M67 (NGC2682) Open Cluster

Beautiful image! I love the colours of the stars.

If I may ask one (possibly sensitive) question: are you sure the back focus between the focal reducer and the camera is correct? The stars in the corners look a bit elongated which suggests the back focus is too small at the moment.

Posted by: wvreeven

Beautiful image! I love the colours of the stars.

 

If I may ask one (possibly sensitive) question: are you sure the back focus between the focal reducer and the camera is correct? The stars in the corners look a bit elongated which suggests the back focus is too small at the moment.

Thank you for the kind words @wvreeven

No, your question is not sensitive at all. I am always looking for ways to improve what I am doing and am open to suggestion and tips.

So, if I give you my setup details please do check if I have things done correctly.

First off I use this formula to calculate my Focal Length & Focal Ratio: http://www.wilmslowastro.com/software/formulae.htm#FR_b

EQUIPMENT

Celestron CPC9.25 SCT

Celestron f/6.3 Focal Reducer

SX OAG & Filter Wheel

Canon EOS 750D

CURRENT SPACINGS

From my SCT to the Focal Reducer 49.25mm (via Baader SCT > 2" Click-Lock & 2" > SCT Nose Piece)

From Focal Reducer to DSLR 53mm (via 3mm Rubber Gasket as Focal Reducer fouls OAG Prism > SX OAG & Filter Wheel > 7.5mm Spacer Tube)

From 7.5mm Spacer to DSLR Image Plane 55mm (via Baader Protective T-Ring 298550 & Canon EOS750D)

Regards..,

Kirk

Posted by: 1CM69

Posted by: wvreeven

Beautiful image! I love the colours of the stars.

 

If I may ask one (possibly sensitive) question: are you sure the back focus between the focal reducer and the camera is correct? The stars in the corners look a bit elongated which suggests the back focus is too small at the moment.

Thank you for the kind words @wvreeven

No, your question is not sensitive at all. I am always looking for ways to improve what I am doing and am open to suggestion and tips.

So, if I give you my setup details please do check if I have things done correctly.

First off I use this formula to calculate my Focal Length & Focal Ratio: http://www.wilmslowastro.com/software/formulae.htm#FR_b

EQUIPMENT

Celestron CPC9.25 SCT

Celestron f/6.3 Focal Reducer

SX OAG & Filter Wheel

Canon EOS 750D

CURRENT SPACINGS

From my SCT to the Focal Reducer 49.25mm (via Baader SCT > 2" Click-Lock & 2" > SCT Nose Piece)

From Focal Reducer to DSLR 53mm (via 3mm Rubber Gasket as Focal Reducer fouls OAG Prism > SX OAG & Filter Wheel > 7.5mm Spacer Tube)

From 7.5mm Spacer to DSLR Image Plane 55mm (via Baader Protective T-Ring 298550 & Canon EOS750D)

Regards..,

Kirk

 

 

This got me wondering if in fact I had an error somewhere in my calculations so I uploaded a single raw frame from this set of data to Astrometry.net and was given a PixelScale of 0.536 arcsec/pixel

Using the formula:

fRatio = (pixel size [um] x 206.3) / (aperture [mm] x arcsecPixel)

fRatio = (3.7 x 206.3) / (235 x 0.536)

fRatio = 763.31 / 125.96

fRatio = 6.1 rounded with an effective Focal Length of 1424mm

This is obviously below the f/6.3 that using the website formula in the above post gave me, so I have slightly too much space between the Focal Reducer & my camera's image plane than is ideal.

I cannot say if the website I used for calculations has incorrect values or if it is in fact a slight variation in my actual Focal Reducer.

I am more inclined to trust the arcsec/pixel value produced by Astrometry.net

With this in mind I have calculated that I was over spaced by 6.25mm and have now rectified this by placing an order for some very fine spacer rings and I will be able to reduce the spacing by around 6.3mm.

When the items arrive & the first available time to image after that, I will upload to Astrometry.net again to check.

Overall though I do not think that I will get a true flat field across the whole frame as this is a limitation of the Focal Reducer not being a true Flattener.

Regards..,

Kirk

I agree that trusting the value given by astrometry.net is a sensible thing to do. I have come across slightly erroneous specs too often (and sometimes very erroneous specs!) and have developed a mistrust for them. My advise would be to take the specs as a guideline and play with the spacing until it is right. The only way to do that is to painstakingly take pics and adjust the spacing until the stars are to your satisfaction 🙁