Back to forums

Reducer or not

Equipment Reducer Refractors
5 replies•269 views
Reducer for Apo Refractors?
Multiple choice poll 15 votes
60% (9 votes)
40% (6 votes)
You must be logged in to vote in this poll.
Anderl avatar
Anderl avatar
Hello fellow astro friends,

out of curiosity, do you think using an reducer for your apo is worth it or not?
why do I ask? 

1. bad weather and I want to do something astro
2. i have noticed that people using high end apos like tak toas seem to only very rarely use reducers. 

cs
Andi
Arun H avatar
Arun H avatar
A reducer simply increases your field of view. Most will reduce optical quality. The question you need to ask is - will your field of view of interest fit on your sensor without the reducer? If so, there is no benefit to using one. Your reducer will increase signal on a per pixel basis, but this advantage goes away if you end up cropping your image to a size that would have been obtainable using just a flattener. With my Astro-physics stowaway, I have both a flattener and a reducer. The choice of which I use depends purely on whether my desired FOV will fit on my sensor.
Well Written Helpful Insightful Concise
Lynn K avatar
Lynn K avatar
I have always used focal reducers/flatteners.  1st on a WO 80 Megrez to bring to focus. Also on a AT111ED and TMB130SS.  Nither of those scopes had a manufactured dedicated reducer/flattener.   The WO reducers I have owend worked OK, but didn't flatten the field well. That may have been caused by spacing issues.  I ended up using the Astro Physics 27TVPH on the TMB which is not a flattener, but It worked well.

I now use the Takahashi QE-0.73x on my FSQ106ED  and the Takahashi RH-QB 0.73x on my FSQ85ED.  Even though both those scopes or flat field, the small 3.76 pixels signal will be increase by 100% according to John Hayes' Excel spread sheet calculator.  Of course the field is increased, but the S/N also with shorter exposure times. According to John Hayes caulator I would need to double my exposure from 5min to 10min at F5/F5.3.

I also use a Astro Physics Quad reducer/flattener on my AP130GTX.  Works exstreamly well.  Without a flattener Iand  with an APS-C chip there is curvature in the corners. A flatten would remove that, but with the current 3.76 pixel in the APS-C chips the S/N is greatly decreased. The flattener is a little less expensive, but bothe require correct back focus.  So, I prefer the reducer over the flattener (which I have, but haven't used yet).

I realize that the three above scopes or expensive with limited avalibility.  But to make a blanket statement that reducers do this or that, is often based of lower priced scopes/reducers with lower tolerances and QC.  Those statements need to be referred to the brands experienced.  Reducer/ Flatteners can have  bad representation.  That is usually with lower priced refractors and reducers. You can't cheaply reproduce either at consistant QC.

The  advantage is wider field, shorter exposures which can improve S/N depending on exposure time.  The disadvantage is getting back spacing correct.  But a flattener will also require that.  They can also be very expensive depending on brand.  Even if you crop the image maintaining the pixel alignment grid, the S/N doesn't change.  Not sure about resampling. It can also be more difficult to control camera tilt, which I am dealing with now on the FSQ106ED  with 3.76 pixels. That's just going to need a few nights with a small hex wrench. 

I must also metion my backyardh has a very limited field of view. Clear nights are few and Bortle 7.  SO, I do narrow band and time is limited.
Lynn K.
Tony Gondola avatar
Tony Gondola avatar
I think you will find that for most refractors with matched reducers, the main role of the unit is to flatten the field with focal reduction as a secondary effect. By using one you get a slightly larger field of view that is sharper out to the corners, you also get the bonus of lower f/ratio. This is a win-win and is why most people use them as part of the basic photographic refractor rig.
Well Written Helpful Insightful
Christian Großmann avatar
Christian Großmann avatar
To me, focal reducers are just another option. During summer and autumn months, they help getting subjects with a wider FOV, which i like. On my EdgeHD, I use it mainly because guiding at a focal length of 2m can be challanging and at 1500mm it is a bit more forgiving. For all my scopes, I also have just a field flattener (1,0x) and so I can use them without a reducer. This is very handy during the galaxy season.

While selecting new targets, I look for the FOV I need and the scope that may fit best. If a reducer is necessary to get what I want, I use it. But thinking about it, I use them about 75% of the time or even more.
Helpful
Simon Pepper avatar
Simon Pepper avatar
That’s a tricky one as I use both! I use my reducer in the summer to gather light quicker and to have a larger FOV, but during galaxy season I use it with just a FF. However if I had to pick one it would be with the reducer specifically for the faster ratio.