Star Elongation at the Edge

[astrochris]

Hello! I just started using a new SV102 reducer on my Stellarvue SV105. Sadly there isn't a reducer made specifically for the SV105 but Stellarvue told me that the 102 is very close and I would probably not notice a difference. I purchased it and finally had a chance to use it last night but the edges of the image have elongated stars. I would imagine that's what it would look like if I didn't use a field flattener. The SV105 dedicated field flattener does not do that. It either doesn't work or I am doing something wrong. 

A friend told me that he has the same set up and that I might need a 1 or 2mm spacer to make the edges come in focus. What do you think I am doing wrong?

Honestly I don't understand the spacing issue if your focuser moves in and out; I thought that is what focusers were for?

[ronin]

Flatteners work when they are a specific distance from the lens or the image plane, when you fit one aand wind it in and out you are altering where it sits along the optical path, so the performance is not 100%.

What will happen (at least in an ideal world) is that you install the falttener, add the exact amount of spacers and when the flatener is at the right position then the filed on the sensor is flat.

I have no idea how good it will ever get. I do get the idea that many items are designed to the condition of good enought not perfect. So the extremes may never be flat.

If the 105 is f/7 as the 102 appears to be then I would have agreed that the flatener would work but the spacing needs to be sorted.

What scopes like the WO GTF series and Star71 do is put the flatener at the right position as part of the machining of the scope, then you attach camera and move the camera to where the flat field is located. So the flatener is fixed and fixed at the right location and the camera is moved, In yours I guess the flatener is added but not at the ideal location so much of the field is flatened but not quite all.

[alacant]

Hi. The field flattener is too close to the sensor. Move it away a mm at a time until the stars are round. Plastic milk cartons are great to make temporary washers and if you're serious about this I'd recommend get calipers so you can then get the exact length spacer. HTH. 

[Waldemar]

I think you have two issues here:

#1 backfocus to your camera is not exact. In or out is hard to say, you will have to establish the right distance by trial and error with a variable spacer like the Baader Varilock or use a set of shims like these: https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html
#2 the star elongation is not even, the left bottom corner is a lot better then the others, that means your camera is not perpendicular to the optical axes, so a little bit tilt.  Again the only way to cure this is by trial and error.

A very nice program is available that will tell you exactly what to do in which direction:  CCD Inspector http://www.ccdware.com/products/ccdinspector/
Not cheap, though. A trial period may be available.

[MarsG76]

And if all fails....option 1. just crop the elongated stars in post. Option 2. Astrophotography tools has a rule set up to make oblong stars round, so after processing, run it through that, and mask in the round stars over the elongated ones.

 

 

[ollypenrice]

If the flattener moves with the draw tube the correct flattener-to-chip distance must be respected. The manufacturer of the fattener should be able to specify this distance.

You then need to add up everything in the system which contributes to this distance. That might be just the adapter + camera backfocus figure for a DSLR or it might be filterwheel + adaper + off axis guider + camera backfocus for a CCD setup. All being well this accumulated 'stuff) will be less than the required backfocus so you add spacers to suit. You can add 1/3 of the filter thickness as well.

If your refractor has a built in flattener in the fixed part of the tube (as in the Petzval design) you can forget all this and just focus.

Olly