refractor / focal reducer

[skytry (Peter)]

hi All,
having just obtained a 102mm F5 SW refactor,

my question,

having been experimenting with DSLR camera astrophotography,

when using the refractor, is it necessary to adapt a focal reducer / flattener for imaging,

if so, what size is required,

thanking you in advance,

regards,
Peter,

[Wavytone]

Hi Peter,

It would help if you said exactly which model of refractor including its focal length or focal ratio.

Basically yes the simpler doublet and triplet types do have field curvature which means you'll need a flattener that is a good match for the specific scope in question.

If you want a focal reducer these are more costly.

The size depends on the type and size of your focusser - some flatteners/reducers are designed to slide into a 2" barrel much like an eyepiece while others have a screwthread intended to screw into the end of the focusser - if it has a thread - and there are several sizes in use.

[mental4astro (Alexander)]

I would imagine that all field flatners come focal reducer for refractors would be tailored for apochromatic refractors, not achromatic refractors. This being due to the chromatic aberration created by the objective being too uncontrolled compared to apo's.

In so far as just a focal reducer goes, again chromatic aberration would remain as too the field curvature, especially with fast refractors, and particularly when using a DSLR, if not even more pronounced.

[dannat (Daniel)]

at f5 don't think you'd ned focal reducer, flattener may be required if you have a decent size chip to image on

i presume you are talking about the 102mm f5 achromatic refractor

[cometcatcher (Kevin)]

I have Bintel's Orion focal reducer and also the field flattener. (The reducer is a flattener also I believe). I find they work well to even out the bright center "hotspot" in all my refractors - ED100 F9, 120mm F5, ED80mm F5.5, 100mm F8 etc. The flattener is necessary on the ED80 but the rest are okay without.

I'm not sure about the 102mm F5. Take a pic through it and see if there's any streaking on the corners. If so you need a field flattener.

Of course they do nothing to help the chromatic aberration in the achro refractors. I find a 48mm screw in light yellow photographic filter to be useful for reducing CA in achros. You can find those on ebay cheap, often sold as NOS New Old Stock.