MN190 and a focal reducer

[Catanonia]

Messing around with the possibilities of reducing the MN190 FL1000mm F5.3 down a step or two with a reducer.

I tried out the WOII reducer / flattener I use with the WO66 and of course it didn't work

Now looking at a

Antares 0.5x focal reducer in the 2inch variety as googling around seems to be the only alternative.

Has anyone had any luck this these with

1. Newtonians ?

2. Skywatcher or Orion MN190 ?

[Catanonia]

Well ordering one from Rother Valley Optics on trail to see if it works.

Thanks Rother Valley

[riklaunim]

You need backfocus for it to work. I forced 0.5x 1,25" reducer to work with 150/750 newtonian some time ago, but at a shorter CCD-reducer distance (less reduction - less backfocus needed). So short camera nosepiece, and the camera as deep in the focuser as possible

[AndyUK]

Hi Catanonia - I'll be very interested in your report on whether that works... If it does, that small expense might enable me to potentially "release funds" to put toward a CCD camera...

[Catanonia]

The only problem I can foresee if focusing travel.

I have about 18mm to play with and the reducer is 10mm thick so that is covered.

However further reading say that the optimal distance for it is 79mm from the chip and I think I will be more like 55mm which according to the blurb will be more than 50% reduction and will probably lead to issues.

Now I can shift it with spacers, but will that affect inward travel on the focuser...

Only one way to find out.

[ollypenrice]

You're, asking a lot to get well below f5! Exciting if it works though.

Olly

[Psychobilly]

After a googling session.. it looks like it may be possible to do it but not cheaply if you want to maintain the field flatness... good luck .

Billy...

[riklaunim]

Here is an image of M 10 from 150/750 Newtonian with the 0,5x reducer on a DMK21 with a short nosepiece:

Pixel scale:3.08 arcsec/pixel

And with DSI III Pro: All sizes | M5... or M10 | Flickr - Photo Sharing!

MN would be coma free and if you don't push the reducer to far away from the CCD you shouldn't get field curvature from the reducer.

[Catanonia]

You're, asking a lot to get well below f5! Exciting if it works though.

Olly

Around F4 would be nice and feesible, depends on spacing and focus.

[Catanonia]

Rik, thanks for the image, I guess might have issues with a larger chip format of the Kodak 8300. But as you say the image is already flat from the MN190 corrector lens, so just a matter of reducing it.

Vingetting might by a problem, but flats should solve that.

Only thing I can't work out is the reduction.

According to websites, 79mm lens - CCD = 50%. More distance = less reduction, less = more reduction.

So at 56mm compared to 79mm I would be pushing past 50% which is bad.

But others say it is the other way around and at 56mm I could be around 35% or less which would in principle be nice. That would be around F3.4

[Merlin66]

Based on 79mm giving you 50% reduction I estimate that the focal length of the reducer lens is 160mm. The shorter the distance between the reducer and the chip the less the reduction.

ie 55mm spacing would give you x0.66 reduction.

Note also that the lens has to be positioned about 160mm inside the original focus (for x0.5)!

I have a spreadsheet which calculates these things (including allowance for the main mirror shift) available if needed.

[Catanonia]

Based on 79mm giving you 50% reduction I estimate that the focal length of the reducer lens is 160mm. The shorter the distance between the reducer and the chip the less the reduction.

ie 55mm spacing would give you x0.66 reduction.

Note also that the lens has to be positioned about 160mm inside the original focus (for x0.5)!

I have a spreadsheet which calculates these things (including allowance for the main mirror shift) available if needed.

Please mate if you could PM or post.

[Merlin66]

I've attached a copy of the generic spreadsheet for reducers.

Just enter "160" into cell A26 and you'll see the required spacing.

Only calculates the reduction factor NOT the optical aberrations!!

reducers_V2a.zip

[Catanonia]

I've attached a copy of the generic spreadsheet for reducers.

Just enter "160" into cell A26 and you'll see the required spacing.

Only calculates the reduction factor NOT the optical aberrations!!

Fantastic mate, by my reckonings, I would be looking at F5.3 to F3.8

Just a question that does concern me,

"What does it mean by INSIDE DISTANCE ?"

Lost at this term and can't picture it.

[GrantEb]

Just for info, I had a TS 0.5x FR 2", and the amount of "glass", was only about 40mm in diameter. The Antares may be completly different though.

[Catanonia]

Just for info, I had a TS 0.5x FR 2", and the amount of "glass", was only about 40mm in diameter. The Antares may be completly different though.

thanks Grant, according to the specs, the Antares is 10mm thickness or so they say

Fingers crossed.

[Merlin66]

The "inside focus" is the distance that the reducer must be placed in front of the original focus. This this sometimes overlooked, and can be difficult to achieve in some optical set ups.....then users complain they can't bring the reducer to a good focus.

HTH

[martin_h]

Focal Reducer Calculator

[Catanonia]

The "inside focus" is the distance that the reducer must be placed in front of the original focus. This this sometimes overlooked, and can be difficult to achieve in some optical set ups.....then users complain they can't bring the reducer to a good focus.

HTH

ah that might be a problem.....

[angusb1]

I've always found this online calculator to be really useful Focal Reducer Calculator

[Catanonia]

Focal Reducer Calculator

Thanks mate, just plugged all this into the calculator and I will need 20 - 30 mm of infocus + the 10mm for the reducer = additional upto 40mm....

Will be very tight, will have a check later and see if this is going to be possible.

Would be great if the reducer would actually fit inside the actual focuser tube.

[Catanonia]

Been doing some measurements etc,

I think, if the reducer works as hoped that I will get the MN190 down from F5.3 to around the F3.4 - F3.8 depending on spacing. There will be a bit of vignetting, but should still be a flat field and nothing flats can't sort out. That will be pushing the AG8 boundries of scope for 50 notes.

I need around 30mm inward travel and just measured up and I am 'close' to this being available.

Just waiting for Rother Valley to get stock in for a trail unless someone will lend me a Antares 0.5 2inch reducer to try out.

Does anyone know what the threads on the Antares 2inch 50% are please. I can't find this information.

[Merlin66]

The reducer thread appears to be the standard 48mm filter thread.

[Coco]

Been doing some measurements etc,

I think, if the reducer works as hoped that I will get the MN190 down from F5.3 to around the F3.4 - F3.8 depending on spacing. There will be a bit of vignetting, but should still be a flat field and nothing flats can't sort out. That will be pushing the AG8 boundries of scope for 50 notes.

I need around 30mm inward travel and just measured up and I am 'close' to this being available.

Just waiting for Rother Valley to get stock in for a trail unless someone will lend me a Antares 0.5 2inch reducer to try out.

Does anyone know what the threads on the Antares 2inch 50% are please. I can't find this information.

I'm sitting on my hands I'm so excited at this experiment..

Any news?

[riklaunim]

Teleskop Express also has the 2" reducer. It's either made by GSO or Antares as they like to brand those vendors as TS.

When I was playing with the 0.5x reducer I've used a 1,25" clamping ring to regain some backfocus:

Those reducers are just like filters. They fit on the nosepieces and go inside the focuser tube with the nosepiece