Prime back focus lengths question...

[DiligentSkies]

Hello All,
I have had my equipment for nearly two months.
The several times that I have been able to set up my scope and mount, "Have been entirely devoted to working out software platform issues."

Namely, plate solving within using Sharp Cap to address some very specific issues related to creating my mount's Celestial Model and polar alignment.

I am happy to report much success.

In that by using plate solving to address issues specifically related to my observational situation/constraints.
Since all I can see is directly South with about 2 hours of RA on either side of the meridian.
No EAST or WEST vantage points and certainly no Polaris North or Zenith.

Plate solving has been an eye opener.

By using plate solving, I been able to build out my mounts Celestial Model using Formalhaut, Diphda, Sadachbia/Sadalsuud(depending on that darn palm tree) and Skat.

I then next complete the mounts alignment workflow by using Celestron's All-Star polar alignment procedures by revisiting Diphda and make manual Azimuth and Latitude adjustments to bring Diphda back into view. Such that when I fire up PHD2 guiding, I am able to achieve reasonable guiding specs of being polar aligned.

Now that I am quite satisfied with these platform issues and polar alignment and things are working quite fine, "I want to attempt to address prime back focus issues, in terms of optimizing my imagining train's physical back focus length.

I am not really sure if, "I am chasing my tail here."

I have achieved an optimal Bahtinov Mask focused on my current physical back focus length build out length as measured off the William Optics spec charts.

In other words, "Best guesstimates on my back focus physical build out length.

The thing is and maybe this the point to fully describe my equipment.

First, I have a William Optics GT 81mm APO triplet with the William Optics adjustable 0.8 reducer.

According to the William Optics spec charts.

The prime back focus length for this setup is, "62.1mm when the reducer is dialed in near the 7.1mm point."

Next my imagining camera is the QHY268c that has a 17.5mm back focus spec. Thus my imagining train has a back focus length of 44.6 mm of wiggle room according to the William Optics spec charts.

To my best measurements, I am in the zone of being prime back focused.

However, this is something not being addressed by the William Optics spec charts on the prime focus back length imagining train build out specs.

In that I have also have incorporated two elements of glass into this imagining train build out.

First, an Astronomik L2 UV-IR filter that is buried in the housing of the William Optics reducer.

Second I am using a ZWO filter drawer system to swap out filters in my imagining train build out.

Namely, a tri-band Optolong L-eNhanced filter, a visual black filter(for taking my darks), an Optolong S-ii filter and a visual blank filter drawer(no filter just the housing) for broad band imaging.

Such that when I get Bahtinov Mask focused with the Optolong L-eNhanced filter in place and then switch out to a no filter arrangement.

I am then out of focus, "As expected."

This is informing me these glass element filters are playing a role in my imagining train's total physical back focus length.

Such that I am questioning...

"Whether I am just Bahtinov Mask back focused optimally on my current imagining train's back focus build out length(and that I may be less than optimal) and there is room for improvement by adding or taking away from the spacers used to build out my imagining train.

So is THERE a means to evaluate this???

As of now... I am noting many, if not all of my stars are eggy in appearance even though I am Bahtinov Mask focused onto my imaging train's current prime back focus build out length (specifically stars in the two second subs I am using for guiding that should be pin point and are clearly not and the dynamical range on imagining the Orion Nebula seems to be washed out and falling well below the expectations of the QHY268c camera.)

Yes, I know there is the plus or minus 3mm fudge factor in building out an imagining train length, but I suspect I may be pushing, "The envelope towards the edge of the physical back focus length optimum."

Essentially, "Is there a systematic means to evaluate these constraints in making fine tune adjustments towards changing the physical length of my imagining train?"

Meaning that, "I may very well be able to achieve an optimal Bahtinov Mask focus according to my current physical imagining train build out."

However it appears to me, "I very well may be NOT optimally in the zone nearing an accurate/optimal physical back focus length in regards to my imaging train's physical back focus length build out that incorporates these additional glass elements. That ultimately affect the specs of building out an imagining trains physical prime focus back length according the William Optics spec charts?"

So essentially, "I am looking for a systematic means to evaluate results."

Such that...

"What should I be looking to measuring results verses making +/- 2mm changes to my physical focus length?"

Is there, "A tried and true evaluation in place, when it comes to physically changing the prime focus back focus length??"

Sincerely,
Mark @ diligenskies@outlook.com

[BlackWikkett]

Hi

Here's a good video about focus and back focus. https://youtu.be/HndMaPsH85I

I struggled with this subject for a long time. First thing to understand is everything in the optical light path of your telescope will affect focus. When you see back focus mentioned in a camera's spec sheet this usually defines the image plane of the sensor measured from some point on the housing of the camera. If you see back focus on telescope this is just the measurement from the back of the telescope (usually threads / clamp) to point where the sensor of the camera will achieve focus. Reducers will also have the back focus distance. This can also be range for some reducers. This type of reducer can provide a variable amount of reduction based on the amount of space from the reducer to the camera's sensor.

Every time you add or change filters you will need to refocus. When attaching the camera, filters, and reducers to the telescope you don't have to be perfectly at x.xxx mm of back focus. Just get in the ball park and make sure your telescope's focus adjustment has enough travel to make up the difference.

Hope this helps
-Chris