Back Focus on MoonLite 3.5" NiteCrawler with the 645 reducer QE 0.72 - Forums

I recently purchased a Tak FSQ106EDX4 along with the 645 reducer QE 0.72 and a NiteCrawler 3.5. The reducer is installed with a WR35 to TAK 645 adapter. It is a 2 part design and comes in 3 output thread sizes, M54, M48, and M68 male.

My question is the back focus requirement from the terminal end of the NiteCrawler with the reducer installed at the M54 male thread. I tried a using a M54 spacer combination 15mm & 7.5mm for a total of 22.5mm which brought the camera & filter wheel up to 56.5mm. It allowed the NiteCrawler to find focus, but certainly not correct. Based on the camera sensor evaluation, it appears to be to far away. That brings me to think that the target distance is 55mm from the terminal end.

Thanks for your response,

Todd Charlson

Hey Todd, Do you mind posting some examples of what it looks like and what the subs look like? Maybe also take a picture of your imaging train. The back focus for the QE 0.72 is 56.2mm, which is quite standard for most Tak reducers I have used. You also have to take into consideration your filter thickness. If you are using 3mm thick filters like the Chromas for example, you need to add 1mm to your backfocal distance requirements. So in that case you would need to be at 57.2mm to achieve optimal backfocus for that reducer and filter combination.

See answers below.

My understanding was that when you are in focus, you are also at the correct back focus, even with the 645 reducer installed.

My understanding was that when you are in focus, you are also at the correct back focus, even with the 645 reducer installed.

I think that only works if you are using the native Tak OEM focuser because the reducer is designed to slide inside at a fixed distance from the objective and independent from the draw tube.

Don't think that works with the WR35.

I stand corrected. It appears that the WR35 has an adapter that allows the reducer to be fixed to the OTA just like the OEM as detailed by others.

The 645 reducer does not have a set back focus. When the camera train is in focus you are good to go.

I had the same issue with my setup and I had a 55mm backfocus on my imaging train (with the OAG and EFW I couldn't remove anything to make it shorter to get focus). Here is what I posted on CN a while back:

"Ron at Moonlite sells a 2 part adapter to use the 645 reducer with the Nitecrawler. The OTA adapter from Moonlite includes the threaded dovetail that screws on the OTA and a 0.7" extender to use at native focal length. If you get the 645 adapter set, you have to remove the 0.7" flange extender. I bought the M54 adapter and it is just a tad bit too long for the Moonlite to come into focus (JUST barely…maybe 1000 more steps with the Moonlite and I would hit focus) with the 645. He is making me a new M54 adapter that is thinner than their stock M54 adapter. If you have an imaging train less than 55mm you shouldn't need the thinner adapter. My ASI2600MM with filter wheel and OAG put me at 55mm and I couldn't shorten it. My full frame setup shouldn't need the thinner adapter once I get it up and running (still waiting on filters from Astronomik)."

If you can remove adapters from your imaging train you should still be able to use the 0.7" extender flange before the Nitecrawler. Since I couldn't remove the EFW or OAG I had to take out that 0.7" extension. Even with that removed, I still needed a thinner M54 adapter to reach focus.

Ashraf AbuSara:
My understanding was that when you are in focus, you are also at the correct back focus, even with the 645 reducer installed.

I think that only works if you are using the native Tak OEM focuser because the reducer is designed to slide inside at a fixed distance from the objective and independent from the draw tube.

Don't think that works with the WR35.

I got the information from Ron at Moonlite.
A call to Takahashi could likely straighten the whole thing out. My scope is at Takahashi right now, so next time I talk to them, I will ask and report back.

Ashraf AbuSara:
My understanding was that when you are in focus, you are also at the correct back focus, even with the 645 reducer installed.

I think that only works if you are using the native Tak OEM focuser because the reducer is designed to slide inside at a fixed distance from the objective and independent from the draw tube.

Don't think that works with the WR35.

I got the information from Ron at Moonlite.
A call to Takahashi could likely straighten the whole thing out. My scope is at Takahashi right now, so next time I talk to them, I will ask and report back.

