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Thinking of going 2" for the first time, have some questions.

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9 replies to this topic

#1 metalpoet

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Posted 26 January 2020 - 06:26 AM

As the title suggests I'm starting to consider going up to 2" for the first time. Of my two scopes only the 6" Maksutov-cassegrain can natively fit 2" parts (the newt would need both a focusser and secondary upgrade so not looking at that now).

But like most commercial cats the Maksutov-cassegrain has a 2" rear opening with a reducer leading into the 1.25" star diagonal.

I have a nice set of 1.25" super plossl eyepieces and I would certainly want to retain those. But I wouldn't mind being able to make use of the 2" capacity for some things.

So first question: what approach should I take ? Just buy a 2" diagonal with a 1.25" reducer ? Perhaps a 2" focusser (combining two upgrades in one) is better value for money ?

And assuming I can probably only buy one 2" eyepiece to start, and will have a budget of 30USD or less - what should I look to get ? Where would it pay off most ? A widefield eyepiece for DSO's or a short one for high magnification?

Finally: is it even worth doing on a 6" scope ? Would I even benefit at that aperture ? If it is, would anything in that budget be worth buying at all ? Or should I leave the 2" world until the day I can go for a bigger scope (I'm hoping to get an 8" or 10" Dobsonian in the next year or two - and a 16" or bigger in a few years after that).

#2 luxo II

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Posted 26 January 2020 - 06:49 AM

Wide field low power for sure - I use a 38mm SWA that has the max field of view possible in a 2” barrel, but its the size of a can of soft drink.

Smaller focal lengths mean smaller fields of view so most eyepieces under 25mm will be 1,25” there’s not much point in them being 2”.

Edited by luxo II, 26 January 2020 - 06:50 AM.


#3 SeattleScott

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Posted 26 January 2020 - 06:59 AM

Yes, a 38SWA will give about 40% wider FOV than a 1.25” Plossl. And it should work well with your Mak. I used to use one in my 6” F12. It would not work well in your Dob. Keep in mind you also have to drill or cut a bigger hole. The 2” draw tube won’t fit in a 1.25” hole. Hope you are good at working with sheet metal. So you might just leave the Dob at 1.25” and get a 2” diagonal and 38SWA for the Mak.

That being said it would cost over $200 retail for a quality 2” diagonal and 38SWA. Even used, you can’t get there with a $30 budget.

Scott

#4 metalpoet

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Posted 26 January 2020 - 07:05 AM

Yes, a 38SWA will give about 40% wider FOV than a 1.25” Plossl. And it should work well with your Mak. I used to use one in my 6” F12. It would not work well in your Dob. Keep in mind you also have to drill or cut a bigger hole. The 2” draw tube won’t fit in a 1.25” hole. Hope you are good at working with sheet metal. So you might just leave the Dob at 1.25” and get a 2” diagonal and 38SWA for the Mak.

That being said it would cost over $200 retail for a quality 2” diagonal and 38SWA. Even used, you can’t get there with a $30 budget.

Scott


The Newt is home made, so I cut the current hole myself anyway (it's a cardboard tube). But the level of change required is prohibitive I think.

Sounds like going 2" eventually will be worth it, possibly combining that upgrade with an external dual speed focusser to get rid of the make/sct focus shift.
But also well out of my current budget. Still useful information to now know when the time comes that I can actually afford it all.

#5 sg6

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Posted 26 January 2020 - 07:51 AM

Ask around if the Mak will realistically take a 2", many seem to have internal baffling that causes vignetting with the increased field from a 2" eyepiece.

 

Would be a fair bit of cost and work to find all you achieved was a wider where the outer 30% on each side was in effected masked out.


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#6 metalpoet

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Posted 26 January 2020 - 08:22 AM

Ask around if the Mak will realistically take a 2", many seem to have internal baffling that causes vignetting with the increased field from a 2" eyepiece.

Would be a fair bit of cost and work to find all you achieved was a wider where the outer 30% on each side was in effected masked out.


Yipe. I will certainly need to determine that, and I'm not even sure where to start. It's a Bosma model, they still make them but this specific one is very old (at least 10 years). I picked it up from a small adds classified for 1/12th of what the cheapest new 6" CAT I've seen goes for (ZAR4K versus ZAR36K for the Nexstar 6SE) and the company that imported it doesn't even exist anymore - as far as I can tell they never even had a website.

So getting accurate information on it has always been challenging. The little I have came from Bosma's site - at least they haven't changed the optical specs. That said it's optics are fantastic and it's a great performer for it's aperture.
It's a fantastic grab-and-go scope (the purpose I bought it for) and I have been very happy with the purchase and consider it a massive bargain.
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#7 SeattleScott

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Posted 26 January 2020 - 09:30 AM

The Bosma is designed for 2” eyepieces.

What F ratio is your Dob? What works well at F12 doesn’t always work well at F5 or so.

Scott
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#8 metalpoet

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Posted 26 January 2020 - 09:53 AM

The Bosma is designed for 2” eyepieces.

What F ratio is your Dob? What works well at F12 doesn’t always work well at F5 or so.

Scott


The Dob is a 6" F8 with primary mirror from Edison Scientific (before they got bought out).

#9 SeattleScott

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Posted 26 January 2020 - 09:55 AM

Ok that is pretty flexible then. The 38mm would still be a reasonable exit pupil and a decent performer at F8.

Scott

#10 noisejammer

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Posted 26 January 2020 - 06:07 PM

The Dob is a 6" F8 with primary mirror from Edison Scientific (before they got bought out).

I'd check the minor axis of your secondary. Most 6" f/8's use a 1.2" (30mm-ish) diagonal. This will be fine with some 2" eyepieces but it will vignette others.

 

Please pm if you'd like some contacts in CT - I know several experienced ATM's in the area.




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