New Off Axis Guider

[Gina]

It looks like it should work with my ED80 FR/FF and Atik EFW2 with 314L+ but I can't try it because another adapter has "cold welded" itself to the FR/FF and being only thin metal and very short, there isn't anything to grip or drill holes into

I've got it off :) Yippee

And... I've looked at the spacing - it will definitely work but may require a spacer - need to make more exact measurements.

[AndyUK]

I've got it off :) Yippee

Congrats, Gina - It's very frustrating to have a twiddly spacer stuck in a place where you can't get your hands into to get sufficient grip... but more than matched by the satisfaction of that split second when you finally feel it budge and you know you've just beaten it .

Steve - A most excellent review, sir . My in-focus issues aside with my MN190, I've decided I'll definitely be getting one of these OAG's later in the season...

[Gina]

Congrats, Gina - It's very frustrating to have a twiddly spacer stuck in a place where you can't get your hands into to get sufficient grip... but more than matched by the satisfaction of that split second when you finally feel it budge and you know you've just beaten it .

Steve - A most excellent review, sir . My in-focus issues aside with my MN190, I've decided I'll definitely be getting one of these OAG's later in the season...

Thank you Andy I've assembled everything and carefully measured the spacing (though only with a 6" rule) and the spacing seems to be spot on. That's (from the front) SW 0.85x FR/FF directly onto the OAG then Atik T2 to 54mm adapter, Atik EFW2 and Atik 314L+. I make it 54mm from FR face to sensor so allowing something like 0.5 to 1mm for filter, I reckon that's within 1mm of the correct spacing. So it looks like I shall be using the OAG and Lodestar with my ED80 Looks like this OAG was designed to work in this setup

I have this OAG working nicely with my MN190 with Lodestar guider and Atik 314L+ with EFW2 with an additional 11mm spacing between EFW2 and OAG to get frocussing with both cameras at the same time.

[AndyUK]

I have this OAG working nicely with my MN190 with Lodestar guider and Atik 314L+ with EFW2 with an additional 11mm spacing between EFW2 and OAG to get frocussing with both cameras at the same time.

Gina - On the MN190, we have similar kit, although I have a Trutek filter wheel, which, including adapters is 24.5mm and I have a moonlite focuser, which I think is where I'm getting stymied with in-focus - With the width of the OAG (inc flanges) I need 15mm in-focus available... and unfortunately I only have 14mm . I do have a 10mm spacer which includes a 1.25" filter holder though, so my plan is to use that instead of the filter wheel (saving 14.5mm ), but of course that then means 1 filter / session. However, just as a matter of interest, (and next time your out) I wonder if you could do me a small favour? What's the light-path distance of your EFW2 (including adapters) and when you're in focus, how much in-focus travel do you have available on your focuser?

Sadly my DIY skills won't stretch to the cheaper option of changing the focuser back (there's no way I'm ever taking that corrector plate off!), but if the EFW2 might get me just a couple of mm. then I think it might JUST work...(?)

[Gina]

Gina - On the MN190, we have similar kit, although I have a Trutek filter wheel, which, including adapters is 24.5mm and I have a moonlite focuser, which I think is where I'm getting stymied with in-focus - With the width of the OAG (inc flanges) I need 15mm in-focus available... and unfortunately I only have 14mm . I do have a 10mm spacer which includes a 1.25" filter holder though, so my plan is to use that instead of the filter wheel (saving 14.5mm ), but of course that then means 1 filter / session. However, just as a matter of interest, (and next time your out) I wonder if you could do me a small favour? What's the light-path distance of your EFW2 (including adapters) and when you're in focus, how much in-focus travel do you have available on your focuser?

Sadly my DIY skills won't stretch to the cheaper option of changing the focuser back (there's no way I'm ever taking that corrector plate off!), but if the EFW2 might get me just a couple of mm. then I think it might JUST work...(?)

OK, I'll check the measurements and let you know.

[Tim]

Andy, if you struggle you can try my TS OAG which is a couple of mm shorter. I know it works with MN190 and Trutek wheel I did get the Trutek guy to make me some shorter fittings though. I'll not be going back to the TS OAG now

[Ibbo!]

so you rate much better than the TS then

Steve

[AndyUK]

Andy, if you struggle you can try my TS OAG which is a couple of mm shorter. I know it works with MN190 and Trutek wheel I did get the Trutek guy to make me some shorter fittings though. I'll not be going back to the TS OAG now

Cheers Tim - That might be very useful - Thanks ! As I noted, I'm suspecting it might be the Moonlite focuser that's hampering me, but at least it will enable me to see if my calculations are correct. If I am right, that couple of mm will make all the difference... and if my calcs are wrong and I find I still have another 2mm spare, then the OVL version might come back into play. Next time I'm due to be passing by I'll drop you a PM (if that's okay?).

[Gina]

The Atik EFW2 is 22.3mm thick (with Atik ‘T’ adapter) I think tht's your couple of mm saved if you want to do it that way. Have to say the EFW2 if a superb filter wheel. Great design and excellently engineered. Review here :- http://www.firstlightoptics.com/user/atik-efw2-filter-wheel-oag-review.pdf

[AndyUK]

Thanks Gina - That would indeed seem to get me back 2.2mm, which may well just be enough! Not wishing to clutter up this thread with my own specific needs I'll PM you .

[dmahon]

The filter pushes the focus out by 0.5 to 1mm too (ie if you need 55 from a reducer normally, with a glass filter you will need 55.5-56), so that may help. The glass in front of the CCD may not have been taken into account too, which will help a little again.

