Attaching filters to diagonals?

[Ziggy]

Hi Guys,

I'm back with another question, this time regarding filters. I've had a look to see if this question's been asked before, but can't see it anywhere.

The set-up I'm looking at for my scope (9.25" SCT), is a WO Crayford Focuser, (which attaches to the back of course), then a WO 2" Dielectric Diagonal, (which will of course connect to the focuser). I figured getting the 2" diagonal and such, as I will most likely want a 2" eyepiece sometime in the future, and it comes with the 1.25" adapter of course. My main eyepiece at the moment is the 22mm Panoptic, which has the dual barrels. Thanks for the advice everybody on that eyepiece too.....she's a beauty :)

While I think of it, Panoptic 22mm users out there, does it perform, or fit better in a 2" or 1.25" diagonal. Does it make a difference which you use? Being so heavy, seems like it might be more secure in the 2" diagonal? Obviously I've never used the 2" before.

I've never used filters before either. So, taking all that into account, I am just wondering if anybody knows whether UHC/O-III filters work better screwed into the eyepiece barrel, rather than into the diagonal? Does it make a difference to the image you see at the eyepiece, or any other difference, depending on where you attach it? Does it affect the performance of the diagonal itself having a filter attched to it?

My reason for asking is, I don't want to get into the situation where I'm buying both size filters. With the Panoptic, I can buy the much cheaper 1.25" filter, but will then have to buy a 1.25" lower power eyepiece in the future, (say a Ultima 30mm or something like that), and forgo quite a bit of the FOV, which may be defeating the purpose a little on such a low power eyepice? If the filters work well attached to the diagonal, then it might make sense to just buy the 2" filter at the start (despite the considerable extra cost), then it won't matter what size barrels I get in the future?

After all that, all I really want to know is how UHC filters work screwed into the diagonal compared to the eyepiece? I'm presuming most people connect them to the eyepiece for convenience. Shouldn't be much difference I wouldn't think visually, but when you're trying to get every edge out of your scope, every little bit counts.

I'll appreciate the thoughts of anyone who has any experience with this sort of thing.

Thanks a lot,

Glen.

[gbeal]

Glen,
what a quandary huh?
While not having exactly the same problem as you, I am finding a similar situation with the two Naglers I currently own, a 22mm (2" only) and 12mm (both 2" and 1.25").
What I usually do is use the 12mm with a 2" to 1.25" adaptor, as you would normally, and this adaptor has a 48mm filter thread on it's front.
So if I do any lunar viewing with a single eyepiece (as opposed to the binoviewer) and I use a filter it goes on the front of the adaptor. Mostly this is a result of using older 48mm camera filters in the colours I wanted, orange etc.
I think logically you are better buying the bigger (albeit more expensive) 2" filters now, rather than later. Just make sure you can use them with the 1.25" eyepieces, possibly with the style of adaptor I use.
Gary

[Robert_T]

Hi Ziggy, I'm not aware that it makes any difference to the quality of your view where the filter sits in the optical train (i haven't noticed myself anyway). I only have 1.25in filters but then again most of my eyepiecs are 1.25in. If I were more into deep sky and wanted a filter for use with multiple eyepieces though I'd go for a 2inch set into the diagonal (I use a WO 2in dielectric) to save all that fussy screwing and unscrewing. Hope the WO crayford works well for you, I had thought to get one for my C9.25 but ended up opting for the Meade Motorfoucs unit - if you want to do planetary imaging then a motor focus makes a BIG difference!


cheers,

[ausastronomer]

Hi Glenn,

"THEORETICALLY", the closer a filter is placed in the optical system to the eyepiece, the better it will work. The reasoning behind this is because the light cone "narrows" as it approaches the eyepiece and there is less chance of the filter introducing aberrations with a narrower light cone. This theory would IMO also be more appropriate to a fast newtonian than an F10 SCT, as the fast newt has a "steeper angle of incidence", with the entering light cone.

