Focal reducer on newtonian

[ribuck]

Hi,

I have a 8" newtonian (1000mm FL @ F5) and was wondering if anyone has seen any tricks that can be done to allow the use of a focal reducer on a newtonian reflector.

I believe the the biggest problem is not having enough back focus, so i was wondering if it was possible to maybe replace the stock focuser on my 8" Skywatcher Explorer 200p with a low profile moonline with a short draw tube to over come this.

I'm looking to use a focal reducer in the region of 0.5 - 0.67 .

Does anyone know if there is a way to calculate the back focus required, i.e. 1.5 iches etc.

regards,

Ribuck

[daz]

Ribuck

Not sure you'd be able to use a FR even with a lower profile focuser..... :scratch:

Could be wrong, but can't say I've heard of anyone using one.

[Whippy]

I did use an Atik 0.5x reducer on my C8N in conjunction with one of my Baader Hyperion EP's and it did come to focus but at f2.5, it didn't look pretty at all. Not sure if a coma corrector like a MPCC or something similar would help TBH.

Tony..

[part timer]

I don't think you would like the view if you could even get it to focus. I doubt even a coma corrector would help.

[Kaptain Klevtsov]

You can use the Atik 0.5X one if you're using a webcam, but you need to slightly shorten the nosepiece and reverse the 1.25" adapter tube on the 'scope fitting. For a DSLR, forget it. You'll have to move the DSLR inwards so far that the FR will be inside the tube, or you'll need to move the mirror up an inch or more, then a lot of light will spill over the edges of the secondary.

Kaptain Klevtsov

[bern]

As KK says it can be done and it depends a lot on how big the chip is as to what's feasible.

It's easy to do with small CCD's (1/2" or less) but you have to be realistic .6x/.66x is probably the bottom line.

Here's an online calculator:

http://timosastro.1g.fi/tools/focalreducer.html

bern

PS I often use reducers with Newtonians especially for Video Astronomy and imaging with cameras like the Atik 16IC.

[ribuck]

Hi All,

I'm only interested in using a focal reducer with my CCD, which is a Starlight Express MX916 which is a small / Medium sized chip. I've already ordered a 0.6x focal reducer, so i'll se where i get with that, hopefully it will work.

Once the skies clear i'll try a few shots to see how it works. Hopefully it will work with my ccd.

[bern]

I've already ordered a 0.6x focal reducer, so i'll se where i get with that, hopefully it will work.

Out of curiosity, which reducer did you order in the end?

bern

[FLO]

I was going to ask the same

It hasn't ever occurred to me to use a focal reducer on a Newtonian; I look forward to your report

[ribuck]

Hi guys,

i bought the astro engineering 0.6x as it is on special at "scopes 'n' skies" for £23.99 with free delivery rediced from £39.99 so at that price i couldn't help myself, had to buy it to see if it would work. I also thought that 0.5x was pushing it, so this is why i went for 0.6x

I'm away this weekend so wont get a chance to use, so hopefully next week when i get back we will have clear skies.

http://www.scopesnskies.com/prod/antares/barlow-lenses/focal-reducer/0.6x.html

[FLO]

Looks good, I look forward to hearing how it performs

[rikyuu]

Just a little tip that Bern told me recently, changing the distance between the reducer and the CCD alters the field of view/magnification. So sticking a spacer or filter between the reducer and CCD zooms out, but requires further wind in of the focuser tube so there's a limit. I was experimenting with this on a terrestrial object last night. In fact I unscrewed the lens off my barlow and attached that in place of the reducer, and hey presto, it zoomed in, requiring the focuser to be wound out of course, but I could use the remaining body of the barlow as a tube extender, cool!

I also tried KKs idea of screwing the 1.25" adapter to the other side of the T-thead adapter so I could effectively get the focuser even further in, this worked great, except the thumb screws on the adapter need shortening as they foul on the focuser. I'm effectively getting the equivalent of about 3mm-12mm lenses using these methods, but I need some stars now to experiment with as my terestrial target is too close!!

