

Weights: yeah get some weight you can hang on the back of the scope when using the big eyepieces.   Could be one of these foam covered exercise hand weights you strap in the back somewhere.  Or Velcro ankle weighs.  Or a water bottle.  Or magnetic weights.  Or some Velcro deal.  I've probably tried it all at one time or another.

 

Wheel chair ramps: yup I've done the same thing to my Obsession 20.  The wheels drop down into the vehicle interior, and then the scope scrapes on the hatch threshold.  I built a more gentle transition for the wheels into the vehicle.   Or cut some 2 x 8 boards and let the wheels run on those two feet into the vehicle.   Gotta keep the wheels high until the scope clears the threshold.

 

Get a scope cover in case the weather takes a sudden change.  The TeleGizmos covers are good, especially the 365 models.  AstroSystems Scope Coat is good too (thinner and light weight).   If you cover the scope in the field, try to set it horizonal so it doesn't blow over.   Have some long straps handy to tie the front of the scope down to something heavy in case of high winds.  Dark sites often have changeable / bad weather.  Try to aim the scope down wind for storage.  Vertical storage doesn't play well with high winds.

 

Sometimes I put a 12' x 12' tarp under the scope -- cuts down on mud / dust.  Can anchor it with camping stakes or big nails.

 

Always keep the primary mirror covered during daytime.  Don't want any spontaneous scope combustion.

 

Eyepieces: Maybe fill the gap around 10mm.  I like that one for galaxy close-ups.

 

Filters: Get an 2" OIII filter for stuff like Veil Nebula.  I like Astronomik.

 

Coma Corrector: yes get one.  I like TeleVue ParaCorr 2.  Others might have different favorite.

 

Encoders: on mine the NGC encoder shafts fit into O-rings set into holes on the scope.  But the O-rings were dried out and shot.  So the encoder shafts only turned intermittently or not at all.

 

Truss pole clamps: go easy tightening them.  Seems like every couple months someone on here busts an Obsession clamp and is looking for a replacement.

 

Dew removal: sooner or later you will probably want a heater on the secondary mirror.  AstroSystems is the one to get.  Get some heaters for eyepieces / finder / Telrad also.  I guess CA is dryer than MD... might not use heaters it too often in CA.  One of these 12v heat guns will be handy in a pinch.


Edited by ngc7319_20, 26 February 2021 - 03:12 AM.

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#4 CHASLX200

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Posted 26 February 2021 - 07:17 AM

Make sure the mirror is not resting on the stop bumps and check the sling. Also make sure the 2ndary holder is placed right.  Collimation at F/4.5 needs to be dead on. I have the same size scope i just bought and needed to do a bit to get things right.


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#5 Voyager 3

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Posted 26 February 2021 - 07:45 AM

Nice looking scope.

It's your forum too!

At f4.5 with all that aperture, yeah a coma corrector is a major asset. The Astro-Tech (GSO) coma corrector is optimized for f4.5 and works. The Paracorr 2 is more precise and adaptable to different eyepieces.

I had a buddy back in the day that was a guitar teacher and all these rich kids with their new Martin D35's came in and had cloths they'd prop their guitars on against the wall. He's hand 'em a 5' cord, tell 'em to tie it around the neck and drag it around a day or two to work the precious out of them. Of course they didn't but a hack or 2 in the wood's not going to hurt your scope.

Glatter collimation tools -  waytogo.gif 

DGM NPB filter is a good one.

Looks like you're going to have some FUN!

+1 

 

 

 I am an avid reader of OUR forums!


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#6 bbasiaga

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Posted 26 February 2021 - 11:35 AM

As far as eyepieces, search around online and you can find a calculator or downloadable spreadsheet where you will enter your scope information and eyepiece information and then output the field of view and power for each combo.  Generally its good to have something around 1degre FOV, and 0.5 degrees.  With those two, you'll be set for most things.  You may choose to add a wider FOV eyepiece that maximizes the exit pupil at around 7 or 8mm, if your 1degree isn't already doing that.  You may also choose to add something higher power for planets or small things.  You have some nice stuff already, so just see where it sits in this continuum.  

 

I'd also add an O3 filter to the list. 

 

I think coma correction is a very personal thing.  I don't often use one with my 22" F4.  But some people can really notice the difference below F5.  Paracor 2 is the gold standard if you go that way. 

 

The scope is less fragile than you think.  Check and make sure there are tabs over the top of the mirror that prevent it from tipping off the supports when pointing low.  If they aren't there, add them.  Besides that, you can check how low the scope can point before the front of the bearing comes off the teflon pad.  With the mirror retainers in place, that will become your lower limit. 

