Using a x0.63 or x0.33 Reducer? - Read on

[Merlin66]

I've always been told the "optimum" back focus distance for a x0.63 reducer was 110mm.....

I've just re-measured the focal length of my Meade x0.63 reducer at 240mm ( from back edge of the main body)

Using this figure I find the back focus distance, to achieve f6.3 on a f10 SCT is more like 90mm!!!!!!!!!

My calculations are in the spreadsheet "Reducers.zip"

Anyone want to correct me or add comment????

reducers.zip

[sgazer]

I read in a book somewhere that the distance to chip on some meade FR's was wrong due to a manufacturing problem. I don't know if this could be the issue you are seeing.

[Merlin66]

There was a batch a few years ago made wrong....

Can anyone with a x0.63 reducer measure it's focal length and report back. (Easy to do - just focus the Sun's image on a piece of card and measure the distance from the body to the card.)

There's another variable which I've just included:

The effective focal length of the SCT varying with backfocus.

Lord published details:

http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EFLMAKCASS.pdf

I've just included this data and get similar but different results???

reducers_lord.zip

[Psychobilly]

Hmm might do this with the FFIII later...

[riklaunim]

Check this links:

- Cloudynights

- undermidnight.com

f/3.3 reducer is by name designed for f/3.3 and should do good on slightly higher focal ratios.

[sgazer]

Can anyone with a x0.63 reducer measure it's focal length and report back. (Easy to do - just focus the Sun's image on a piece of card and measure the distance from the body to the card.)

QUOTE]

....made me laugh, easy to do.....just finding the sun is the hard bit!

[Merlin66]

Riklaunim,

Thanks for the links.

At this stage I'm NOT interested in the optical properties of the reducers but the degree of magnification ( plate scale/ EFL/ f ratio) they give at various distances between the CCD and the reducer.

My need is to focus an on axis star image, so edge coma etc etc doesn't mean anything to me ( one of the additional benefits of spectroscopy!!!)

I have a need to match the effective focal ratio of the telescope and the f ratio of the spectroscope.

The immediate challenge is to see which reducer ( if any) can give me a f7 ( or f8) entrance beam to the slit. The minimum distance between the slit and the reducer is 120mm. So far it appears the standard reducers won't do the job; I may need a achromatic doublet of about 400mm focal length to get the f7 I'm looking for.

The problem of varying plate scale affects all imagers with reducers.

Ken

[Peter Drew]

Hello Ken. The Sun's just come out! Meade F6.3 reducer backfocus 235mm, F3.3 reducer backfocus 90mm (+/- imm due to hand shaking with the cold!)

[Merlin66]

Peter,

Appreciate that, your 235mm is getting close to my 240mm so, they are in the same ball park ( on the Cloudy Night link they talk about 260mm for the Meade and 285mm for the Celestron version???)

The 90mm for the x0.33 is the same as I measured, against the "quoted" 88mm.

Members: Are there any more measurements available???

Has anyone got some plate scale data on reducer distances with a SCT.....??

[Macavity]

Can't you take a few terrestrial shots - Chimney pots and the like, under varying conditions. If need be, scale-compared to a full moon, measured on a printout. I sense such would be more accurate than e.g. measuring focal length of reducers. The question alway remains too, where exactly any such "length" originates - The lens holder, the node point(?) of the non-zero thickness glass etc. etc.

Sorry if I've misunderstood the question.

[Merlin66]

Yes is the basic answer... it doesn't matter what you image as long as you can calculate the plate scale ie arc sec/mm.

(The calculations may be ?? affected by the position of the primary focus when imaging/ viewing nearby objects - the focus moves rearward- shouldn't be an issue if the reducer is in a fixed position on the rear cell -I think:icon_scratch:)

The reducers are normally measured to the rear body part inboard of the thread, this is pretty close to the optical centre of the lens, and accurate enough for this exercise.

What I'm seeing is that the previous "recommended" distance of 110mm between the reducer and the CCD doesn't give f6.3 ( on a f10 SCT) and a distance of 90mm seems more realistic, IF the f ratio / plate scale is important.

[riklaunim]

Use maybe filter threaded reducer? Easy to make desired reduction.

[Merlin66]

Riklaunim,

The filter reducers are not a long enough focal length ( about 80mm), they don't allow the back focus I need (120mm). They're OK for webcams but IMHO not much use for anything other.

[SpookyKatt]

Hi Ken,

Just checked my Antares F6.3 reducer backfocus and it was 240mm.

kathleen

[Merlin66]

Sounds like the "average" is going to be around 240mm.

If you use the first "basic" spreadsheet:

95mm = f6.3

110mm = f5.4

120mm = f5

whereas incorporating the Lord formula:

95mm = f7.5

110mm = f7.1

120mm = f6.9

Quite a difference!!

Which one is more correct????