William Optics IV versus Riccardi Reducer

[PeterSEllis]

Hi Guys
With reference to my earlier uploaded picture "Lagoon M8 through a Williams Optics IV Reducer", my Riccardi finally arrived. I did a couple of quick shots of the Lagoon with the new Riccardi, "between cloud banks", the results where a little disappointing to say the least. Below is the Top Left Hand Corner of the Lagoon magnified up a fair bit, for both the WO and Riccardi reducers. I might add, that the center of the field is fine for both reducers.
I haven't given up on the Riccardi yet, when I measured the distance from the back of the Riccardi to the CCD chip it was 73.5mm, the manufacturer quotes 74mm for the Esprit 150mm, maybe that extra 0.5mm makes a big difference. I will test this when the moon makes astrophotography difficult again (as it will), but for now it is back to the 12" and Galaxy hunting.
At my age you only have a limited amount of darks night left, I don't like to waste them.

Cheers
Peter

[RickS]

Hi Peter,

It's a bit of a crap shoot when you're using a reducer or flattener not specifically designed for your scope (assume that is the case?) and tweaking the spacing somewhat differently from the specs is sometimes necessary. Also, a reducer doesn't necessarily flatten a curved field. Hope you get it sorted out...

Cheers,
Rick.

[Slawomir]

Hi Peter,

I also have a small Riccardi (there are two versions) and spacing and orthogonality need to be very accurate. Having said that, I think Riccardi works best when used with slow long focal length refractors. My 105mm f/6 gets reduced to f/4.5, and even with a small ICX814 stars are slightly but noticeably elongated in the corners.

First thing to check would be whether stars in all four corners are symmetrically / equally elongated away from the centre. That would indicate accurate orthogonality. Then you could add a 1 or 2mm spacer and see if star shapes improve or get worse. Unfortunately, it takes time and patience to fine-tune a 0.75x reducer.

As Rick mentioned, there may be often tweaking required because in reality focal length of each scope may vary a bit, position of the sensor in a camera can vary slightly from unit to unit and each Riccardi reducer has some variation for sure (these are assembled in Taiwan I believe).

[gregbradley]

Reducers often don't give round stars to the corners.

I originally used a Tak .6X reducer meant for the FS102. It worked up to APSc sensors but not full frame.

The larger 4 inch Tal reducer and flattener worked better. The Tak flattener worked flawlessly and it worked on other scopes besides my FS152. It worked on a TEC180 as well. The reducer though worked reasonably but the corners with a 16803 sensor (36mm square) were a tad weak close to the corners.

Dedicated TEC flatteners worked flawlessly though.

The AP telecompressor for the AP155 that was also meant for the AP140 worked well but it has a tolerance of +/- 1mm. It has a decent backfocus though so you could use it with the usual image train of camera, filter wheel and OAG.


I have read the specs on most of these Teleskop Services Kunming based scopes and the backfocus for their reducers and flatteners including the Riccardi can be quite tight like 55mm in some scopes. 75mm would be one of the longer backfocus distances and TS usually specify the backfocus with different focusers, it varies.

TS also sells a tilt adapter so you probably need that as the first image shows tilt more than uncorrected coma.

The 2nd image I agree is a mess and does not bode well. I think TS sells a variable length adapter tube which may be the go to test various distances.
Greg.

[PeterSEllis]

Hi Rick
Thanks for your reply. You are right it is a bit of a crap shoot, I try not to think about the amount of money I have wasted in Astronomy trying things that didn't work, but I haven't given up on the Riccardi yet. The first time I tried the the WO reducer it was bloody horrible, even the center of the field was really bad, once I got the spacing close, it improved remarkably and now is usable, I'm hoping the same will apply to the Riccardi, time will tell.

Cheers
Peter







Hi Suavi,
Thanks for your reply, I had a look at your last picture and it looks like you have it nailed on the back focus distance. I hadn't thought about the orthogonality, it is something I better check, the lesson I learnt from the WO reducer, was that the back focus had to be accurate for it to vaguely begin to work properly. The Esprit 150mm is F7, hopefully that will work in my favour.
When the moon start to be painful again, I will start experimenting with different spacing, as well as checking orthogonality.
Many thanks for your comments, you have given me something to think about.

Cheers
Peter






Hi Greg,
Thanks for your reply. I searched the NET to see if anyone with a 150mm Skywatcher Esprit was using a reducer/flattener, but came up empty, which is very unusual. I also started a thread on Cloudy Nights, no information there either, "which is even more unusual". I think I'm in virgin territory with this one, which means a lot of experimenting.
I must admit tilt, was something I had't thought about. It might be time to lash out again, and get a TS tilt adapter as you suggested.
The three responses have have given me a lot to think about.
Once again many thanks for your comments, I will ponder this information, before I start the next round of experiments.

Cheers
Peter