-Brian 


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#7 Dobserver

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Posted 26 February 2021 - 01:04 PM

"The Astro-Tech (GSO) coma corrector is optimized for f4.5 and works." Thank you havasman, I didn't know about this one; it is certainly much more affordable the the Televue. 

 

NGC7319_20: "Weights: yeah get some weight you can hang on the back of the scope when using the big eyepieces.   Could be one of these foam covered exercise hand weights you strap in the back somewhere.  Or Velcro ankle weighs.  Or a water bottle.  Or magnetic weights.  Or some Velcro deal.  I've probably tried it all at one time or another" Great suggestions, thank you. 

"Wheel chair ramps: yup I've done the same thing to my Obsession 20. Gotta keep the wheels high until the scope clears the threshold." Yes! That's what happened. Oh, the pain...

"The TeleGizmos covers are good, especially the 365 models.  AstroSystems Scope Coat is good too (thinner and light weight)". Nice. So Cal is a very dusty place; I will look into these. Also want a tarp.

 

CHASLX200: "Make sure the mirror is not resting on the stop bumps and check the sling. Also make sure the 2ndary holder is placed right" Yeah, what's with the sling? That is not something I have seen before, and I'm not sure I understand its purpose or where it's supposed to go. As for stop bumps, what are these? I can't find any info on this topic on google. I can tell something's not quite right with the position of the mirror, and collimation can be a struggle, because the bottom screw needs to be very tight... I love the secondary and once I figured out how it worked have not had any problems. 

 

bbasiaga: "As far as eyepieces, search around online and you can find a calculator or downloadable spreadsheet where you will enter your scope information and eyepiece information and then output the field of view and power for each combo.  Generally its good to have something around 1degre FOV, and 0.5 degrees.  With those two, you'll be set for most things.  You may choose to add a wider FOV eyepiece that maximizes the exit pupil at around 7 or 8mm, if your 1degree isn't already doing that.  You may also choose to add something higher power for planets or small things.  You have some nice stuff already, so just see where it sits in this continuum. 

I'd also add an O3 filter to the list."

Thank you, Brian. Yes, I was thinking of getting a 9mm 100° eyepiece (probably APM), which would give almost half a degree FOV and a bit over 200X, which should fill a nice niche. I know longer focal length hyper-wide eyepieces on fast scopes can be problematic, but I'm assuming I should be ok with 9mm (tell me if I'm wrong).  Many people are recommending an OIII; thank you, it will go on the list. 

 

Much appreciated guys!



#8 bbbriggs

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Posted 26 February 2021 - 10:33 PM

1) Balance: I spent the time to precision balance the counterweight for the heaviest case and put a magnet on the top ring opposite the focuser where I would stick a weight to balance the lighter stuff, a 2.5# barbell weight.

2) Convenience: moved the collimating to the top of the mirror box with some belts, rods and pulleys. No more squatting and reaching under the scope while craning my neck to see the back of the laser.

3) Best accessory purchase ever: Denkmeier binoviewer with power X switch.

You might also want to make an off axis mask.


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#9 CHASLX200

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Posted 27 February 2021 - 06:31 AM

"The Astro-Tech (GSO) coma corrector is optimized for f4.5 and works." Thank you havasman, I didn't know about this one; it is certainly much more affordable the the Televue. 

 

NGC7319_20: "Weights: yeah get some weight you can hang on the back of the scope when using the big eyepieces.   Could be one of these foam covered exercise hand weights you strap in the back somewhere.  Or Velcro ankle weighs.  Or a water bottle.  Or magnetic weights.  Or some Velcro deal.  I've probably tried it all at one time or another" Great suggestions, thank you. 

"Wheel chair ramps: yup I've done the same thing to my Obsession 20. Gotta keep the wheels high until the scope clears the threshold." Yes! That's what happened. Oh, the pain...

"The TeleGizmos covers are good, especially the 365 models.  AstroSystems Scope Coat is good too (thinner and light weight)". Nice. So Cal is a very dusty place; I will look into these. Also want a tarp.

 

CHASLX200: "Make sure the mirror is not resting on the stop bumps and check the sling. Also make sure the 2ndary holder is placed right" Yeah, what's with the sling? That is not something I have seen before, and I'm not sure I understand its purpose or where it's supposed to go. As for stop bumps, what are these? I can't find any info on this topic on google. I can tell something's not quite right with the position of the mirror, and collimation can be a struggle, because the bottom screw needs to be very tight... I love the secondary and once I figured out how it worked have not had any problems. 

