Eyepiece collection with different focal points

[dabbeldi]

Hi,
I’m after some ideas how to deal with my eyepiece collection having annoyingly different focal points.
The telescope is a Skywatcher 10 inch collapsible dobsonian (1200 mm focal length, f4.7).
My eyepieces are ES 8.8mm 82 deg, a 13 mm T6 Nagler and the standard 10 mm and 25 mm Plossls the telescope came with. There’s an ES 24 mm 68 deg on its way as well. They are all 1.25 inch eyepieces.
So, the skywatcher eyepieces and the ES 8.8 mm are within focus when I have the standard 1.25 inch tube attached (the 8.8 mm is just in focus). For the 13 mm Nagler, I need the 2 inch extension tube plus the 1.25 inch tube or I need to pull out the Nagler by a bit over 1 cm when only the 1.25 inch tube is attached, which is obviously not how I’d like to handle any eyepiece.
Have any of you come across a similar issue and if so, what might be the solution? Different length extension tube? New focusser? Replace the ES eyepieces with Naglers/Panoptics (no Ethos) ? I would appreciate any thoughts and experiences.
Thanks
Daniel

[MortonH]

Tele Vue eyepieces tend to reach focus further 'out' than other EPs.

In my 8" f/5 Newtonian I use a 2" 35mm extension tube and all my eyepieces can reach focus. I have a mixture of Tele Vue, Explore Scientific, etc. I agree that the standard 1.25" tube that came with the scope didn't work for the Tele Vues.

You can also get 'hi hat' 2"-1.25" adapters that push the focus out a bit more.

http://myastroshop.com.au/products/d...sp?id=MAS-034E


I'm a bit surprised that the Nagler requires both extension tubes. One possibility is that the primary mirror has been adjusted as far up the tube as possible (towards the eyepiece end) which pushes the focal point further out. If you loosen the collimation bolts to allow the mirror to move back a bit towards the bottom of the tube this might be enough to get all your eyepieces in the range of your 2" extension.

[dabbeldi]

Thanks, I will check the primary mirror first.
Daniel

[astro744]

Have a look at column f at http://www.televue.com/engine/TV3b_page.asp?id=214

This is the focal plane position; refer diagram for an explanation.

You can see that most are at 0.25" with the most in travel being -0.38" and the most outward travel being 1.16". I'm not sure how that compares to others on the market but I would think most eyepieces available would fit nicely within the range of a standard Newtonian focuser with say 1.5" of travel. Note you can measure your own relative positions but allow for the thickness of 2-1.25" adaptor when calculating/measuring your own set.

If you had say the two eyepieces at the extreme end then you would need a focuser with 1.6" of travel. I think the Skywatchers scopes are designed for photo and visual but the latter needs and extension. Many focusers have 1.5" of travel but this figure does vary amongst makes and models. Some makes allow you to change the length of your draw tube.

To answer your question; yes, get all Tele Vue and you won't have that problem.

[Wavytone]

Originally Posted by astro744

To answer your question; yes, get all Tele Vue and you won't have that problem.

But far from being the ONLY solution. You don't have to pay the Televue Tax to get a set of par-focal eyepieces.

Many sets are par-focal - even Edmund RKE's were par-focal way back in 1980 long before TeleVue existed.

In modern sets...the Vixen LV were par-focal, so were LVW's (though not the same focus as LV's), Vixen SLV and SSW are par-focal. This is the main reason to buy a set, not a mixed bag of mongrels.

I'd expect no less from Takahashi and Masuyama.

Even cheapies like ProStar or GSO are par-focal as long as you stick to the same series.

[bigjoe (JOSEPH)]

Originally Posted by Wavytone

But far from being the ONLY solution. You don't have to pay the Televue Tax to get a set of par-focal eyepieces.

Many sets are par-focal - the Vixen LV were par-focal, so were LVW's (though not the same focus as LV's), Vixen SLV and SSW are par-focal, even Edmund RKE's were par-focal. This is the main reason to buy a set, not a mixed bag of mongrels.

I'd expect no less from Takahashi and Masuyama.

Even cheapies like ProStar or GSO are par-focal as long as you stick to the same series.

Looking at Wavy's VIXEN SLV'S Side by side with one of my TELEVUE delites I'd give build Quality and appearance to the VIXENS!!. They also have loverly field stops! Very underrated IMO; and by all accounts excellent lanthanum EP'S .
AND thats coming from someone almost exclusively using Televue EP'S!! Which I love. So stick to the same good EP brand as suggested .

bigjoe.

[astro744]

Yes but the so called tax you pay also gets you highly corrected eyepieces suitable for a fast Dob and when combined with a Paracorr provide highly corrected views to the edge without eyepiece or mirror aberrations visible (provided the mirror is good too of course). Tele Vue also offer free eyepiece specifications not only to those who pay the 'tax' but to all for you to be able to design or customise your telescope to better suit the eyepieces you intend on using. Newtonians are very adaptable depending on which decisions have already been made, i.e. tube diameter, secondary size, focuser height and even then changes are possible.

