Meade series 4000 FF/FR back focus distance

[lensman57]

Hi,

Sorry to ask this question as I am sure it has been posted before, but I do have a Meade f6.3 reducer, it is a later model marked CHINA, does any one know the correct distance for this reducer from the seat of the male SC ( back of the reducer ) thread to the chip. I am a bit confused as it is normally quoted as either 110mm, for Antares, same design, 85 mm for Celestron, same design and 95 mm for Meade, same design. I have also read once in a post on this forum that the later CHINA marked ones were 55 ~ 56 mm. I would really appreciate any light on this matter as I have to machine an adapter tor my ATIK and the correct distance is obviuosly crucial.

Many thanks,

A.G

[Oddsocks]

Hi A.G.

The older series Meade 6.3 reducer was 100mm to 110mm glass to chip distance.

The newer series Meade 6.3 reducer was/is 45mm glass to chip distance (reduced to help camera clear the base of the fork mount).

Interestingly, this was published on the Meade web site in the support section until last week when Meade appeared to do a do a rehash of the site, that page has now disappeared.

I had to get an adaptor made for a SXV H9 and the older long length reducer a few years back, sold off long ago, but got the spacer made in two halves, one half threaded tube screwing down into the other.

This gave me a range of adjustment of 70mm to 140mm with no sag. Just a couple of simple grub screws used to lock the two halves together.

Camera rotation angle was set by using shim washers under "T" mount thread.

Might be an idea for you to consider?

Here is a link to a long thread discussing the Meade reducer spacing:

http://tech.dir.grou.../message/119130

William.

[lensman57]

Hi A.G.

The older series Meade 6.3 reducer was 100mm to 110mm glass to chip distance.

The newer series Meade 6.3 reducer was/is 45mm glass to chip distance (reduced to help camera clear the base of the fork mount).

Interestingly, this was published on the Meade web site in the support section until last week when Meade appeared to do a do a rehash of the site, that page has now disappeared.

I had to get an adaptor made for a SXV H9 and the older long length reducer a few years back, sold off long ago, but got the spacer made in two halves, one half threaded tube screwing down into the other.

This gave me a range of adjustment of 70mm to 140mm with no sag. Just a couple of simple grub screws used to lock the two halves together.

Camera rotation angle was set by using shim washers under "T" mount thread.

Might be an idea for you to consider?

Here is a link to a long thread discussing the Meade reducer spacing:

http://tech.dir.grou.../message/119130

William.

Good afternoon William,

Many thanks for your reply, I measured the focal length of the reducer and it is about 200 mm so I am puzzeled now as to which camp this one belongs to. The 45mm I presume is from the bak end of the glass element if I am wrong please correct me.

Thanks and Regards,

A.G

[Oddsocks]

Hello A.G.,

Thats right, the length is measured from the optical surface.

The reason often given is that the the Q.C. during manufacture could be a bit lax and the glass elements could be positioned with some variability in the corrector barrel.

I'm not very clued up on the optical maths involved but I believe when you determine the natural stand-alone focal length of a flattener/reducer using the focussed star or moon projected on to white card then the length measured is halved for the working point in the image chain, i.e. the reducer is not placed at the prime focus point of the telescope but is placed in the focal plane in front of the prime focus.

At least that is what I read elsewhere but I couldn't explain why and the document wasn't clear if this was true of a flattener, reducer, or both.

When I had my original Meade f6.3 reducer it came with a camera connector tube of 100mm length but the thread at the camera end was not a "T" thread hence I had to get one made as when I tried a 48mm? to "T" adaptor it pushed the total length too far and the image had some coma, in the end I remember 98mm being the final distance used (but that was fifteen years go and couldn't swear to it now).

So for your reducer the distance would be 100mm making it one of the older Meade flatteners.

I can't think that Meade would design a flattener with a 200mm back-focus distance on purpose as the camera would collide with the mount too easily?

There were a few posts about that further down in that tech group thread I linked to.

Hopefully an SGL member with some optical expertise will confirm whether this is correct or not, focal reducers do seem to be a bit of a black art.

William.

[lensman57]

Hello A.G.,

Thats right, the length is measured from the optical surface.

The reason often given is that the the Q.C. during manufacture could be a bit lax and the glass elements could be positioned with some variability in the corrector barrel.

I'm not very clued up on the optical maths involved but I believe when you determine the natural stand-alone focal length of a flattener/reducer using the focussed star or moon projected on to white card then the length measured is halved for the working point in the image chain, i.e. the reducer is not placed at the prime focus point of the telescope but is placed in the focal plane in front of the prime focus.

At least that is what I read elsewhere but I couldn't explain why and the document wasn't clear if this was true of a flattener, reducer, or both.

When I had my original Meade f6.3 reducer it came with a camera connector tube of 100mm length but the thread at the camera end was not a "T" thread hence I had to get one made as when I tried a 48mm? to "T" adaptor it pushed the total length too far and the image had some coma, in the end I remember 98mm being the final distance used (but that was fifteen years go and couldn't swear to it now).

So for your reducer the distance would be 100mm making it one of the older Meade flatteners.

I can't think that Meade would design a flattener with a 200mm back-focus distance on purpose as the camera would collide with the mount too easily?

There were a few posts about that further down in that tech group thread I linked to.

Hopefully an SGL member with some optical expertise will confirm whether this is correct or not, focal reducers do seem to be a bit of a black art.

William.

Hi William,

Many thanks again for your informative reply, I found the formula last night while researching for this but I don't know where it was, it calculated about 77 mm, and it then said something that the reducer had to be placed a further 30 mm away from the prime focus of scope, ie in total it is about 107 mm which sounds about right I am going to use it with my SW 100mm F9 ED scope so I am glad in a way that it is not one of the 45 mm ones as these will be more difficult to adapt. If I get it going then I will post about it.

Regards,

A..G