MN190 and a focal reducer

[Catanonia]

Ordered the Antares 2incher from Rother on a trail to see if it will work

Watch this space

[nightvision]

Really interesting thread, I just got a Ant 0.5 reducer (bought on ABS from Greece). I planned to get the 100ED from F9 to 4.5. All the issues mentioned with back focus and reducer to CCD distance are apparent. Visually it works well on my Nagler 17/T4 but I will need to come up with some other nose-piece arrangement to get enough focus for the 300D. Removed the focuser assembly and held the reducer against a M42/EOS adapter and it worked but with some vignetting.

Also tried it in the 12" F5 dob, definitely needs a low profile focuser to get close to making it work.

I see a lot of DIY focuser and adapter fiddling in my future.

[Catanonia]

Postie has been and the reducer and t thread extenders are sat at home awaiting me

Now for some measuring and waiting for clear skies when I get home.

Just need a few stars

[nightvision]

I'll be interested in the size/depth of T thread adapters you use and your focus challenges. My EOS adapter seems quite long so the filter is about 80mm from the sensor. Focus on my ED100/F9 will work if I can get the reducer a little closer to the sensor. Your scope has a nice low profile focuser so maybe you won't have any focus travel problems.

On the vignetting effects, I reckon if you have a big sensor (anything bigger than about 1mp) then its not that much sky to degrade a little.

[nightvision]

I am curious about the reducer to sensor distance in your calculations. How did you arrive at 46mm? My EOS adapter gives about 80mm, do you have a lower profile adapter?

My calcs below:

Current distance Reducer to Sensor and what I hope to achieve by moving it closer so that the inward-focus problem goes away.

[Catanonia]

I am curious about the reducer to sensor distance in your calculations. How did you arrive at 46mm? My EOS adapter gives about 80mm, do you have a lower profile adapter?

My calcs below:

Current distance Reducer to Sensor and what I hope to achieve by moving it closer so that the inward-focus problem goes away.

Ok here are my calculations

CCD chip - Camera Edge = 19mm

Camera Edge to filter wheel edge = 19mm

2inch Nosepiece 40mm ++

So from CCD to the filter wheel edge = 38mm +- 2mm + 40mm for noisepiece and 10mm reducer width = way to much.

So I want to place the reducer as close to the filter wheel as possible.

If I screw it directly onto my 2 inch nose piece I will have CCD chip to reducer of 80 - 90mm and F2.4 ish massive inwards and a visible image region of a few mm.......... Problem. So need to get the reducer much much closer to the chip.

Now I have the reducer I have a problem so a question

1. Does it matter which way around I place the reducer ??????

My plan is to place it on the filter wheel side of the 2 inch nose piece hence close to the wheel and CCD chip (it has threads)

This will place it closest as possible to the filter housing and hence give me about 40 - 42mm from the CCD BUT the reducer will be on backwards.

The nice thing is that the threads for the QHY are M54 and the reducer nicely fits into this radius of the 2inch nosepiece (M42)

So with a M54 extender I should be able to get the reducer attached to the filter wheel pretty much flush to it and give me around 40mm CCD to reducer distance with inward travel needed on focuser of 12 - 19 ish I have available about 20mm

See the attached mock up.

CCD = camera housing, FW= FilterWheel

1. The reducer screws into the filter threads backwards on the 2inch nosepiece at the filterwheel end. This actually sits inside the M54 threads

2. The M54 11mm extender shrouds the reducer and allows 2inch nosepiece M54 threads to extend over the reducer and attach to the filter wheel on its M54 threads.

3. The reducer is now as close to the wheel as possible BUT BACKWARDS

The reducer is 10mm deep and the M54 extender is 11mm plus I have about 4mm place where the reducer can sit inside the filter wheel before being obstructive

[Beyond_Vision]

Not sure if this will end up causing chromatic abberation and vignetting problems if 1.25" filters were used.

[Catanonia]

Not sure if this will end up causing chromatic abberation and vignetting problems if 1.25" filters were used.

I am using 2inch filters

[Catanonia]

I'll be interested in the size/depth of T thread adapters you use and your focus challenges. My EOS adapter seems quite long so the filter is about 80mm from the sensor. Focus on my ED100/F9 will work if I can get the reducer a little closer to the sensor. Your scope has a nice low profile focuser so maybe you won't have any focus travel problems.

On the vignetting effects, I reckon if you have a big sensor (anything bigger than about 1mp) then its not that much sky to degrade a little.

That image is exactly my problem, screwing the reducer on the end of the nosepiece == way to much reducer to ccd distance.