 

bbasiaga: "As far as eyepieces, search around online and you can find a calculator or downloadable spreadsheet where you will enter your scope information and eyepiece information and then output the field of view and power for each combo.  Generally its good to have something around 1degre FOV, and 0.5 degrees.  With those two, you'll be set for most things.  You may choose to add a wider FOV eyepiece that maximizes the exit pupil at around 7 or 8mm, if your 1degree isn't already doing that.  You may also choose to add something higher power for planets or small things.  You have some nice stuff already, so just see where it sits in this continuum. 

I'd also add an O3 filter to the list."

Thank you, Brian. Yes, I was thinking of getting a 9mm 100° eyepiece (probably APM), which would give almost half a degree FOV and a bit over 200X, which should fill a nice niche. I know longer focal length hyper-wide eyepieces on fast scopes can be problematic, but I'm assuming I should be ok with 9mm (tell me if I'm wrong).  Many people are recommending an OIII; thank you, it will go on the list. 

 

Much appreciated guys!

Not sure what kind of mirror cell you have. With the Obsession a sling is used to hold the mirror and can be adjusted with a bolt on each side. The bump stops are some kind of plastic with what should be clips to help keep the mirror from flopping over if the scope is pointed to low in the sky.

 

You don't want the mirror resting on the stops so the sling is to help keep it just off the stops.  The mirror can deform and give a bad image if it is resting on the stops.


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#10 Dobserver

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Posted 02 March 2021 - 03:49 PM

Thank you all for the great advice! NGC7309_20, I will be getting a scope cover next (think I'll go with the Astrosystems; the price difference is significant and the cost of this hobby is starting to hurt pretty badly). 


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#11 bbasiaga

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Posted 02 March 2021 - 04:22 PM

"The Astro-Tech (GSO) coma corrector is optimized for f4.5 and works." Thank you havasman, I didn't know about this one; it is certainly much more affordable the the Televue. 

 

NGC7319_20: "Weights: yeah get some weight you can hang on the back of the scope when using the big eyepieces.   Could be one of these foam covered exercise hand weights you strap in the back somewhere.  Or Velcro ankle weighs.  Or a water bottle.  Or magnetic weights.  Or some Velcro deal.  I've probably tried it all at one time or another" Great suggestions, thank you. 

"Wheel chair ramps: yup I've done the same thing to my Obsession 20. Gotta keep the wheels high until the scope clears the threshold." Yes! That's what happened. Oh, the pain...

"The TeleGizmos covers are good, especially the 365 models.  AstroSystems Scope Coat is good too (thinner and light weight)". Nice. So Cal is a very dusty place; I will look into these. Also want a tarp.

 

CHASLX200: "Make sure the mirror is not resting on the stop bumps and check the sling. Also make sure the 2ndary holder is placed right" Yeah, what's with the sling? That is not something I have seen before, and I'm not sure I understand its purpose or where it's supposed to go. As for stop bumps, what are these? I can't find any info on this topic on google. I can tell something's not quite right with the position of the mirror, and collimation can be a struggle, because the bottom screw needs to be very tight... I love the secondary and once I figured out how it worked have not had any problems. 

 

bbasiaga: "As far as eyepieces, search around online and you can find a calculator or downloadable spreadsheet where you will enter your scope information and eyepiece information and then output the field of view and power for each combo.  Generally its good to have something around 1degre FOV, and 0.5 degrees.  With those two, you'll be set for most things.  You may choose to add a wider FOV eyepiece that maximizes the exit pupil at around 7 or 8mm, if your 1degree isn't already doing that.  You may also choose to add something higher power for planets or small things.  You have some nice stuff already, so just see where it sits in this continuum. 

I'd also add an O3 filter to the list."

Thank you, Brian. Yes, I was thinking of getting a 9mm 100° eyepiece (probably APM), which would give almost half a degree FOV and a bit over 200X, which should fill a nice niche. I know longer focal length hyper-wide eyepieces on fast scopes can be problematic, but I'm assuming I should be ok with 9mm (tell me if I'm wrong).  Many people are recommending an OIII; thank you, it will go on the list. 

 

Much appreciated guys!

I'm not sure what you mean about longer focal length wide field eyepieces being 'problematic'.  The wider the field the more coma they will show.  Honestly on my 22" f4 I rarely even use a coma corrector, and my go-to eyepiece is a 30mm 82degree.  I just don't see the coma as much, or at least don't perceive it as a problem.  Others really notice it.  But with a paracor in, any issue there is solved.  If coma is what you are referring to, then how big of a problem it is is somewhat subjective.   If you're thinking of something else, let me know and if I know anything I'll comment. 