Putting the theory aside and considering the practicalities, I own 22 filters in both 1.25" format and 2" format and have used them in all positions known to man. I have never been able to detect any difference in optical performance resulting from different placement of the filter in the system. Thats not to say that sensitive optical instruments couldn't pick a difference, just that my eye can't do it :)

I figure, use the filter wherever it takes your fancy. Just be warned that if you use a 2" filter on a 48mm threaded 1.25"/2" adaptor, you may need a parfocalising ring or some other physical stop to prevent the eyepiece protruding too far into the adaptor and physically damaging the filter. I find it convenient to stick the filters on the end of the adaptor and not have to unscrew them when you change eyepieces. I use an old aluminium parfocalising ring as the physical stop as my Pentax XW's have longish barrels and it works great.

CS-John B

[Rodstar]

John, sounds like you have read more of the Kama Sutra than I. :wink2:

[Ziggy]

Nice riposte Rod!

Thanks guys for the responses. If you guys can't tell any difference where the filter's placed, then there's no way I'm going to. So that's good to know.

Just looking at the prices of these filters, I find it hard to accept buying one 2" filter, when I could buy two 1.25" filters for the same price, or less!

And as the 22mm Pan is going to be my main eyepiece for a while, maybe that's the way I should go. But I'm always trying to look ahead, and what if I get 2" eyepieces?? Will have to give it some more thought.

Still, as I said, it's good to know that it doesn't affect performance greatly where the filter is placed.

I like the sound of your motor focuser Rob, but for the time being I'm happy to stick with visual observing. I did think about an electronic focuser actually, but it seemed convenient to just get both diagonal and focuser together from WO.

Rod, I see you have the WO Crayford, have you received it yet? If so, how does it work for you?

Anyway, once I decide which size to get, then I have to decide which make to purchase :confuse2:

But it's all fun ;)

Glen.

[ausastronomer]

Rodd,

If I keep going the way I am in the domestic stakes, the Kama Sutra will be but a distant memory :)

Glenn,

I own the Astronomiks OIII and UHC in 1.25" format and they are my 2 most used filters. I really wish for practicalities sake they were both 2" filters but at $400 each in 2" format that compensates for a lot of unscrewing IMO :)

If you go for some cheaper filters like the DGM Optics NPB, its definately worth going to the 2" version as the larger filter is only another $50. I own the 2" DGM NPB filter also BTW and its very good, not quite as good as the Astronomiks IMO but at 1/3rd the price its much better value.

CS-John B

[Rodstar]

Glen, the WO Crayford is due to arrive any day (Daniel Beringer.....I hope you are reading this ;) ).

I am buying it having already had a really good observing session using Louie's (Atalas). It is a wonderful gadget....so good I sold a Nagler to help finance its purchase. You might look for Louie's input as well on the subject.

I, like you, am sticking to the visual observing at this stage, and I am striving for as fine a focuser as I can, especially as I pursue my interest in double stars. I can't stand the mirror flop from the manual focuser on my scope, and I find the LX200 microfocuser is not at all intuitive or easy to sharply focus compared with the WO crayford. I might add, the WO crayford is a mighty fine looking piece of gadgetry too! Its main drawback is that it extends back too far from the back of the OTA in my scope to enable viewing around the zenith (it hits the bottom of the fork). For me, I am happy to put up with that, when I weigh it against the wonderful performance of the focuser.

[h0ughy]

is he importing 2 of these, I would like one too.

Like John I haven't found much difference visually having the filter elsewhere in the optical train but I would think it may make a difference imaging.. my 5 cents worth

[Rodstar]

Yeah, HOughy, I believe Daniel (Frontier Optics) is getting a supply of them, as part of a January shipment from the US. He is doing a special IIS price discount, so make sure you mention ISS! ;)

[h0ughy]

Can you PM me the "special price" I need to see if my budget can handle it. :lol:

[Rodstar]

I have PM'ed you HOughy with the figures.