8" RC - Can I use a reducer & Flattener at same time

[Northernlight]

Hi All,

I've been struggling with what i thought was collimaiton issues with my 8" GSO RC for several months now, and now seriously considering just buying a MN190 and just about to pull the trigger, but Then for some reason it dawned on me last night that I'm not using a flattener only a reducers as i have the AstroPhysics 0.67x reducer.  So it got me thinking that this is probably why i'm getting elongated stars in the corners.

So i asked one of my buddies if i can use the AstroPhysics Reducer & a Falttener at the same time, and he suggested that this was not possible.

Can anyone tell me if this is correct or if there is a combined Reducer & Flattener available for a 8" RC

 

NL

[michael.h.f.wilkinson]

I thought the reducer also acted as a flattener, but might be wrong. It is important to get the spacing between camera and reducer just right, to avoid odd star shapes in the corners

[johnrt]

The AP x0.67 reducer has a very tolerant spacing - you can vary it by up to 25mm to change the amount of reduction it provides. The star shapes being out are not likely to be caused by this. What spacing do you have from the reducer to chip? Have a search around and you will find lots of users of the 8"RC and this reducer without poor star shapes.

Have you checked you have no tilt in the focuser / imaging train? How are you collimating the the scope?

[derrickf]

The AP 0.67X reducer is exactly what its name implies: a focal reducer - it has no flattening effect.

As with all reducers, spacing between the camera and the reducer optics has only the effect of changing the reduction factor unlike field flatteners which have a critical spacing requirement. The CCD-T67 has an unvignetted field that is approximately the size of your ccd chip diagonal at nominal reduction ratio so the worst effect you might expect is some light fall off at the edges. If you have misshaped stars without the reducer there is no reason to expect that you will not have them with the reducer in the imaging train.

I cannot recall seeing a combined flattener/reducer for an RC - there are reducers and there are flatteners.

How are you collimating your RC? I use a Tak collimation scope on my 12" and it is quick, accurate and repeatable.

 

I've never owned an MN190 but I have some vicarious experience trying to help a friend get his set up - don't assume that collimation issues will go away when moving to this scope :-) You might be better trying to remedy whatever issues you have with the RC

 

HTH

 

Derrick

[r3i]

1 hour ago, derrickf said:

The AP 0.67X reducer is exactly what its name implies: a focal reducer - it has no flattening effect.

According to the documentation provided with the CCDT67 they have a "little bit of a flattening effect", but I would imagine nothing like what a proper flattener would achieve.

1 hour ago, derrickf said:

I cannot recall seeing a combined flattener/reducer for an RC - there are reducers and there are flatteners.

 

IRIC when GSO first brought out these scopes they also offered a companion reducer/flattener lens to go with the scope, but they were soon withdrawn and have never re-appeared, I guess it didn't do exactly what it said on the tin.

[derrickf]

"According to the documentation provided with the CCDT67 they have a "little bit of a flattening effect", but I would imagine nothing like what a proper flattener would achieve. "
I would say that it depends on how you interpret the loose language used - the documentation says:

"Finally, the 27TVPH and CCDT67 do not add any field curvature. In fact, they have a little bit of a flattening effect. However, the
telecompressor takes a larger field and compresses it into a smaller area. If you have a 20 mm square CCD chip, and you are
compressing at 0.75X, you are effectively taking a 27 mm square field ( 20 / 0.75 ) and compressing it onto your 20 mm square chip.
The question to ask yourself is this: Would I show field curvature on a 27 mm square without the telecompressor? If so, then you
may show some on the 20 mm square at 0.75X compression. It won't be caused by the telecompressor, but will simply be your
instrument's curvature squeezed into a smaller area. To calculate an equivalent uncompressed chip size, take each chip dimension
and divide by the compression factor."

At best the flattening effect is coincidental (if flattening was a design intent there would be more restriction on the permissible spacing otherwise, as with all mis-spaced flatteners aberrations would be introduced - you can't change the laws of optics)  but I interpret it to mean that the effect of the compression is make it appear that there is a minor flattening effect.

IIRC GSO announced the reducer/flattener but never actually delivered any - of course I could be wrong :-)

Derrick

 

[Northernlight]

Hi all,

To check the collimation I have both the Tak Collimation scope and a Howie Glatter laser collimator with the holographic attachment. According to those devices, the collimation looks spot on. I also have the extra focuser collimation ring.

In terms of the spacing it's about 85.5mm from the sensor, as i have a QSI 683 WSG8, which is 50mm from sensor to flange and i added another 5mm spacer.

I know it sounds silly but i've never thought to try the scope without the reducer.

My focuser is a Starlight Feathertouch, so there shouldn't really be any slop in that beauty.

Well if it sounds like it's going to be more hassle, i might just end up selling it and getting the MN190, where i dont need to worry about reducers, flatteners or correct spacing.

 

[johnrt]

The spacing for the ccdt67 should be 85mm, and can be varied from around 70-90mm. You are well out at 55mm.

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4955_Astro-Physics-CCDT67-0-67x-Reducer-2----e-g--for-GSO-RC.html

[Northernlight]

i edited my post as i got mixed up with my apo reducer which is 55mm. definately got 85mm spacing.