Focusing with or without a Bahtinov mask

[Avocette]

I appreciate that there are some planetary imagers who metaphorically throw their hands up in horror when people mention focusing their scopes with a Bahtinov mask. However, in my experience I’ve had much more success by using a mask to focus, either on a nearby star or else on one of the planet’s moons, than using the video image on my laptop screen. I’m using a SkyMax 150 with a Barlow and use the normal focusing knob manually to make adjustments. I do own an add-on Crayford focuser fitted with a SW DC electric motor drive, but using this adds significantly to the focal length of the arrangement, and the manual control motor focuser isn’t precise in use.

Am I missing something here?

[SkyMax 150 (effective aperture 140mm) with Barlow measured focal length 3445mm = ~f25. ASI533MC with pixel size 3.76um.]

[Ian McCallum]

I use two different Bahtinov masks on my set up. A small one for the finderscope/guidescope and the larger one on my two 120mm achromatic refractors. 

I do a bit of planetary and DSO imaging. Your mileage may vary. 😉

[newbie alert]

I used to focus with a mask on a nearby star , but now I focus on the planet, think they call it jogging, in and out of focus until you reach the point of best focus... Bahtinov masks are very close but it can be refined abit further..

[Avocette]

The idea of jogging probably requires using the Crayford accessory - there is a lot of backlash in the Mak primary mirror focus control so I generally approach the point of focus from one direction only.

[Space Cowboy]

Changes in planetary elevation, seeing conditions and temperature mean bmasks are unreliable at the high magnification needed.

[newbie alert]

5 hours ago, Avocette said:

The idea of jogging probably requires using the Crayford accessory - there is a lot of backlash in the Mak primary mirror focus control so I generally approach the point of focus from one direction only.

Yeah good call.. I use a crayford with my SCT 

[Elp]

The easiest method I've found is set your gain and/or your exposure high but not too high so it updates quickly on screen, the planet will be a bright dot, adjust the focus until it's as small as possible, some to and froing will be needed, then dial back the gain and exposure until you see surface detail then repeat. No mask needed.

[Tommohawk]

My current set up for planetary means I have to focus manually - which at high mag is pretty much impossible - so Bahtinov on a nearby star is really the only option.

For the life of me I can't see why it wouldn't be appropriate, from an optical standpoint, for planetary imaging - the Bahtinov image lines are nice and crisp and tiny focusing adjustments throw the alignment quite markedly. Yes, focus will vary due to turbulence etc  - but it will for any method.

I know some of the top planetary imagers prefer to focus subjectively, but I'm not sure that means a Bahtinov is inappropriate or innacurate. The only issue is if your scope/train suffers flexure such that moving from the star to the planet shifts things - and this may be a real problem in some cases.

[Space Cowboy]

18 hours ago, Tommohawk said:

My current set up for planetary means I have to focus manually - which at high mag is pretty much impossible - so Bahtinov on a nearby star is really the only option.

For the life of me I can't see why it wouldn't be appropriate, from an optical standpoint, for planetary imaging - the Bahtinov image lines are nice and crisp and tiny focusing adjustments throw the alignment quite markedly. Yes, focus will vary due to turbulence etc  - but it will for any method.

I know some of the top planetary imagers prefer to focus subjectively, but I'm not sure that means a Bahtinov is inappropriate or innacurate. The only issue is if your scope/train suffers flexure such that moving from the star to the planet shifts things - and this may be a real problem in some cases.

I think you would be better investing in an electric focuser. It's not impossible to focus manually but it is impossible to expect focus to stay constant for more than 10 mins after using a bmask given changes in seeing and temperature.  Are you going to then slew back to a star to refocus and possibly miss a period of good seeing or even cloud moving in? 🙂

[Tommohawk]

On 30/12/2022 at 13:59, Space Cowboy said:

I think you would be better investing in an electric focuser. It's not impossible to focus manually but it is impossible to expect focus to stay constant for more than 10 mins after using a bmask given changes in seeing and temperature.  Are you going to then slew back to a star to refocus and possibly miss a period of good seeing or even cloud moving in? 🙂

Yeah I have electric focusers and would certainly prefer this - however I can't do this with my latest planetary rig. It's a Newt but with camera mounted on a helical focuser where the secondary would normally be.

The idea is that I have a smaller central obstruction (43mm from memory cf a 250mm primary) It works nicely except for the focuser headache and for that reason I will likely abandon this design. (It was a prototype for a larger version)

I have checked focusing before and after doing a run and I cant see any changes - and I'm pretty OCD about this! Of course this doesn't prove that it stays in focus when slewing.