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tokens: of linking lengths is somewhat generous compared to previous studies. We use the FoF algorithm only as a basis for an X-ray-detected cluster catalog in the HectoMAP region. We would rather include false candidates than exclude real ones. We next show that this approach recovers the relevant set of X-ray clusters in a separate densely surveyed region with essentially full X-ray coverage (Starikova et al. 2014 ). We test the FoF on the SHELS survey (Geller et al. 2010 , 2012 , 2014 , 2016 ), a dense complete redshift survey with no color selection and covering two well-separated 4 deg 2 fields (F1 and F2) of the Deep Lens Survey (DLS, Wittman et al. 2002 ). We use the F2 survey alone for this investigation because it has essentially complete X-ray coverage of all of the possible massive systems in the appropriate redshift range. The F2 survey is ∼95% complete to R ≤ 20.6 with a number density of ∼3200 galaxies deg −2 . In total, there are 26 X-ray-extended sources detected based on Chandra and XMM X-ray data (Starikova et al. 2014 ); 18 of these X-ray sources are in the redshift range 0.2 z ≤ 0.6 of interest for testing our approach to HectoMAP X-ray clusters. Here, we select 12 X-ray sources within 0.2 z ≤ 0.5 for testing our cluster identification procedure because the F2 magnitude limit R ≤ 20.6 is ∼0.4 mag brighter than the HectoMAP limit. The brighter F2 limit precludes FoF cluster detection at z ≳ 0.5. The X-ray flux limit for the 12 X-ray-extended sources with z 0.5 is 4.3 × 10 −15 erg cm −2 s −1 within 0.5–2.0 keV energy band. This X-ray flux limit is nearly two orders of magnitude fainter than the one we reach for the HectoMAP systems. Starikova et al. ( 2014 ) showed that 10 of these sources have optical counterparts identified as clusters in the SHELS spectroscopic survey (Geller et al. 2010 ) or in the DLS weak lensing analysis (Wittman et al. 2006 ). Here, we examine the number of F2 X-ray extended sources recovered by the FoF algorithm with the HectoMAP choice of linking lengths. Figure 4 shows the cone diagram of the subset of F2 galaxies selected according to the HectoMAP red galaxy prescription within 0.2 z ≤ 0.5. In Figure 4 , the cyan circles display the locations of seven X-ray-extended sources with spectroscopically identified counterparts from Geller et al. ( 2010 ). We also show the X-ray extended sources (yellow circles in Figure 4 ) matched with weak lensing peaks (Wittman et al. 2006 ) or with optical concentrations on the sky in the DLS images (Starikova et al. 2014 ). The redshifts of these X-ray sources with photometrically identified counterparts may not be accurate because they are based on photometric redshifts. Figure 4. Cone diagram for the SHELS F2 survey subsample with R ≤ 20.6, ( g − r ) fiber,0 > 1.0, ( r − i ) fiber,0 > 0.5 projected in the R.A. direction. The cyan and yellow circles indicate extended X-ray sources in F2 with spectroscopic counterparts (Geller et al. 2010 ) and photometric counterparts