-Brian


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#12 Sheol

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Posted 02 March 2021 - 07:11 PM

                         The Ultrabloc is probably a better filter for 1.25 EPs. I love mine. But 2 inch EPs, I do not know.. I mean I could get one but I do not know that I would really use it. I wonder if a 2 inch OIII would be a good filter?

 

                              Clear Skies,

                                  Matt.


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#13 StevenYood

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Posted 03 March 2021 - 04:13 AM

On the subject of filters, there’s nothing finer that using an Astrocrumb filter slide on a big dob.  I have my three position slide loaded with a UHC, an NPB and an OIII.  H Beta stays in the EP case since it gets used so infrequently.  I’m a lazy soul and I hardly ever bothered with filters before I got my slide.  Now, I use them regularly.  
If you are only getting one filter, I’d make it a UHC.  It’s good on a very wide variety of targets.


Edited by StevenYood, 03 March 2021 - 04:15 AM.

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#14 GeneT

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Posted 07 March 2021 - 04:38 PM

Weights: yeah get some weight you can hang on the back of the scope when using the big eyepieces.   Could be one of these foam covered exercise hand weights you strap in the back somewhere.  Or Velcro ankle weighs.  Or a water bottle.  Or magnetic weights.  Or some Velcro deal.  I've probably tried it all at one time or another.  

A lot of good advice in the above post. When I had a balancing issue with my 12.5 inch Portaball, I bought some ankle weights that came in a plastic material that I could cut with a regular pair of scissors. I just tucked the material in the base of my mirror assembly box. I put some of the material all around the box, with more in one area following some trial and error. That solved my problem with the telescope slowly slipping with heavier eyepieces. I found that I did not have to move the material around when viewing with lighter eyepieces. There are a lot of other possibilities for keeping a telescope stable and free from such slippage.  


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#15 Dobserver

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Posted 09 March 2021 - 02:31 AM

I'm not sure what you mean about longer focal length wide field eyepieces being 'problematic'.  The wider the field the more coma they will show.  Honestly on my 22" f4 I rarely even use a coma corrector, and my go-to eyepiece is a 30mm 82degree.  I just don't see the coma as much, or at least don't perceive it as a problem.  Others really notice it.  But with a paracor in, any issue there is solved.  If coma is what you are referring to, then how big of a problem it is is somewhat subjective.   If you're thinking of something else, let me know and if I know anything I'll comment. 

-Brian

HI Brian, yes, I meant the coma. So, is it the true FOV that determines how much coma is visible? So far things have looked good: the 35mm Panoptic shows it somewhat at the edges, but I can live with it. But I went ahead and purchased a 9mm APM XWA 100° eyepiece. It will have an FOV of .44° at 228X. Can I expect it to be free of coma?



#16 bbasiaga

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Posted 09 March 2021 - 10:37 AM

HI Brian, yes, I meant the coma. So, is it the true FOV that determines how much coma is visible? So far things have looked good: the 35mm Panoptic shows it somewhat at the edges, but I can live with it. But I went ahead and purchased a 9mm APM XWA 100° eyepiece. It will have an FOV of .44° at 228X. Can I expect it to be free of coma?

I'm not an optician, so I can't really state factually that its TFOV that determines the coma.  I can say that my observing experience is that the narrower the TFOV, the less I see coma - at least assuming that the two eyepieces are similar in their level of quality/correction.  So my 9mm nagler and 27 panoptic i would call roughly equal in quality, and I see more coma in the 27 towards the edge of the field.  Try it out and see if you like it.  I'm not familiar with the 9mm XWA angle eyepiece you mention and how well corrected it is.  But I think you'll probably be fine at f/4.5 if you don't see anything you don't like in the 35 panoptic.  

A guy I observe with at an annual star party has a 24" F4.2, and I don't think he even owns a paracor.  There is another guy there who swears that you need one at F5, but then raves over the clean views in the 24" (perhaps he can't see there isn't a paracor there in the dark? :) ).  So its a pretty personal, and you can always add it later.  

 

-Brian


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#17 brentknight

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Posted 09 March 2021 - 11:09 AM

I notice coma when I'm looking at "two-in-the-view" objects.  Smaller objects that can stay near the center of field might be less of an issue.  There are quite a few double-clusters out there where the main cluster looks pretty darn sharp, but the nearby one is unresolved.  A good coma corrector will make both objects sharp.

 

If you do decide to get a CC, I highly recommend the Paracorr 2.  Buy once, cry once - it's so much easier to use than any other CC.


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