A hypothetical question about dob...

[spacedobsonian]

If you had to buy a small reflector dob (<6 inches) would you pay £20 more for a parabolical mirror BUT lose the ability to collimate?

Would you buy the cheaper, spherical mirror, collimatable reflector dob or a £20 more expensive, parabolic mirror, not collimatable reflector dob?

[Steve Ward]

Both are 'collimatable' ... 

[cajen2]

I think SD is referring to the new Ursa Major tabletop dobs, which have spherical mirrors. My own instinct would be to go for the parabolic, but I've never used or looked through an UM, so I don't speak from experience. One of my dobs is a tabletop with a parabolic mirror (Heritage 150p) and it has very good optics.

[Carbon Brush]

Can you provide links to the actual scopes?

[wesdon1]

41 minutes ago, spacedobsonian said:

If you had to buy a small reflector dob (<6 inches) would you pay £20 more for a parabolical mirror BUT lose the ability to collimate?

Would you buy the cheaper, spherical mirror, collimatable reflector dob or a £20 more expensive, parabolic mirror, not collimatable reflector dob?

Do you mean one 'scope is broken, so can't be collimated? To my knowledge, all Dob's-i.e. Newtonian Reflectors, are collimatible? ( Remember the name "Dob or Dobsonian" only refers to the way the telescope is mounted )

[Mandy D]

39 minutes ago, spacedobsonian said:

If you had to buy a small reflector dob (<6 inches) would you pay £20 more for a parabolical mirror BUT lose the ability to collimate?

Would you buy the cheaper, spherical mirror, collimatable reflector dob or a £20 more expensive, parabolic mirror, not collimatable reflector dob?

Given how well both my Newts hold collimation and the fact that spherical mirrors just do not match up to parabolic ones, I would choose the latter. I've never needed to adjust the collimation on my 250PX and only collimated my 200P when I got it because it was so far out, you could not even see the primary mirror in the eyepiece.

Personally, I would also avoid any Newt on which the collimation cannot be adjusted. I'm sure both of mine would benefit from having the collimation tweaked, but I am happy with things as they are, right now.

[cajen2]

I believe it's this one:

https://www.firstlightoptics.com/beginner-telescopes/ursa-major-102mm-tabletop-dobsonian-telescope.html

and it's being compared to a SW tabletop like this:

https://www.firstlightoptics.com/heritage/skywatcher-heritage-100p-tabletop-dobsonian.html

 

[Mandy D]

20 minutes ago, wesdon1 said:

Do you mean one 'scope is broken, so can't be collimated? To my knowledge, all Dob's-i.e. Newtonian Reflectors, are collimatible? ( Remember the name "Dob or Dobsonian" only refers to the way the telescope is mounted )

The Skywatcher Heritage 76 is not collimatable. I have one here and have just checked it, both the primary and secondary mirrors are fixed in place.

[Peter Drew]

A "fixed" collimation Newtonian like the Heritage 76 theoretically shouldn't need subsequent collimation.  However, at F4, if the factory setting is not very close to perfect, images will suffer at higher powers.  There are three screws securing the end unit that contains the primary mirror, if the unit is removed and the three clearance holes are slightly slotted then the complete unit can be carefully adjusted by hand and then the screws tightened in place.  I fine tuned a Heritage 114mm to good effect by doing this.    🙂

[spacedobsonian]

1 hour ago, wesdon1 said:

Do you mean one 'scope is broken, so can't be collimated? To my knowledge, all Dob's-i.e. Newtonian Reflectors, are collimatible? ( Remember the name "Dob or Dobsonian" only refers to the way the telescope is mounted )

No, some scopes like the Skywatcher 130 starquest can't be collimated!

[wulfrun]

I have a Heritage 114P, it has a collimatable secondary but the primary is fixed. I've tweaked the primary as per @Peter Drew's method above and made sure the secondary is correct. I've checked it now and then and it's not needed any adjustment since, so I don't think the fixed primary is an issue. Probably helps that the mirror is relatively small and lightweight.

[spacedobsonian]

1 hour ago, cajen2 said:

I believe it's this one:

https://www.firstlightoptics.com/beginner-telescopes/ursa-major-102mm-tabletop-dobsonian-telescope.html

and it's being compared to a SW tabletop like this:

https://www.firstlightoptics.com/heritage/skywatcher-heritage-100p-tabletop-dobsonian.html

 

That's were I got the original idea for this post!