[bern]

Just to (try to) make things clear, reducers can be called .5x, .6x, .6598x or whatever BUT the value will only be true with a specific distance between the chip and reducer which hardly ever applies!

The AE .6x reducer is probably about the same as the Atik .5x reducer, because of the laws of physics, it's really just what they choose to call it for marketing purposes.

The whole art of using reducers, especially with Newtonians, comes down to a balance of the initial F ratio, size of CCD and focuser in-travel available. Experience helps here but experimentation is often called for too.

bern

[Ant]

It works exactly the same for Barlow.

I have a 2" 2x barlow. But the lens part unscrews and I can screw it into the 2" adapter on the Canon. This changes it to about a 1.4x barlow (that figure is an educated guess).

The closer to the chip / film / EP the less of an effect.

Ant

[bern]

Dead right Ant, and if you use a 2x Barlow for imaging the real amplification ratio is more likely to be 2.3x or thereabouts because the CCD is set back further than the focal point of an eyepiece.

bern

[beamish]

Just out of interest, I had mentioned to KK my plan to see what effects/results from trying the WO 0.8x R/F on my f6 200mm Cape, which ought to bring it close to an f4.8 ?

Karlo

[ribuck]

Well I just got back from climbing in the lakes, so i will try the reducer this week. I had planned on attaching the reducer to 1.25" nose piece of the camera. If this doesn't work, i may look to getting a 2" reducer and thread straight into the 2" thread og my ccd, this shortening the distance between ccd and reducer.

[ngc2403]

the ASA reducer should work. It will illuminate a 20mm field. A canon SLR has a 20mm length on one side. So it should flatten axross 20mm , but vignetting may be present. You need a scope of good optical quality, and make sure you have plenty of back focus. The 2" corrector is around 600 euros and offers 0.73x reduction. Its probably the only thing that will work with a decent corrected image circle

[ribuck]

I'm wondering if we could ask FLO or Scopes 'N' Skies etc, if they can loan us a 2" inch focal reducer and a low profile Crawford with a short drawtube, such as a Moonlite.

I think this would make a very interesting test for Netwtonian owners. I already bought the 0.6x Astro Engineering reducer, so i could use that as a test as part of the testing Vs 2" reducer.

I cannot afford to buy the 2" reducer & Moonlite Focuser, so this is why i am wondering if we would get a temporary loan as an experiment. If we managed to get a winning combination, i think this would create a lot of excitement in the Newtonian camp.

I would be more than happy to pay all the postage to carry out this test.

FLO / Scopes 'N' Skies etc, can you help us ?

[Kai]

I would be very surprised if any company would want to lend out that kit to that amount.

Would not talking to one of the dealers directly and maybe come to a agreement ref returning the product if they didn't work together be a better solution ???

[ribuck]

Kai,

If you dont ask, you dont get. A lot of manufacturers send samples of products to dealers on request. I'm not asking to get stuff for free just to see if we can get a short loan of equiptment. Besides, if we were successful, this would probably generate a decent amount of business from the Newtonian camp.

[Kai]

Thats why i suggested that you contact the dealers directly rather than on open forum .

[FLO]

FLO doesn't mind loaning kit to SGL for review; I like to think its one of the ways FLO is different to regular dealers. In fact, since the day we started there has never been a time when something hasn't been out for review (if you check the Sponsor section today you'll find there is a Skymax 127 SupaTrak being sold at clearance after being reviewed). However, FLO is not a hobby or second income as is the case with some dealers and each time we loan something it cannot then be sold at its normal price so we need to be reasonably sure that it will generate some sales. In our experience, people aren't buying focal reducers for Newtonians, perhaps because they are already typically f5-f6. However, as Andrew already has a low-profile focuser and has offered to test a focal reducer for SGL, we shall send one out to him tomorrow.

In future though, if anybody thinks a review or report would be of benefit or interest to SGL, please PM James or myself or SGL's ADMIN team directly.

HTH

[GazOC]

Will the Celestron FR connect to a Newt focuser?

[FLO]

I'm not sure exactly how it will be tested, I'll leave that to Andrew, but if anyone has suggestions on how to make this test more useful or relevant then I am sure he'll be open to suggestions.