Celestron C6 0.63 reducer/flattener back focus

[alexbb]

Hello all,

I bought relatively recent a SCT 0.63 reducer/flattener for my C6. I'm planning to use it with the ASI1600 camera, but it seems that I can't figure out the proper spacing distance between the reducer and the sensor. That or there's another problem with the scope that I can't figure out.

The first distance that I tried was ~105mm. I achieved this by using a SCT adapter to T2 which is 50mm + a Baader VariLock set to ~28mm + the filterwheel 20mm + 6.5mm the distance from the sensor to the camera thread. + the filter width, it should be ~105mm.

I understood that the closer the sensor is placed to the flattener, the better the field is flattened, but the reduction is weaker. I didn't mind that. I then tried to use an M48 (has also a T2 thread inside and deeper) to T2 adapter which was about 15-20mm.

In both cases the stars were not in focus away from the center, with an out of focus elongation towards the edges (I don't know which word describes this).

The following M13 is with the Baader VariLock and the Crescent was with the shorter adapter. I didn't have time to test without adapter at all and now it's going to stay cloudy for a while.

Don't mind the quality, the M13 is unguided with ~20min (6s subs), the Crescent is ~20min (1min subs), guided. Very bad seeing.

Did anyone use a C6 with a 0.63 reducer/flattener and APS-C or 4/3 sensor with success? Or does somebody know which is the correct backfocus distance and how to get rid of the distorted stars towards the edges, but still using the same scope + flattener?

Many thanks and clear skies,

Alex

[Davey-T]

The 6.3 f/r is not particularly suited to DSLR sensors, I use one and it causes extreme vignetting.

As far as spacing goes, unlike some f/rs  they're pretty forgiving so I don't think that's your problem.

Have a read of this regards spacing

What are your stars like without the f/r ?

Dave

 

[alexbb]

6 minutes ago, Davey-T said:

The 6.3 f/r is not particularly suited to DSLR sensors, I use one and it causes extreme vignetting.

As far as spacing goes, unlike some f/rs  they're pretty forgiving so I don't think that's your problem.

I understood the same too by reading reviews, discussions, debates, etc. The vignetting should be less for the 4/3" sensor and it's possible to correct it with flats. I don't plan to use it with the DSLR. I also understood that the closer you place the sensor to the reducer, the better the field is flattened for this design.

 

9 minutes ago, Davey-T said:

Have a read of this regards spacing

What are your stars like without the f/r ?

This is how my stars look without the f/r and with a DSLR

I was also aware of the info in the link you provided, but, as I mentioned before, I read that getting the sensor closer yields a flatter field. And it also seems that the stars in the Crescent frame look better than the ones in the M13 frame. The Crescent was shot with a shorter distance.

I have a 7.5mm extender so I could replace the VariLock adapter or the other one with the 7.5mm extender to shorten the distance or I could add it to the VariLock and increase the distance. But I don't have clear skies anymore.

I took this information as correct: https://www.cloudynights.com/topic/305891-back-focus-question-for-c6-sgt/

Thanks, Dave!

Alex

[Davey-T]

Stars without focal reducer don't look right either, have you checked your collimation ?

Dave

[alexbb]

Yes, I tweaked it a little since then. At center the out of focus star had concentric disks/circles in and out, as much as I could tell visually. I didn't use any collimation aids, just that the disks/circles looked concentric from just a little defocused to very defocused.

Edited April 11, 2017 by moise212

[michael8554]

Hi Alex

SCT's suffer from coma at the edges of the field - stars start to look like tadpoles pointing towards the centre of frame.

The 0.63 Reducer/Flatener does what it says on the tin, but it is not a coma corrector, though it does to an extent.

Michael

[alexbb]

Hi, Michael

I'm expecting to have coma to some extent, but there's not only coma in my pics. If I had only coma, I'd expect the stars to look like in my uncorrected newton. Stretched quickie with my 130 from last year is attached.

It seems that I might see some stars tonight through high clouds, could be fine for figuring out the spacing.

Fingers crossed, I'll keep you informed.

Alex

[alexbb]

I made some tests with various distances and, indeed, the results seem better with a shorter distance. I rechecked the collimation too and I tried to get it as close as possible, using the APT collimation aid tool this time. I couldn't take proper flats so I'll post the results later after I'll take them. There's also a possibility for some clear skies tonight.

[Frogliza]

On 14/04/2017 at 03:47, alexbb said:

I made some tests with various distances and, indeed, the results seem better with a shorter distance. I rechecked the collimation too and I tried to get it as close as possible, using the APT collimation aid tool this time. I couldn't take proper flats so I'll post the results later after I'll take them. There's also a possibility for some clear skies tonight.

Hey, this is a really old thread but I have had the same issue recently, did you find a good back focus distance to get round stars across the entire image?

[alexbb]

9 hours ago, Frogliza said:

Hey, this is a really old thread but I have had the same issue recently, did you find a good back focus distance to get round stars across the entire image?

Hello,

No, I did not find a better backfocus distance, neither pursued this idea, but neither found very good images on the internet with this combo.

I have now also a C9.25 and a Starizona SCT IV Reducer/Corrector and find this combination usable. The Starizona's corrector does a much better job than the Celestron's, which I found useless.