My plan is to put the reducer inside the nosepiece at the CCD end but BACKWARDS. I ca do this as my external threads are M54 compared to the M42 size of the reducer.

[riklaunim]

Not sure if this will end up causing chromatic abberation and vignetting problems if 1.25" filters were used.

I did a bahtinov mask + parfocal Orion RGB filters test with a simple filter threaded reducer and it didn't shifted focus between R and B. SCT f/6.4 Hirsh reducer at ~115mm did shift them a bit. Short distances seem to produce less aberrations.

[Catanonia]

mmmm the online calculator assumes a refractor... Does this follow the same logic for reflectors ???

[Beyond_Vision]

I am using 2inch filters

But what about CA as you are introducing lenses into the optical path

[Catanonia]

But what about CA as you are introducing lenses into the optical path

Hence the testing and giving it a go. To be honest I am winging this as I go along.

The plan is with a simple screw filter I MIGHT be able to get a MN190 from FL1000mm F5.3 to FL600 - 700mm and around F3.x

That would put a £1000 scope + £60 parts in the league of an AG8 for a fraction of the cost.

I got the idea from those fast newts that actually incorporate reducers inside thier focusers.

[Beyond_Vision]

If it were that easy then it would have most likely been done before. Hence the focal reducer for an APO refractor costing £300+ due it also needing to be apochromatic and hence carefully designed.

Still worth a try though

[Catanonia]

If it were that easy then it would have most likely been done before. Hence the focal reducer for an APO refractor costing £300+ due it also needing to be apochromatic and hence carefully designed.

Still worth a try though

spoilsport

Well if it works, I will be FAMOUS

[nightvision]

Done some testing and I think it does matter which way round, the lens appears to be asymmetric like most objective lens groups. But I think I will try some more tests to confirm.

[Catanonia]

Done some testing and I think it does matter which way round, the lens appears to be asymmetric like most objective lens groups. But I think I will try some more tests to confirm.

Well it is held in with cir clips, so I can always flip it if need be

[nightvision]

Just completed some tests, the field flatness is very different so the reducer is definitely asymmetric, produces bad images reversed, tested as an objective lens with a 50mm eyepiece.

I have the same challenges that you face but easier to deal with on a refractor.

[Catanonia]

Just completed some tests, the field flatness is very different so the reducer is definitely asymmetric, produces bad images reversed, tested as an objective lens with a 50mm eyepiece.

I have the same challenges that you face but easier to deal with on a refractor.

Thanks mate, i was wondering what the difference would be. So I need to circlip it out and flip it.

[Catanonia]

If it were that easy then it would have most likely been done before. Hence the focal reducer for an APO refractor costing £300+ due it also needing to be apochromatic and hence carefully designed.

Still worth a try though

So different focal points per RGB filter then ?

If so, easy enough to do ?

[nightvision]

So different focal points per RGB filter then ?

If so, easy enough to do ?

That's a very good point, makes mono CCD AP look like the way to go when you consider you don't really need to buy an Apo!

Thinking this through.... ST 150mm Achro F5, add reducer for F2.5 to F3.5.... £475+reducer

Or even cheaper an ST80 Achro F5 for £90

[Catanonia]

That's a very good point, makes mono CCD AP look like the way to go when you consider you don't really need to buy an Apo!

Thinking this through.... ST 150mm Achro F5, add reducer for F2.5 to F3.5.... £475+reducer

Or even cheaper an ST80 Achro F5 for £90

LOL surely it can't be that easy can it

[riklaunim]

Carina Nebula - NGC 3372 | Flickr - Photo Sharing! - f/5 vixen flat achro reduced with a flattener to f/4 It's hard to reduce the f-ratios, but slight reduction are doable without a disaster or advanced optical constructions.

[nightvision]

Carina Nebula - NGC 3372 | Flickr - Photo Sharing! - f/5 vixen flat achro reduced with a flattener to f/4 It's hard to reduce the f-ratios, but slight reduction are doable without a disaster or advanced optical constructions.

Nice image. Moderation does seem to be the way when reducing. With that in mind I have devised a solution (well on the road to one).

Took apart the EOS adapter and fitted the reducer inside the adapter. There is enough space around the reducer to allow for a collar to join the 2" tube back on. This gives me only a 25mm inward focus requirement and a moderate .67 FL reduction; F6 is an improvement over F9. The 29mm imaging circle also means no vignetting on the 28mm 300D CCD.

[Catanonia]

Sweet mate

I agree moderation, i don't want 0.5 reduction more like 0.3 or thereabouts hence me wanting to get it as close to the ccd chip as possible.

Ok need some images before and after