Focal reducers question
Posted 08 September 2013 - 10:00 PM
I've been working with a Celestron 9.25 SCT and a Meade 6.3 focal reducer with no success. I have moved its distance for the sensor in and out but it appears always elongated stars in the corners. My chip is small, Atik 16HR, so I think it must be easy to correct my problem, but not success. I'm planning to buy a better focal reducer and I need your advice. What focal reducer do you use and what results have you got? Anybody working with an Optec 0.5X? AP67? ASA SCT reducer? Starizona reducer?
Posted 09 September 2013 - 01:45 AM
Is your scope a standard SCT or an EdgeHD ?
Posted 09 September 2013 - 07:33 AM
Even with the Celestron focal reducer, the field is curved. It is just not curved as much as without the focal reduceer.
When using the focal reducer, try focusing on a star half way to the edge of the field.
Also, ensure that whatever is happening is happening equally around the field. If the probelm is the same in all corners, again try refocusing to half way out.
If it is not an even amount of aberration around the field, suspect collimation or de-centered secondary mirror.
The decentered secondary can make it hard to get all corners of the chip sharp.
Posted 09 September 2013 - 07:43 AM
I'm happy with the Starizona reducer, I don't have an 9.25 SCT, but have good results with my 8", some distortion with my 6 and 11 inch scope which was easy to correct. I tried all kinds of spacing's with the standard 6.3 corrector and was never happy. YMMV
Posted 09 September 2013 - 09:17 AM
Frobi: my OTA isn't EdgeHD, it's the standard model.
Edggie: very interesting point, I always focus on center. I'll try to take a half way out star. Thanks!
Mike: the Starizona reducer was one of my finalists and the price is reasonable. What I like more of the Starizona type reducers is that I can use an external focuser.
This is an image I got with a Meade f/6.3 + C9.25 + TSOAG9 + Xagyl FW + Baader 7nm H-a + Atik 16HR. Maxim DL5 says I was working at 1666 mm FL.
Posted 11 September 2013 - 11:49 AM
Posted 11 September 2013 - 01:05 PM
A better approach for more folks is to collimate the scope using the minimal amount of accessories and then check the accuracy as you add reducers, cameras, filter wheels, etc. You may find that the new configurations negatively affects things, and if the affects are severe enough then you will have to troubleshoot the different setups.
Nice tight barrel fittings (those with compression rings instead of just thumb screws are often helpful here) and threaded couplings are usually a good bet. Pay close attention to safety undercuts, some are angled which can lead to fittings coming slightly loose over time (which can drive you crazy). Sometimes just using a different brand adapter that fits better will be a good fix. Removing wedge with threaded couplings can involve using shims, this is also often a good fix when barrel fitting sag a bit under the extra weight from cameras.
All of this takes time and patience, but the results are usually consistently better quality images and a scope that performs better in all configurations.
Posted 12 September 2013 - 04:13 PM
Posted 14 September 2013 - 09:09 AM