[spacedobsonian]

2 hours ago, Steve Ward said:

Both are 'collimatable' ... 

How?

[spacedobsonian]

2 hours ago, Carbon Brush said:

Can you provide links to the actual scopes?

For example:

https://www.firstlightoptics.com/beginner-telescopes/ursa-major-102mm-tabletop-dobsonian-telescope.html

And 

https://www.firstlightoptics.com/heritage/skywatcher-heritage-100p-tabletop-dobsonian.html

[spacedobsonian]

6 minutes ago, wulfrun said:

I have a Heritage 114P, it has a collimatable secondary but the primary is fixed. I've tweaked the primary as per @Peter Drew's method above and made sure the secondary is correct. I've checked it now and then and it's not needed any adjustment since, so I don't think the fixed primary is an issue. Probably helps that the mirror is relatively small and lightweight.

A bit off-topic but they made a heritage 114p?

[spacedobsonian]

Just now, spacedobsonian said:

A bit off-topic but they made a heritage 114p?

I mean Skywatcher 

[Sunshine]

i'm not sure about whether the skywatcher can be collimated or not but I would choose it simply because I have no idea who Ursa Major is, Skywatcher is very well known.

[cajen2]

Ursa Major is the brand name that FLO uses for scopes it has sourced from the likes of GSO, along with StellaLyra and StellaMira. GSO make good quality stuff.

[Louis D]

At f/8 and above, a spherical primary could be considered acceptable for non-planetary observing.  No amount of collimation will improve an f/4 spherical all that much.

[Peter Drew]

True, but poor collimation will make matters worse.     🙂

[markse68]

20 minutes ago, Louis D said:

At f/8 and above, a spherical primary could be considered acceptable for non-planetary observing. 

non- planetary? @Siegfried1969 seems to be getting quite remarkable planetary detail with his 11cm f7.3 spherical Tal 1! I have one too and a 6” Tal 2 version- need to give them another go before Mars gets too far away again. but f4 doesn’t sound a good idea

Mark

[wulfrun]

53 minutes ago, spacedobsonian said:

A bit off-topic but they made a heritage 114p?

Yes, the Heritage 114P Virtuoso. Not sure if the OTA is available in any other mounts:

https://www.firstlightoptics.com/heritage/skywatcher-heritage-114p-virtuoso.html

(Mine has red tube, not black as shown)

 

[Mandy D]

1 hour ago, Peter Drew said:

A "fixed" collimation Newtonian like the Heritage 76 theoretically shouldn't need subsequent collimation.  However, at F4, if the factory setting is not very close to perfect, images will suffer at higher powers.  There are three screws securing the end unit that contains the primary mirror, if the unit is removed and the three clearance holes are slightly slotted then the complete unit can be carefully adjusted by hand and then the screws tightened in place.  I fine tuned a Heritage 114mm to good effect by doing this.    🙂

I did notice the three screws and figured that would be possible, but I only have that scope because it cost just a few pounds at a car boot sale and I liked the artwork on it.

[Siegfried1969]

23 minutes ago, markse68 said:

non- planetary? @Siegfried1969 seems to be getting quite remarkable planetary detail with his 11cm f7.3 spherical Tal 1! I have one too and a 6” Tal 2 version- need to give them another go before Mars gets too far away again. but f4 doesn’t sound a good idea

Mark

It took me about one year to get to this detail. First time when I started observing again, I could not see much (just a small disk with two stripes on it) but by continuing observing, more and more detail came apparent. Jupiter is rather straightforward. Mars is much harder. 

I also must tell that the two filters are helping a lot. I read so many good things about them and so I bought into the hype. 

Neodymium works really well on the sun and Jupiter while the contrast booster works very good on the moon and Mars.

My TAL telescope is currently undergoing a huge overhaul. The mirrors are now with OOUK and should come back within around 2 weeks. I also flocked the tube, replaced the TAL 6x30 finder with a bigger 8x50 RA finder + a rigel. I also took the entire mount apart, cleaned and regreased everything. It was an intensive project that is still ongoing.

[spacedobsonian]

11 minutes ago, Siegfried1969 said:

the contrast booster works very good on the moon and Mars.

Going really off topic! Does it work on Jupiter and if not which simple (colour or any) filter will work for planetary?

Edited January 3, 2023 by spacedobsonian