BACKFOCUS for ASA 2" Reducer Corrector 0.73x

[-Amenophis-]

Hello,

 

I own the ASA 2" reducer 0.73 and I can not find the right measure for the BackFocus.

 

Here is my equipment:

Newton telescope : focal 766.94mm

 

I tried with the table in the documentation of the correction but I can not be the measure precisely.

 

Thank you in advance for your help.

 

Have a good day

[-Amenophis-]

Edit : more information about telescope

254mm diameter

Focal lenght : 766.94mm with ASA reducteur (astrometry)

FD 3

[MarkS]

From your figures, your telescope has an f ratio close to 4.1.

 

The backfocus from the table in the data for the reducer is 65.5 mm for f/3.8 and 65.4 mm for f/4.5 for an aperture of 250mm.

 

Therefore the backfocus for your set-up should be very close to 65.45 mm.

 

Mark

[-Amenophis-]

Thank you very much for your help

[-Amenophis-]

Sorry, but my focal length is 1000mm (Newtonian 254/1000 FD4).

So i calculate 65,5mm is that correct ? 

 

Thank you

[MarkS]

That should be correct.

 

Mark

[-Amenophis-]

thank you mark !  

 

Just to be sure : 

Backfocus CCD moravian and filter wheel : 33,5mm

Connected rings : 15mm

AOG : 15 mm

Filter astrodon : - 1mm

 

So : (33,5 + 15 + 15 - 1) = 62,5 mm

 

In conclusion, add 3mm extension rings to obtain 65,5mm

[MarkS]

This looks correct.

 

Presumably you could adjust the 3mm spacer if you need to fine tune the distance. The focus is very critical at ~f/3!

 

Mark

[-Amenophis-]

Thank you

 

How do you check that? (the good distance for Backfocus)

Do you use software like CCDInspector to see if the distance is good? (see coma) 

[MarkS]

You can use CCD Inspector, but I have found that the best way is to take a set of, say, 2min images of a good star field at slightly different focus settings. If there is a setting that gives good round stars all over the specified image circle, then the backfocus is correct - or very close.

 

If there is no focus setting that gives round stars everywhere, change the backfocus distance by 1/10mm or so and repeat the process. Keep doing this until you have the best you can get.

 

This is time consuming and tedious, but have found it very worthwhile. I did it with my Barlow when I changed cameras from a 8300 size to 16200. I found that if I increased the backfocus 0.3mm beyond the specification (!) I could get very good images right to the corners of the bigger chip - but the focuser had to be very precise and stable.

 

Mark

[-Amenophis-]

Hello,

 

So, Please find my test image.

I tried to adjust the collimation as best as possible with the option "real time" and then I took a break of 20s BIN1x1.

 

Technical specifications include:

 

  - Newton 254/1000

  - Paramount MyT

  - ASA 0.73x corrector

  - Moravian G2-4000

  - Image scale: 2,032

  - Focal: 751.12

  - RA 15 04 05.392

  - DEC +26 48 25.53

 

The goal for the moment is to find the correct distance of the backfocus for the corrector.

The TILT will be the next step.

 

For Downloading Image .fit :

http://Link to Download image .fit

 

This is the field :

   

 

With the four corners and the center of the image to see the appearance of the stars : 

  

 

Zoom on the center of the image :  

    

 

With CCDInspector :  

   

 

It is true that there is a lot of Tilt but this will correct later.

 

Here are the questions:

 

1 °) So what do you think of these results regarding my bacfocus for the corrector?

 

2) Is the accuracy of the collimation sufficient?

 

3 °) In "settings"> Collimation screw direction> Clock-wise = pull is selected. I can not understand. In particular with the following image : 

 

Thank you very much in advance for your help

Edited June 23, 2017 by -Amenophis-

[MarkS]

There appears to me to be two issues.

 

First, there appears to be significant coma on the upper right and right of the image.

 

Secondly, there are elongated stars on the lower parts of the image.

 

Because the left side of the image looks basically OK, I would guess that you may have a backfocus AND a tilt problem. It is certainly worthwhile to shoot several images with small focus shifts - say 0.05mm steps for a start. This should give you some idea of what to do next.

 

In my experience, the collimation results from CCDInspector are not always very reliable. I think careful visual inspection is the best, if not the easiest, indicator.

 

Best of luck with your investigation!

 

Mark