Oh that's interesting! I am not sure how that is possible but I'd defer to Ron for sure.

Lum @ 240 secs This image looks to close

Ha at 300sec but at the same focus as the Lum. Subsequently, it looks too far away.

Tak mounted, No spacers. Just the initial test

Daniel Carter:
The 645 reducer does not have a set back focus. When the camera train is in focus you are good to go.

I had the same issue with my setup and I had a 55mm backfocus on my imaging train (with the OAG and EFW I couldn't remove anything to make it shorter to get focus). Here is what I posted on CN a while back:

"Ron at Moonlite sells a 2 part adapter to use the 645 reducer with the Nitecrawler. The OTA adapter from Moonlite includes the threaded dovetail that screws on the OTA and a 0.7" extender to use at native focal length. If you get the 645 adapter set, you have to remove the 0.7" flange extender. I bought the M54 adapter and it is just a tad bit too long for the Moonlite to come into focus (JUST barely...maybe 1000 more steps with the Moonlite and I would hit focus) with the 645. He is making me a new M54 adapter that is thinner than their stock M54 adapter. If you have an imaging train less than 55mm you shouldn't need the thinner adapter. My ASI2600MM with filter wheel and OAG put me at 55mm and I couldn't shorten it. My full frame setup shouldn't need the thinner adapter once I get it up and running (still waiting on filters from Astronomik)."

If you can remove adapters from your imaging train you should still be able to use the 0.7" extender flange before the Nitecrawler. Since I couldn't remove the EFW or OAG I had to take out that 0.7" extension. Even with that removed, I still needed a thinner M54 adapter to reach focus.

I have an QHY OAG L Pro being delivered tomorrow. The back space diagram indicates 55mm for the image train using the exact equipment I am using. So, no room for adjustment. I will contact Ron if an issue with the back space arrises. Thanks for sharing your experience.

I wonder if its sagittal astigmatism.
This is a demanding setup, and it's not really meant to support a full-frame chip and that small of a pixel. However, I still roll with it myself for the speed.

My understanding was that when you are in focus, you are also at the correct back focus, even with the 645 reducer installed.

Yup. This is the correct answer. The 645 threads onto the rear lens cell of the FSQ and unlike other Tak reducers it doesn’t have a back focus requirement per se. Once you reach focus you are good to go. The total back focus can still be an issue however. With a camera such as the 6200mm pro, you’ll need to add a 10mm or so spacer so that the OAG camera cup doesn’t interfere and once that is added it barely reaches focus.

I have this exact setup. It’s never been an issue. I remove the spacer between the NiteCrawler and the OTA and then screw in the Moonlite adapter and reducer, then the rest of image train. I don’t have enough space for a OAG so need a seperate guidescope for that. Get focus and away I go.

Matt Hughes:
I have this exact setup. It’s never been an issue. I remove the spacer between the NiteCrawler and the OTA and then screw in the Moonlite adapter and reducer, then the rest of image train. I don’t have enough space for a OAG so need a seperate guidescope for that. Get focus and away I go.

It appears that the original poster has the same setup as mine which is using the full 55mm backfocus that comes with using the OAG and EFW. When using an image train with 55mm of backfocus, getting a focus point was a challenge. I had to remove the 0.7" extension before the Nitecrawler and needed a M54 adapter that was narrower than Ron's normal adapter.

Since you didn't have the OAG it isnt the exact setup the original poster is using. You may not have ran into issues finding focus with the shorter spacing between the Moonlite adapter and your imaging train since you didnt have the OAG.

@Todd Charlson - try removing the 0.7" extension before the Nitecrawler and use the M54 adapter that came with your 645 kit. If you still aren't able to reach precise focus, you may have to reach out to Ron and get the narrower M54 adapter.

