Hyperstar focus

[Paul Byrne]

Hi guys,

I am just starting with a Hyperstar on my wedge mounted CPC11 with a Canon 6D full frame DSLR. On most of the guides I see that are related to focusing Hyperstar using a Bahtinov they show the usual nice Bahtinov diffraction pattern you get when focusing at the prime focus position. However I just seem to get an image that matches the pattern on the mask regardless of how far either side of the focus limit I go. I can get the image sharp and bright and when I remove the mask my stars look ok, but it is certainly not the 6 point diffraction pattern. Is this normal, sorry probably a very basic question! I have a collimation issue too but I believe its a case of one step at a time with Hyperstar!!

[Star101]

First, I would sort out the collimation issue. Imaging would be pointless if collimation is out!

Then try the bahtinov again

 

[ollypenrice]

You will certainly need to sort out collimation as Star101 says above.

However, the Bahtinov pattern problem might also arise from the extent to which the camera body lies in the light path of the telescope. Any intrusions will create diffraction effects of their own. I had a look for B-mask examples and found this: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p8592_Farpoint-Bahtinov-Mask-for-Celestron-HYPERSTAR-SC-EHD-9-25-.html

Here we see a mask with clear aperture for the secondary mirror and camera. If this is what you have your camera may be intruding into active part of the mask. It might be worth making an additional circular mask (like a large washer) to block off the camera's intrusion completely and create a larger opaque centre for the mask. Compass cutters available from graphics outlets online or on the high street are ideal.

So steep is the light cone at F2 that I'd be surprised if a B-mask would be adequate for finding critical focus, though it should give a good rough guide. I'd use a capture program that could give an FWHM reading. Unless the Celestron SCTs have changed you might also want to note that the final focus adjustment should be anticlockwise, pushing the primary up the tube. Going the other way there can be a lag as the mirror settles back down.

Olly

Edit. when making masks or blackening tubes against reflection it is best to use heat resistant stove or barbecue mat black paints because they use pigments rather than dyes. Dyes can be surprisingly reflective in some wavelengths.

[AKB]

Just wondering if your mask is designed for the SCT at f10, rather than f2?  Not au fait with mask design, but this ought to be a factor. 

[Paul Byrne]

Thanks for the replies guys. I will try and sort the collimation issue on the next clear night (hopefully tonight!!). I am using the Farpoint Astro Bahtinov "Hyperstar Variant".

https://www.astroshop.eu/focusing-masks/farpoint-bahtinov-focus-mask-for-11-celestron-sc-hyper-star-variant/p,45166

 

The images below are as follows.

 

Faint stars showing the diffraction pattern when at "best focus" after focusing with Backyard EOS FWHM and putting the mask back on to see what pattern it would show.

 

Peladies at that focus.

 

Polaris, the irregular shape on the left of the disk is the USB cable cover of the Canon 6D.

 

The Pelaides shot shows a focus and collimation issue, I just hope its not a corrector plate alignment issue aswell!

 

 

[ollypenrice]

Well, I think that Starizonas's claims about 'easy' astrophotography should land them in court and banned from the forums. I have thought this for a long time and hope that one day it will happen. This doesn't help you, of course, and I would like to do so, so first off I'd send these results to Starizona for comment.

This is a detailed guide to setting up which you might find useful.  https://www.cloudynights.com/articles/cat/articles/the-amazing-hyperstar-a-guide-to-optimize-perf-r3013

Olly

[Star101]

How are you fitting the mask to the scope ?

 

I have similar mask. I cut mine in half so I can rest it on the glass apature around the hyperstar. Placing both halves into a circle ⭕️ 

[Paul Byrne]

24 minutes ago, Star101 said:

How are you fitting the mask to the scope ?

 

I have similar mask. I cut mine in half so I can rest it on the glass apature around the hyperstar. Placing both halves into a circle ⭕️ 

This is it here. It sits at the glass.

[Paul Byrne]

https://www.astroshop.eu/focusing-masks/farpoint-bahtinov-focus-mask-for-11-celestron-sc-hyper-star-variant/p,45166

[Star101]

Yep, definitely weird. 

Do you have a collimating image of bright stars out of focus. Showing round doughnuts ?

[Paul Byrne]

55 minutes ago, Star101 said:

Yep, definitely weird. 

Do you have a collimating image of bright stars out of focus. Showing round doughnuts ?

It sure is, collimation is ok on a defocused star, not perfect but ok. I know focus tolerance is very tight with hyperstar but it’s strange the fact I can’t even get a “recognizable” diffraction pattern with the mask even after focusing as best I can with Backyard Eos. I have contacted Starizona so hopefully they will have some suggestions. Thanks for your help.