There are two issues here; one is to try and get all eyepieces par focal and whilst some manufacturers have eyepieces in their range that are par focal it is also not possible to have all eyepieces from a given manufacturer to be par focal, including Tele Vue. You can get both 1.25" and 2" par focalising rings that sit around the barrel to raise the eyepiece from a few different suppliers. Tele Vue sell barrel extenders for their 2" eyepieces that need a lot of outward travel.

The other issue is choice of focuser. I think mass produced telescopes these days try and cover all bases i.e. visual and photo and whilst this may work well with some designs it also is subject to some compromise. Perhaps the OP should look at a new focuser with a bit more travel if only for visual and positioned such that the focal plane is between the eyepiece with the most in travel and the one with the most outward travel. Many focusers have different length draw tubes to choose from.

The OP could also choose a zero depth 2"-1.25" adaptor for eyepieces that need more in travel and even leave this on the eyepiece and effectively convert the eyepiece to 2". There is also a 2"-1.25" Hi-Hat adaptor for 1.25" eyepieces that require more out travel (and leave it on the eyepiece if desired to convert to 2"). There are many options but yes having eyepieces that are as close to par focal to start with is handy. Note if you do intend on using a Paracorr one day then this too will change the focal plane which will need to be allowed for. It is possible to have a telescope that can use many different eyepieces with and without a Paracorr in place without any change required.

Whatever you choose, enjoy!

[Wavytone]

Originally Posted by astro744

... not possible to get all eyepieces par focal

Not true, though rare.

Some examples of sets that are fully parfocal across the series are Vixen LVW and SLV and SSW, and Masuyama. The vixen LVW series were unique in spanninga 10:1 range of focal lengths 42 down to 3.5mm, and also have an aspect which is unique - they all weigh roughly the same so those with refractors or newtonians don't need to rebalance the scope between low and high power. Someone really thought about all aspects of the design of those, not just the lenses.

Another series that are precisely parfocal across the set are Brandon's...

[astro744]

Yes within a series of eyepieces having them parfocal would of course be possible but making all eyepieces parfocal from a given manufacturer across different series types, e.g. Plossl, wide field, ultra wide field would be difficult if not impossible. How does one allow for the 2"-1.25" adapter thickness in ones calculations? e.g. If the 42mm LVW is parfocal with the LVW 3.5mm then what thickness 2"-1.25' adaptor must one use or must one have a zero thickness adaptor?

Zero thickness adaptors come in two types; the one in which the 1.25" lock screw slots into the 2" focusser and then there is the one that has an allen screw that locks onto the barrel of the 1.25" eyepiece effectively making it a 2" eyepiece.

You can add Clave Plossl to the parfocal list as this was a major selling point for me when I bought some in the '80s plus the fact that they claimed 0.5mag extra gain due to their highly polished surface and fine coatings and of course 4 elements only in a true Plossl (asymmetrical) design. (They have certainly NOT disappointed!) This of course is the subject of a different debate but suffice to say having parfocal eyepieces or at least within a few mm of each other is very handy when observing but I don't expect say a 31mm Nagler to be parfocal with other eyepieces and as long as I have the inward travel I'm OK with that.

It would be good if all suppliers post their detailed eyepiece specifications but they don't so most of the time one has to either find out by buying the given eyepiece or asking here on the forum if anyone experiences any drastic inward or outward focus issues with a given eyepiece.

[Wavytone]

In the LVW series the 3.5 ... 22 have dual barrels - 2" and 1.25". No adapters required if you want to use them all as 2" eyepieces. The 42 has a fairly long barrel too to accomodate the backfocus.

As I said, the designer gave these more thought than most and I can't help suspecting he must have been a stargazer with firsthand experience of what grates in practise.

[bigjoe (JOSEPH)]

Originally Posted by astro744

Yes within a series of eyepieces having them parfocal would of course be possible but making all eyepieces parfocal from a given manufacturer across different series types, e.g. Plossl, wide field, ultra wide field would be difficult if not impossible. How does one allow for the 2"-1.25" adapter thickness in ones calculations? e.g. If the 42mm LVW is parfocal with the LVW 3.5mm then what thickness 2"-1.25' adaptor must one use or must one have a zero thickness adaptor?

Zero thickness adaptors come in two types; the one in which the 1.25" lock screw slots into the 2" focusser and then there is the one that has an allen screw that locks onto the barrel of the 1.25" eyepiece effectively making it a 2" eyepiece.

You can add Clave Plossl to the parfocal list as this was a major selling point for me when I bought some in the '80s plus the fact that they claimed 0.5mag extra gain due to their highly polished surface and fine coatings and of course 4 elements only in a true Plossl (asymmetrical) design. (They have certainly NOT disappointed!) This of course is the subject of a different debate but suffice to say having parfocal eyepieces or at least within a few mm of each other is very handy when observing but I don't expect say a 31mm Nagler to be parfocal with other eyepieces and as long as I have the inward travel I'm OK with that.