Todd Charlson:
My question is the back focus requirement from the terminal end of the NiteCrawler with the reducer installed at the M54 male thread. I tried a using a M54 spacer combination 15mm & 7.5mm for a total of 22.5mm which brought the camera & filter wheel up to 56.5mm. It allowed the NiteCrawler to find focus, but certainly not correct. Based on the camera sensor evaluation, it appears to be to far away. That brings me to think that the target distance is 55mm from the terminal end.

The 56.5mm is for the .6 reducer not the .72 reducer as per Tak. If using the .72 reducer it needs to be 73.5mm between the reducer and the chip, then use the focuser to
bring it into focus. I use the FLI Atlas focuser not the NiteCrawler, but the math is the same and works for my setup.

This chart is from Tak website.

Bob Lockwood:
The 56.5mm is for the .6 reducer not the .72 reducer as per Tak. If using the .72 reducer it needs to be 73.5mm between the reducer and the chip, then use the focuser to
bring it into focus. I use the FLI Atlas focuser not the NiteCrawler, but the math is the same and works for my setup.

This chart is from Tak website.

Thanks for the information, Bob. This gives me a detailed starting point and potentially room to add a Moonlite Sidewinder tilt adjuster.

Todd Charlson:
Bob Lockwood:
The 56.5mm is for the .6 reducer not the .72 reducer as per Tak. If using the .72 reducer it needs to be 73.5mm between the reducer and the chip, then use the focuser to
bring it into focus. I use the FLI Atlas focuser not the NiteCrawler, but the math is the same and works for my setup.

This chart is from Tak website.

Thanks for the information, Bob. This gives me a detailed starting point and potentially room to add a Moonlite Sidewinder tilt adjuster.

I should clarify that the 73.5mm is not a fixed distance, but it should be where the chip comes to focus between the reducer and the chip.

Good point Bob. It’s worth pointing out also that Tak’s distances are measured from the back of their stock focuser. The Moonlite replaces the stock focuser and is thinner, so it increases those numbers and enables a longer imaging train - one of the benefits of using the Nightcrawler.

For those interested in this setup with a ZWO 6200mm pro. There isn’t much room to spare. With the Moonlite two piece adapter, the 645, a 10mm spacer to fix the OAG interference problem, and an ASG Photon tilt adapter, there isn’t a whole lot of room left to reach focus. Maybe a few mm and not enough room to add say a Gerd Neumann CTU.

I'd also point out that you need to be very careful when everything is being made, adapter wise. As Mark pointed out, there is very little room, and as in my setup using the FLI Atlas focuser, the red line in the image below shows about where the 645 reducer sits, and with the focuser in as far as it can go, it's only about 1mm from hitting the pick-off mirror in the OAG. There's a lot of people out there doing what you are trying to do, it will work.

Bob Lockwood:
I'd also point out that you need to be very careful when everything is being made, adapter wise. As Mark pointed out, there is very little room, and as in my setup using the FLI Atlas focuser, the red line in the image below shows about where the 645 reducer sits, and with the focuser in as far as it can go, it's only about 1mm from hitting the pick-off mirror in the OAG. There's a lot of people out there doing what you are trying to do, it will work.

The 2-piece adapter for the 645 Reducer from Moonlite is made for this specific application and fits with no issues on internal clearance from the end M54 adapter (or M68 adapter - whichever is chosen when ordering). If adapting for other uses (like your Atlas or I've heard of people using the TCF-LEO or a Gemini) then you would need to be careful on clearance.

Todd Charlson:
I recently purchased a Tak FSQ106EDX4 along with the 645 reducer QE 0.72 and a NiteCrawler 3.5. The reducer is installed with a WR35 to TAK 645 adapter. It is a 2 part design and comes in 3 output thread sizes, M54, M48, and M68 male.

My question is the back focus requirement from the terminal end of the NiteCrawler with the reducer installed at the M54 male thread. I tried a using a M54 spacer combination 15mm & 7.5mm for a total of 22.5mm which brought the camera & filter wheel up to 56.5mm. It allowed the NiteCrawler to find focus, but certainly not correct. Based on the camera sensor evaluation, it appears to be to far away. That brings me to think that the target distance is 55mm from the terminal end.

