Out of focus or something else?

[emyliano2000]

Hi guys.

I have a problem with my 10" quattro that I never managed to solve.

Even though I am focusing as good as I possibly can the diffraction spikes on the stars are quite fat, blurred and even double.

I used the Focusing aid in APT and I focused step by step until I brought the FWHM to the lowest possible value.

I collimate it to the best of my abilities using a Cheshire collimator and testing on a unfocused star it looks quite good.

I have no clue what might cause it.

Here's a 3sec photo of Capella and a 5sec photo of one of the double clusters. I didn't take a photo of the unfocused star, will do it next time.

Thanks.

Emil

[vlaiv]

There can be a number of reasons for this:

- mount / tracking / guiding, but in above images that is pretty much eliminated if they are 3 and 5s exposures

- You are quire oversampling with 10" Quattro and QHY183. At F/4 it is 1000mm FL scope, and with 2.4um pixels of 183 chip, that will give 0.5"/pixel - that is very high resolution for any kind of atmospheric seeing. Try binning your subs at least 2x2 and even 3x3 in poor seeing

- Seeing will have impact on star size and also star spikes - only in the best seeing conditions will you have thin lines for diffraction spikes

- What sort of coma corrector are you using? Some coma correctors will introduce spherical aberration when used with fast scopes - like F/4. Here is interesting discussion on the topic:

https://www.cloudynights.com/topic/554686-coma-corrector-compariosn/

My guess is that you have all three points (we excluded mount / tracking / guiding as a cause) contributing to star shapes in your image. First step would be to bin subs. Second would be to check type of CC you are using and if it is feasible to replace it if it creates problems. Third is to monitor seeing conditions and accept that in some cases stars will be bloated due to seeing.

[emyliano2000]

3 minutes ago, vlaiv said:

 

Thank you for your quick reply vlaiv.

I know I am oversampling. These are test shots that I took to see how the camera works. The trouble is that I had the same problem with the qhy10 which gave me a 1.25"/pixel resolution.

I'm using a baader mpcc but with the sensor of the 183m being so small I thought I wouldn't see the effect as I do now. Maybe it would be the time to invest in a aplanatic coma corrector which is supposed to work great with the quattros.

Emil

[alacant]

Hi. JTOL. I had that with my f4 too. The spider vanes  on opposite sides of the secondary stalk aren't at exactly 180°. On my ES f4, there is a small amount of play where the adjusters meet the tube. You can correct it easily by laying a steel rule across the open end of the tube; with the adjusters loosened a little, move them to make the vanes straight by line of sight. Another thing to check is that the flat surfaces of the vanes are not twusted. A good time to check that the spider is centered too...

Coupled with @vlaiv 's theory above it may get you there.

HTH.

[emyliano2000]

31 minutes ago, alacant said:

Hi. JTOL. I had that with my f4 too. The spider vanes  on opposite sides of the secondary stalk aren't at exactly 180°. On my ES f4, there is a small amount of play where the adjusters meet the tube. You can correct it easily by laying a steel rule across the open end of the tube; with the adjusters loosened a little, move them to make the vanes straight by line of sight. Another thing to check is that the flat surfaces of the vanes are not twusted. A good time to check that the spider is centered too...

Coupled with @vlaiv 's theory above it may get you there.

HTH.

I've done that last night thinking it might be that causing it but it looks like it's not.

Thanks

 

[david_taurus83]

The second image looks out of focus. Silly question but did you try with a bahtinov mask? I've found the focus aid in APT to be a bit fiddly. B mask rarely lies...

[emyliano2000]

6 minutes ago, david_taurus83 said:

The second image looks out of focus. Silly question but did you try with a bahtinov mask? I've found the focus aid in APT to be a bit fiddly. B mask rarely lies...

I tried to eyeball it with a bahtinov mask before and it didn't come right, close but not perfect.  I didn't use the Autofocusing aid because I never got anything good out of it, I just did it manually in APT checking the FHWM values and actually looking at how the stars are in the test photos.