It would be good if all suppliers post their detailed eyepiece specifications but they don't so most of the time one has to either find out by buying the given eyepiece or asking here on the forum if anyone experiences any drastic inward or outward focus issues with a given eyepiece.

0.5 MAGNITUDE GAIN ...Where can I get some!!!
bigjoe.

[dabbeldi]

Hi,
Thanks for everyone’s comments and help. It is much appreciated.
To give an update, I dropped the primary mirror right back and then collimated the scope again. This has not had the desired effect as the eyepieces still focus at different points and I need to switch extension tubes. Maybe I pushed the primary too far up again - I will need to go back and re-check its position during the next collimation.
In the meantime, I have followed MortonH’s advice and purchased the 1.25 inch extension stub, which based on the current focal planes should do the trick.
In regards to the eyepiece discussion, I will play with my current eyepiece collection for a while (ES 24mm 68 is on its way) to see how it all works for me. Clearly there are many different views out there with regards to the best eyepiece. I am very much enjoying the 13 mm Nagler and also the 8.8 mm ES, but haven’t had a chance yet to try other brands, including the longer focal length ES. I don’t mind refocusing, but switching extensions or slightly pulling eyepieces out is getting old real quick.
Cheers

[bigjoe (JOSEPH)]

Originally Posted by dabbeldi

Hi,
Thanks for everyone’s comments and help. It is much appreciated.
To give an update, I dropped the primary mirror right back and then collimated the scope again. This has not had the desired effect as the eyepieces still focus at different points and I need to switch extension tubes. Maybe I pushed the primary too far up again - I will need to go back and re-check its position during the next collimation.
In the meantime, I have followed MortonH’s advice and purchased the 1.25 inch extension stub, which based on the current focal planes should do the trick.
In regards to the eyepiece discussion, I will play with my current eyepiece collection for a while (ES 24mm 68 is on its way) to see how it all works for me. Clearly there are many different views out there with regards to the best eyepiece. I am very much enjoying the 13 mm Nagler and also the 8.8 mm ES, but haven’t had a chance yet to try other brands, including the longer focal length ES. I don’t mind refocusing, but switching extensions or slightly pulling eyepieces out is getting old real quick.
Cheers

The 13mm Nagler T6 is a dream EP , I have one and will not sell it; its that good!!
bigjoe.

[astro744]

Originally Posted by bigjoe

0.5 MAGNITUDE GAIN ...Where can I get some!!!
bigjoe.

See http://www.cloudynights.com/topic/48...clave-plossls/

The claim was made in an ad in the US S&T. I don't think Clave ever made that claim. I bought direct from Clave as it was cheaper against the French Franc. I remember once testing against a 15mm Panoptic on a globular and the 16mm Clave went deeper with a 10.1" f6.4.

I also tested a 6mm Clave against 6mm Brandon and 6mm Radian on the same 'scope. The target was Jupiter and I went back and forth many times between eyepieces and in the end concluded that each was showing the same amount of detail but the Radian was by far the most comfortable with its 20mm eye relief.

[Wavytone]

If that 0.5mag claim were true, the transparency of "brand X" eyepieces would have to be less than 63%.

It could be true if "brand x" didn't have multicoated surfaces on all air-glass interfaces, a distinct possibility in the 1970's.

But virtually all decent modern eyepieces are fully multicoated apart from the crap Meade MA ones, in which case the transparency will be better than 95% and that claim is simply untrue.

[bigjoe (JOSEPH)]

À

Originally Posted by Wavytone

If that 0.5mag claim were true, the transparency of "brand X" eyepieces would have to be less than 63%.

It could be true if "brand x" didn't have multicoated surfaces on all air-glass interfaces, a distinct possibility in the 1970's.

But virtually all decent modern eyepieces are fully multicoated apart from the crap Meade MA ones, in which case the transparency will be better than 95% and that claim is simply untrue.

I thought as much!
The usual ridiculous claims by some manufacturers .Nothing can break the laws of physics/optics it seems; and no, not even Clave, Tak ,AP etc.
bigjoe

[astro744]

Originally Posted by bigjoe

À

I thought as much!
The usual ridiculous claims by some manufacturers .Nothing can break the laws of physics/optics it seems; and no, not even Clave, Tak ,AP etc.
bigjoe

I don't believe Clave ever made the claim but rather a US dealer ( perhaps so impressed after using the eyepieces). As I said I have NOT been disappointed and quite impressed in fact when I first got them and now 30 years later. They go very deep are very sharp, are par focal are lightweight and are a joy to use.

And to the original post; yes having parfocal eyepieces is very handy but over time you will accumulate an assortment and they won't all be parfocal so it's the focuser that has to be correctly positioned and have enough range.