Thanks for your response,

Todd Charlson

If I get a chance tonight, I will take a picture of my setup (without the imaging train - my imaging train has 55mm backfocus like yours). If I recall, I may have had an issue with the ZWO OAG hitting the housing of the Moonlite adapter for the 645 reducer.

This is quickly becoming a great thread about the FSQ106. Great information. Definitely something I will be referencing to in the future if I acquire one.

Daniel Carter:
Todd Charlson:
I recently purchased a Tak FSQ106EDX4 along with the 645 reducer QE 0.72 and a NiteCrawler 3.5. The reducer is installed with a WR35 to TAK 645 adapter. It is a 2 part design and comes in 3 output thread sizes, M54, M48, and M68 male.

My question is the back focus requirement from the terminal end of the NiteCrawler with the reducer installed at the M54 male thread. I tried a using a M54 spacer combination 15mm & 7.5mm for a total of 22.5mm which brought the camera & filter wheel up to 56.5mm. It allowed the NiteCrawler to find focus, but certainly not correct. Based on the camera sensor evaluation, it appears to be to far away. That brings me to think that the target distance is 55mm from the terminal end.

Thanks for your response,

Todd Charlson

If I get a chance tonight, I will take a picture of my setup (without the imaging train - my imaging train has 55mm backfocus like yours). If I recall, I may have had an issue with the ZWO OAG hitting the housing of the Moonlite adapter for the 645 reducer.

The ZWO OAG contacts the Moonlite adapter, preventing it from being fully threaded. I have rotated the flange on the adapter towards the OAG, and it's never been a real issue for me, other than the OCD aspect of it.

Daniel Carter:
If I get a chance tonight, I will take a picture of my setup (without the imaging train - my imaging train has 55mm backfocus like yours). If I recall, I may have had an issue with the ZWO OAG hitting the housing of the Moonlite adapter for the 645 reducer.

@Daniel Carter

Quick question: What effect does removing the 0.7" extension before the Nitecrawker have on the image train?

Last night I installed the QHY L Pro OAG to the filter wheel. Unfortunately, I ran into an issue with the mounting. The tilt was very severe, but I could tell that the image train was too close. I ordered a series of spacers to help determine the actual back space.

Ron isn't available until June 3rd.

Removing the 0.7" extension moves the focus point of the Nitecrawler farther away from the end of the focus tube. The actual focus point doesn't change since it is a fixed point at a given distance from the end of the 645 reducer. For example, if the focus point happened to be 2 inches from the end of your Nitecrawler 645 adapter, Removing the extension now puts the focus point 2.7 inches from the end of the adapter. The actual focus point doesn't physically change in relation to the reducer, you just move the whole focuser assembly forward towards the OTA around the reducer.

It isnt raining tonight so I will try to take pictures.

After thinking about it. I'm fairly certain that I had clearance issues with the ZWO OAG and the end of the 645 Nitecrawler adapter. I ended up taking a Dremel to the ZWO OAG and rounding the base a few millimeters to make that clearance. It was a tight fit but it worked. Make sure not too much material is removed so you don't create a light leak.

Daniel Carter:
Removing the 0.7" extension moves the focus point of the Nitecrawler farther away from the end of the focus tube. The actual focus point doesn't change since it is a fixed point at a given distance from the end of the 645 reducer. For example, if the focus point happened to be 2 inches from the end of your Nitecrawler 645 adapter, Removing the extension now puts the focus point 2.7 inches from the end of the adapter. The actual focus point doesn't physically change in relation to the reducer, you just move the whole focuser assembly forward towards the OTA around the reducer.

It isnt raining tonight so I will try to take pictures.

After thinking about it. I'm fairly certain that I had clearance issues with the ZWO OAG and the end of the 645 Nitecrawler adapter. I ended up taking a Dremel to the ZWO OAG and rounding the base a few millimeters to make that clearance. It was a tight fit but it worked. Make sure not too much material is removed so you don't create a light leak.

Thanks for your quick reply, Daniel. Much appreciated.