Bahtinov Issues

[gnomus]

In an earlier thread (http://stargazerslounge.com/topic/231139-first-attempt-with-zwo-asi120mm-s-moon-many-questions/) I gave a particularly poor written account of an issue I had with "blobs" when trying to focus using a Bahtinov mask.  I've been out again and have now taken a snap of the "blobs".  It is attached below. (Taken with my ZWO ASI 120-MM.)  The star is Capella.

Is this what I would expect to see when using the mask?  If so, how am I supposed to see where the lines intersect?  If not, what is going on with my mask?

I did try using a program called Bahtinov grabber (suggested by a responder to my earlier thread).  This was not all that successful.  I could not get the numbers to go to zero and I could not get to a state where the program consistently said "YES" to the critical focus question (it flickered between yes and no).  Nevertheless, I persisted until I had the lowest number I could get, and until the flickering between Yes and No tended to the Yes state.  I then locked focus on my Edge HD 8.  The numbers still looked OK on Bahtinov Grabber.  I slewed to the Moon, however, and focus was a mile off - I ended up focussing on screen by eye (with poor to moderate results - I think the seeing was again very bad last night).

I am attaching the mask to the end of my home-made dewshield.  I don't know if this is OK (someone said it shouldn't be a problem).  One thing that did occur to me is that it is possible that because the mask is on the end of the dewshield it may not be parallel to the front glass of the scope.  Again, as a beginner, I don't know if this is critical or not.

Apologies for plaguing you all with these questions.

Whilst here I may as well ask a second question, this time regarding captures with the ZWO - I can adjust exposure using the "Gain" and "Exposure" controls.  Is it best to aim to have the lowest possible exposure time, or should I be aiming for lowest gain?  The moon was fairly bright and I set exposure to 1ms and had gain at (from memory) somewhere between 30 and 40%).

[PeterCPC]

That image looks fairly standard -what you need to do is to try to get the gaps between the spikes to be the same distance. In your image the centre spike needs very slight adjustment towards the top right. I use my mask on the end of the dew shield and it works fine.

I can't help on the ZWO question but I expect someone will be along to address that soon.

Peter

[gnomus]

Thanks Peter.  That is reassuring.

As a follow up to my ZWO question, I attach one of the snaps that I got last night.  It is a mono image taken through a red filter.  It is around 1-2% of a 3000 frame SER file. 

Obviously the image is not all that sharp.  However, my question relates to the noise in the sky surrounding the moon.  I attach also a 100% crop of just the sky.  (In the cropped version I have done a curves boost to more easily display the noise - I found that I could not see the noise in the un-boosted shot when I previewed my post).

I did play around with levels for a bit in PS using the Alt key to show where the clipping was occurring.  I could get the sky to go to a solid black, but this also meant losing a fair bit of detail on the shadow areas on the moon's surface.

Does anyone have any suggestions for reducing noise?   

[ronin]

The Bahtinov mask image looks like it is missing a set of lobes on one side - not sure why as I know little about what is seen through one under different conditions. What you have looks very much like the "bunny" symbol.

You do not give the scope (refractor assumed), is the focuser a 2 speed lump?

Just thinking that a single speed may mean you go in and out of ideal focus too quick to manage to stop at the right position.

Hopefully an imager that uses a mask more often will wander along with a greater depth of knowledge.

But what you show and what should be apparent is nor correctly symetric.

[Carina Lass]

Mask image is correct.

focus is slightly out as mentioned

centre spike needs moving towards top right

[lensman57]

In an earlier thread (http://stargazerslounge.com/topic/231139-first-attempt-with-zwo-asi120mm-s-moon-many-questions/) I gave a particularly poor written account of an issue I had with "blobs" when trying to focus using a Bahtinov mask.  I've been out again and have now taken a snap of the "blobs".  It is attached below. (Taken with my ZWO ASI 120-MM.)  The star is Capella.

Is this what I would expect to see when using the mask?  If so, how am I supposed to see where the lines intersect?  If not, what is going on with my mask?

I did try using a program called Bahtinov grabber (suggested by a responder to my earlier thread).  This was not all that successful.  I could not get the numbers to go to zero and I could not get to a state where the program consistently said "YES" to the critical focus question (it flickered between yes and no).  Nevertheless, I persisted until I had the lowest number I could get, and until the flickering between Yes and No tended to the Yes state.  I then locked focus on my Edge HD 8.  The numbers still looked OK on Bahtinov Grabber.  I slewed to the Moon, however, and focus was a mile off - I ended up focussing on screen by eye (with poor to moderate results - I think the seeing was again very bad last night).

I am attaching the mask to the end of my home-made dewshield.  I don't know if this is OK (someone said it shouldn't be a problem).  One thing that did occur to me is that it is possible that because the mask is on the end of the dewshield it may not be parallel to the front glass of the scope.  Again, as a beginner, I don't know if this is critical or not.

Apologies for plaguing you all with these questions.

Whilst here I may as well ask a second question, this time regarding captures with the ZWO - I can adjust exposure using the "Gain" and "Exposure" controls.  Is it best to aim to have the lowest possible exposure time, or should I be aiming for lowest gain?  The moon was fairly bright and I set exposure to 1ms and had gain at (from memory) somewhere between 30 and 40%).

You are out of focus there to start. The mask is best placed as close to the objective as possible but if you manage to get decent focus then all is well. Personally I have had a lot of problems using a mask for critical focus, I prefer to use the FWHM focusing routine with a 3s loop in the capture software. Leave the gain at about 75~80% and adjust the exposure.

A.G

[gnomus]

Thanks again for the responses.  

Ronin: The scope is an Edge HD 8".  It came with a one speed focusser, but I have changed this to a Feathertouch two speed (but still manual) focusser.  I do have quite fine control therefore.

AG: Not sure what FWHM is exactly.  I am using Firecapture (but have also got SharpCap if necessary).  Is that something I do with software - and, if so, can I do it with either of those programs?

Inside in the warmth, and on my widescreen desktop PC, it is easy to see in the picture above that the middle spike is slightly off-centre.  I'm finding it a little more difficult to do this on my laptop screen out in the cold (and with the laptop balanced precariously on top of a wheelie bin).  

[lensman57]

Thanks again for the responses.  

Ronin: The scope is an Edge HD 8".  It came with a one speed focusser, but I have changed this to a Feathertouch two speed (but still manual) focusser.  I do have quite fine control therefore.

AG: Not sure what FWHM is exactly.  I am using Firecapture (but have also got SharpCap if necessary).  Is that something I do with software - and, if so, can I do it with either of those programs?

Inside in the warmth, and on my widescreen desktop PC, it is easy to see in the picture above that the middle spike is slightly off-centre.  I'm finding it a little more difficult to do this on my laptop screen out in the cold (and with the laptop balanced precariously on top of a wheelie bin).  

Full Width at Half Maximum. It is a software implemented measure of a width of the bump of a waveform in mathematical terms but for imaging you just watch the number and when it hits its minimum the star is at its sharpest. I don't do planetary but I doubt if Fire Capture or SharpCap support this function. For DSO imaging almost any capture software is capable of doing FWHM measurements.

A.G