[swag72]

The filter pushes the focus out by 0.5 to 1mm too (ie if you need 55 from a reducer normally, with a glass filter you will need 55.5-56), so that may help. The glass in front of the CCD may not have been taken into account too, which will help a little again.

I hope that someone can clarify this - I understand the filter spacing differently.

My understanding is that if you need a spacing of 55mm, then you actually need 54mm physically as you add 1mm in for the filter. You are saying that if you need 55mm spacing you actually have 56mm physically if I understand your post correctly?

[Sp@ce_d]

I hope that someone can clarify this - I understand the filter spacing differently.

My understanding is that if you need a spacing of 55mm, then you actually need 54mm physically as you add 1mm in for the filter. You are saying that if you need 55mm spacing you actually have 56mm physically if I understand your post correctly?

That's the way I work it out too?

[AndyUK]

This spacing query would probably be better in the main imaging section to get a balanced input(?), but I too got a little confused about this recently and although I received conflicting information( ), the consensus was that you need to ADD in 1/3 of the filter width (which many estimate is 1mm due to filters being c.3mm deep). So to obtain 56 mm spacing for a reducer you might have:

13mm - Sensor chip to camera shoulder

25mm - Filter wheel (inc adapters)

1mm - Filter allowance

TOTAL 39mm

Additional spacing requirement = 17mm

[opticalpath]

My information agrees with Sara, that the addition of a filter to the optical path *reduces* back focus distance by 1/3 of the filter thickness.

I take this info from the guidance document on this very subject that QSI publishes for its cameras .... and they are reliable in my experience.

http://www.qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

The document helpfully tabulates the different reduction amounts for Astronomik, Baader and Astrodon filters.

Adrian

[opticalpath]

OK, now I've just read some posts on a forum by Terry Platt and Don Goldman that contradict what I just said! They say that adding a filter to an otherwise empty slot in your filter wheel pushes the focal plane further *out* by 1/3 of the filter thickness - i.e. you would in effect need an additional thin spacer to maintain focus.

I don't know what to believe, now ....... will have to try it out for myself!

Adrian

[swag72]

I emailed Don about this last week as I was getting conflicting information. I thought Don should know!

I'm sure that he said that if you need a spacing of say 55mm, then you attain a physical spacing of 54mm and that allows for 1mm for the filter, taking you to an overall 55mm. I need to find his email!!! Back in a bit!

Here you go ........ a copy of what Don emailed me back.

Yes, a 3 mm filter will add 1 mm of backfocus from the reducer to the detector, so if you need to be 50 mm away (this is an optical distance specified by the telescope maker) with a 3 mm intervening filter, the physical separation should be 49 mm

[AndyUK]

Hi Adrian - Welcome to the club ! This is where I was a week or two ago - I thought I knew, and then I came across conflicting information

However, the way I read that QSI link and the Terry Platt / Don Goldman comment, I think they're actually agreeing with each other. The latter are stating that if you DON'T have a filter in then you have to add (say) an extra 1mm spacing, which therefore means that if you DO add a filter, then you need 1mm less spacing.

... And QSI are saying that "the addition of a filter to the optical path *reduces* back focus distance by 1/3 of the filter thickness"... which means if you didn't have a filter in, then the spacing requirement would need to be 1mm more(?)

[Oh dear - It's been a long week - My head hurts...!]

[dmahon]

Borrowed from another forum:

here is a quote from Don Goldman posted on the ccd-newastro yahoo news group awhile ago...he knows a thing or two about filters:

>>

Re: Filters & Back Focus Distance

Camera manufacturers show a smaller optical backfocus than mechanical backfocus

because they are measuring the distance from the imaging CCD focal plane. This

includes detector chamber window and sometimes coverslips on the detector.

HOWEVER, that is not where WE measure backfocus from. We want to know how much

space we need to add from the metal back of a telescope or from a field

corrector/reducer to the imaging focal plane. This is how we select spacers,

etc. So, our starting point is the scope.

THEREFORE, the addition of a 3 mm filter ADDS 1 mm [t * (n-1)/n] of backfocus

between the scope and your camera (t is the filter thickness and n is the

refractive index of the substrate of the filter - typically 1.5) So it ends up

being t / 3. 3/3 = 1mm in this discussion. You need to ADD 1mm of space between

your scope and your camera.

This is often confusing. When you place a filter in a beam of light that

converges from left to right, the focus is extended FURTHER right INCREASING the

backfocus distance as measured from the scope.

All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED.

<<

you can find the full thread here:

http://tech.dir.groups.yahoo.com/group/ccd-newastro/message/67993

[dmahon]

I read it as:

The focal point is 55mm from the back of the reducer. You add a (3mm thick) filter. The focal point gets moved out by 1mm (3mm / 3) to 56mm. You need to add a 1mm spacer to obtain the correct focus.

[Zakalwe]

Are these measurements really that critical, where we are talking about fractions of a millimetre?

The Televue TRF gives the tolerance as +/- 4mm, for example.

[opticalpath]

This is often confusing. When you place a filter in a beam of light that

converges from left to right, the focus is extended FURTHER right INCREASING the

backfocus distance as measured from the scope.

All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED.

This is exactly the forum posting I was referring to. The conclusion seems the opposite of what the QSI document is saying ..... hence my total uncertainty now.

Adrian!

[dmahon]

The QSI document is talking about the amount of backfocus space taken up by the QSI camera. Draw the light path diagram (the blue lines are what would happen without the filter)

[opticalpath]

OK, I'm convinced ...... the focus of a converging beam is lengthened by the effect of inserting a plane window/ filter

Adrian

[opticalpath]

Haha - same diagram at the same moment!

Adrian