Beginner Question

[fcwilt]

I recently purchased my first scope:

Orion 150mm Maksutov-Cassegrain Telescope Optical Tube

In the description they make this remark:

 The telescope's 2" visual back and included 1.25" step-down adapter allows use of both 2" and 1.25" size diagonals, eyepieces and accessories (all accessories sold separately). 

But I was reading, in another post, that seemed to be saying that small aperture scopes, like this one, would likely have problems with 2" diagonals/eyepieces. Something about the scopes "baffle size"?

So why make a scope that can accept 2" devices if they likely won't work well?

Thanks.

Frederick

 

[D4N]

It is a lot easier to put a 1.25" ep into a 2" tube than put a 2" ep into a 1.25" tube, at least you have the choice

[billyharris72]

It's not a question of aperture - 2 inch eyepieces can be used quite happily with 80mm scopes for wide angle views. I guess with some models of Mak there might be a problem with vignetting (effectively the eyepiece field stop could be larger than the light conre from the scope) but it seems unlikely. I use a 68 degree Explore Scientific 34mm eyepiece (2 inch) in a 150 Skywatcher Mak and it performs very well indeed. Overall, a long focal length 2 inch eyepeice is a good thing to have for a Mak, as it means you can get a half decent field of view (very much at a premium with this type of scope).

If in doubt, buy from a reputable dealer and ask - they will tell you if there is a problem.

Billy.

[Stu]

26 minutes ago, fcwilt said:

I recently purchased my first scope:

Orion 150mm Maksutov-Cassegrain Telescope Optical Tube

In the description they make this remark:

 The telescope's 2" visual back and included 1.25" step-down adapter allows use of both 2" and 1.25" size diagonals, eyepieces and accessories (all accessories sold separately). 
 

But I was reading, in another post, that seemed to be saying that small aperture scopes, like this one, would likely have problems with 2" diagonals/eyepieces. Something about the scopes "baffle size"?

So why make a scope that can accept 2" devices if they likely won't work well/right?

Thanks.

Frederick

 

Reasons why would include the fact that 2" fittings can feel more solid and secure than 1.25", and perhaps there is a quality benefit to using only the centre area of a mirror rather than the parts nearer the edge which may deviate from flat.

2" eyepieces will work, and can be more comfortable to use than 1.25", having larger exit lenses for example. However, once the field stop size of the eyepiece exceeds the baffle tube diameter (the part that runs from the visual back towards the secondary), then the view will start to vignette i.e. darken around the edges. This is not always easily detected visually if it is not extreme, so 2" eyepieces can still be used.

It can be of use if you have eyepieces which work well in another scope and you just want to avoid buying more 1.25" in the same focal length, for instance.

[fcwilt]

Thanks to all for the information.

I believe I understand the issues.

Mention was made of  "baffle tube diameter" - is this a specification that should be available for my scope?

Frederick

 

[John]

As I understand the design, the secondary baffle with these maksutov-cassegrains actually is attached to the front corrector lens and is wider at the end nearest the primary mirror. What may cause vignetting of a wide field 2" eyepiece would be the inside diameter of the port at the back of the scope that the light cone exits from. You can measure this by removing the diagonal and the visual back.

[Stu]

2 hours ago, John said:

As I understand the design, the secondary baffle with these maksutov-cassegrains actually is attached to the front corrector lens and is wider at the end nearest the primary mirror. What may cause vignetting of a wide field 2" eyepiece would be the inside diameter of the port at the back of the scope that the light cone exits from. You can measure this by removing the diagonal and the visual back.

The Mak-Casses I've seen John have just a small shield around the secondary, and the baffle tube is fixed through the primary to the visual back. The baffle tube is the limiting factor I believe. I don't think the shield around the secondary can be restrictive as other wise it would block the light path.

[Louis D]

Any vignetting would most likely only be visible in widest field eyepieces like a 41mm Panoptic.  Having a 2" visual back opens up the possibility of using 2" only wide field eyepieces like the 17mm ES-92 or the 9mm ES-120, both of which should not vignette in the least because their effective field stops are less than 28mm, way smaller than the diameter of the baffle tube.  Another thing to consider is that the human eye doesn't pick up on vignetting nearly as well as camera sensors, so you'd be unlikely to notice it.

[John]

1 hour ago, Stu said:

The Mak-Casses I've seen John have just a small shield around the secondary, and the baffle tube is fixed through the primary to the visual back. The baffle tube is the limiting factor I believe. I don't think the shield around the secondary can be restrictive as other wise it would block the light path.

Depends on the design Stu. There are several different mak-cass designs.

The Skywatcher and Meade mak-casses have quite a long, flared shield that extends from the secondary spot on the inside of the meniscus down the tube. I agree that this would not cause vigneetting because of the flared design although it does mean that the effective secondary obstruction is larger than the secondary itself in these.

 

[Stu]

2 minutes ago, John said:

Depends on the design Stu. There are several different mak-cass designs.

The Skywatcher and Meade mak-casses have quite a long, flared shield that extends from the secondary spot on the inside of the meniscus down the tube. I agree that this would not cause vigneetting because of the flared design although it does mean that the effective secondary obstruction is larger than the secondary itself in these.

 

Do those designs not have a baffle tube in the same position as the diagram I posted?

Interesting, I'd not seen that before

[John]

12 minutes ago, Stu said:

Do those designs not have a baffle tube in the same position as the diagram I posted?

Interesting, I'd not seen that before

I'm wondering now if the Skywatcher maks actually have both a narrow baffle around the perforation of the primary and the flared baffle around the secondary spot

It would be the rear baffle / exit port internal diameter that creates a risk of vignetting with eyepieces with large field stops anyway.

 

[Dave In Vermont]

I have the identical telescope - mine is branded 'Skywatcher' and is the very same from Synta - and it does great with a 2" William Optics 2" diagonal & 2" eyepieces. The Maksutovs have gone through some major design revisions a few years ago to address many problems they used to suffer from. From focusers to to clearences. So perhaps what you read was from a few years back?

The only vignetting I've had was trying to use a 2" 50mm Super-Plossl eyepiece. Too low a power for it to handle. But my 32mm & 38mm 70° eyepieces work very well IHMO. Maksutovs are known to not be willing to go for too low power, but mine also does great with my 1.25" 40mm Plossl eyepiece. Which gives 45X.

Great little scopes -

Dave

[fcwilt]

Just now, Dave In Vermont said:

I have the identical telescope - mine is branded 'Skywatcher' and is the very same from Synta - and it does great with a 2" William Optics 2" diagonal & 2" eyepieces. The Maksutovs have gone through some major design revisions a few years ago to address many problems they used to suffer from. From focusers to to clearences. So perhaps what you read was from a few years back?

The only vignetting I've had was trying to use a 2" 50mm Super-Plossl eyepiece. Too low a power for it to handle. But my 32mm & 38mm 70° eyepieces work very well IHMO. Maksutovs are known to not be willing to go for too low power, but mine also does great with my 1.25" 40mm Plossl eyepiece. Which gives 45X.

Great little scopes -

Dave

Hi,

Thanks for the info.

The specs for my scope list the range of reasonable magnifications as 50 to 300. With the listed focal length of 1800 this works out to 36mm to 6mm for the eyepieces.

So your results seem to be consistent with those numbers.

Frederick